Chartwork

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Skill Development Course

Chartwork and

Position

Position Fixing

Fixing

Instructor Manual

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Issue 1.2, February 2010 Issue 1.2, February 2010

Published in the United Kingdom Published in the United Kingdom

British Sub-Aqua Club, Telford’s Quay, South Pier Road, Ellesmere Port, Cheshire CH65 4FL British Sub-Aqua Club, Telford’s Quay, South Pier Road, Ellesmere Port, Cheshire CH65 4FL

Telephone:

Telephone: 0151-350 0151-350 6200 6200 Fax: Fax: 0151-350 0151-350 6215 6215 www.bsac.comwww.bsac.com

All rights reserved. This Instructor Manual may not, in whole or part, be copied, photocopied, reproduced or All rights reserved. This Instructor Manual may not, in whole or part, be copied, photocopied, reproduced or translated, or converted into any electronic or readable form without prior written consent of British Sub Aqua translated, or converted into any electronic or readable form without prior written consent of British Sub Aqua

Course Aim

To instruct members in the basic techniques of marine navigation and dive site location to a level which will help them as active divers in planning and managing dives undertaken by a typical BSAC branch.

Course Overview

This is a practical course which has the students using charts and chartwork instruments right from the start. It culminates in a day at sea, navigating to two dive sites and locating them by echo sounder, directed by a voyage plan built up gradually over the rst day. The rst part of the course teaches the basics of charts, measurement and tides, interspersed with exercises. This is followed by position xing techniques and a session on the specic problems involved in locating dive sites. The last part of day one looks at how all the techniques are brought together to create a voyage plan.

The focus of the course is developing practical navigation skills, not boat handling or dive management, which are covered thoroughly on other BSAC courses. The course is aimed primarily at daylight navigation, and makes extensive use of GPS and echo sounders. On the practical day students receive coaching in the mental approach that good navigators employ in addition to mastery of specic techniques.

Because the course uses actual data from ofcial nautical reference materials to plan and execute navigational exercises at sea, it requires a small amount of tailoring of the visual aids and class exercises to make them specic to the local area where the practical navigation aoat is to be run. Full instructions for tailoring are included in Appendix A of this manual.

Session Contents Mins Day 1 Introduction Assemble, introductions, administration 15 Chart Basics Chart availability, major brands paper and digital, Admiralty, updates available from website; scale; general layout; colours - land, intertidal, submerged; chart symbols - depths (spot, drying, isobath), wrecks, rocks etc, nature of seabed, buoys and lights, buoyage, landmarks; lat/long - what it is, how to write it down, how to determine lat/ long of a point on chart; 60 Chart Exercises Exercises 30 Measurement Basics Terminology; bearings, variation, compass rose, measuring at sea; drawing on charts, measuring angles, reciprocals, measuring distance; speed distance & time, calculations and tables; measuring with GPS; water v ground track Exercises 60 Tides Tidal terminology; cause of tides; springs and neaps; tide

tables; tidal range; calculating depth of water; rule of twelfths; tidal diamonds, tidal stream atlas, local effects, calculating slack water; chart symbols - tides and currents; implications for launch/recovery, diving, navigation Exercises

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Position Fixing Methods

Simple x; position lines – compass

bearing, transits, uses of position lines -clearing lines, leading lines, x; position x; visual x accuracy; GPS; GPS limitations, geodetic datum; other methods - depth sounding, radar.

30

Position Fixing Dry Practical

Take transits. Exchange with other groups and use to relocate hidden objects. Outdoors. 45 Finding Dive Sites Accuracy v precision; problems with positions, types of error/inaccuracy; dive site information needed for searches, echo sounder basics, approaching the dive site; search patterns, pros and cons

30 Voyage Planning Waypoints, waypoint navigation, using GPS; choosing waypoints; techniques - aiming off, attack points, buoy hopping, straight to GPS; preparing the voyage plan, planning process, documentation; allowing for tide; practical navigation; charter boats;

navigation Zen; model voyage plan

45

Voyage Planning Exercise

Work in groups to build up complete voyage plan to be used on Day 2 75 7h30' Day 2 Practical Navigation Aoat 6h45' Open Forum Course debrieng; issue documentation; disperse 15

Duration

Classroom lessons may be split over a number of (evening class) sessions if required. This approach allows time for additional exercises to be set, which some students may nd helpful. It would also be possible to split the practical day into two sessions. A Voyage planning exercise should immediately precede every practical session aoat.

Entry Level

BSAC member.

Previous experience is not essential, but it will enhance students' experience of the course if they have experience of diving from boats, and if they are able to drive prociently the type of boats used on the course.

Qualications Awarded

Qualication card conrming course attendance, supplied by BSAC HQ on payment of fees and conrmation of attendance on the course. There are no examinations or assessments.

Course Registration

Branches wishing to run a Skill Development Course must order the packs two weeks in advance from the BSAC Shop and pay the relevant course pack/ registration fees for all students on the course. This can be done online via the BSAC website, by telephone, or by post.

Instructor Requirements

Qualications ●

Each instructor should be a BSAC Open Water Instructor or above, with practical experience of navigating dive boats. All instructors should be familiar with the entire contents of the course, and should be capable of performing all elements of the practical exercises

competently and condently at the chosen location. Knowledge of the content of previous versions of CPF alone is not sufcient.

If necessary, instructors should rehearse the practical skills with other instructors at the location chosen for the practical day before teaching/supervising other students.

Each boat should be manned by a person suitably qualied to take charge of it (see Equipment, below) and with a thorough knowledge of how to operate the electronic navigation aids. Normally this should be the instructor.

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Student:Instructor ratio ●

The maximum recommended ratio is four students to each instructor for all parts of the course. One lecturer may address any number of students in the theory lessons, but additional instructors should be on hand to assist with any exercises.

For the at sea exercises it is preferable to have two instructors in the boat because the physical layout of most RIBs makes it difcult to instruct more than two students at a time whilst the boat is making way. While one student is carrying out an exercise, with a second student observing if appropriate, the second instructor can coach the others.

Equipment

Up-to-date charts of the area to be used during the practical lessons, plus dive guides, almanacs, tide tables, reference books as appropriate, and chartwork instruments. Students should work from full colour versions of charts and similar navigational publications. Internet access for planning sessions if possible. Specic requirements are shown under Additional visual aids at the start of the notes for each lesson.

Boats tted with a GPS, echo sounder and steering compass for practical lessons - one boat for each group of four students and instructor. Any type of dive boat may be used, including large charter boats driven by a professional skipper. However, to ensure that students get sufcient attention and hands-on experience, there should not be more than four students per boat unless the duration of the course is extended accordingly.

Club and private boats used on this course should be operated in accordance with BSAC Safe Diving and any other applicable BSAC or BSAC-recommended guidelines. Check the BSAC website for the latest information.

Charter boats should be operating under the appropriate MCA Codes of Practice.

Venue Facilities

Adequate classroom facilities with a suitable table to lay out charts for each pair of students. Spacious outdoor area for position xing exercises.

Coastal site with boat access to an area suitable for the practical activities. Preferably the area should contain known dive sites, but if not it should contain underwater features which can

they were dive sites. Inland sites will not normally make suitable venues for this course because they lack tides. Refer to Appendix A of this manual for full instructions on tailoring the course to local conditions.

Administration

HQ will supply the appropriate course packs and a Course Report form, on receipt of an order. After the course the organiser should return the completed form without delay to BSAC HQ, who will issue qualication cards to students and record their course attendance on the membership database. For regionally run courses: on completion of the course, instructors should forward outstanding fees, accounts and expense claims etc, to the course organiser (some Regions may operate different procedures).

Costs

BSAC course fees cover the cost of course packs and certication cards. Fees must be paid to HQ at the time of ordering, in order to register and obtain student packs.

For branch run courses, instructor expenses, launching fees, boat fuel, boat hire, and any other costs are the responsibility of the students and their organisers.

For regionally run courses, students must pay the appropriate regional course fee to the Skill Development Course Organiser at time of booking.

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INTRODUCTION

Lesson Objectives

This lesson sets the scene for the course overall. It briey outlines the course content, domestic/logistics and timetable.

Achievement Targets

At the end of this lesson students should:

understand the objectives and structure of the course ●

understand what is required of them during the course ●

understand the domestic and logistical arrangements for the course ●

understand the course timetable ●

Additional visual aids

On courses where the instructors and students do not know one another, it may be useful to insert an extra VA introducing each instructor.

Paper copies of blank voyage plan formsCPF09 VPslate v1.2.doc (or later version)

Introduction

Chartwork and Position Fiing

Introductions

Introduce the instructors, and get the students to introduce themselves. Introductions can be minimised or omitted on courses where the participants already know one another.

Ask the students to describe any previous experience of navigation and boat handling.

Course aims

The aims of the course are to instruct members in the basic techniques of marine navigation and dive site location to a level which will help them as active divers in planning and managing dives undertaken by a typical BSAC branches from RIBs. This means that you can read a chart to identify dangers and things of interest to divers, work out slack water and the approximate depth, get to the dive site on time, and then nd the exact spot on the seabed.

basic techniques of marine navigation ●

suitable level for dive planning & management ●

typical BSAC branch RIB dives ●

If planning a walk in open country or even a day long car tour, it is most likely that you would consult maps to see what and where the attractions are, and listen to the weather forecast. Diving requires similar preparation, to nd out how to get to and locate the site, and to determine what is likely to be found on the seabed. Divers also have to contend with a third factor - tides. This course teaches you how to read and employ charts and tide tables to safely navigate small boats, and as aids to dive planning. Point out that this course focuses on the techniques of chartwork, position xing, and

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navigation. It is not a boat handling course, or a dive management course. To take full charge of diving from small boats you need those skills as well.

Domestic arrangements

Fire exits, catering facilities, house rules, etc

Locations

Cover the different venues that will be used during the course.

Timings

Outline the timetable for the course, including breaks, lunch, and nish time. Either issue a printed timetable, or add it to this VA.

Guideline durations for each session are provided in the Course Arrangements section of these notes.

Course Outline

Day one

chart basics ● measurement basics ● tides ●

position xing methods ●

nding dive sites ●

voyage planning ●

Day two

navigation at sea - all day on the boats ●

course debrief ●

Voage plan

Explain how the course is centred around the production and use of a voyage plan, the key output from chartwork and position xing. The voyage plan will be built up progressively during the course.

Hand out blank copies of the voyage plan forms to each student.

Understand charts

Select dive sites

Plot courses

Write it down – VP

build up in stages ●

nish off in last session ●

use ‘tomorrow’ ●

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Navigate to dive site

Locate the dive site

For us as divers, this is the extra step without which all the others are pointless [and the bit other navigation courses do not tell you about].

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CHART BASICS

Lesson Objectives

This lesson describes the types of chart that students are likely to encounter, how to understand the information that is shown on them, and how to plot and record positions using a chart.

Achievement Targets

be aware of the types of chart useful to divers ●

be able to extract information from a chart with the aid of 5011 ●

be capable of plotting a position on a chart ●

be capable of reading a position from a chart ●

Additional visual aids

Local charts, examples of different paper chart formats, 5011.

CPF09_exercises handout.doc (see Appendix A for tailoring instructions)

Chart Basics

Outline

Objectives

introduction to nautical charts ●

learn to extract information relevant to divers ●

plot and record positions ●

Availability

Scale

Chart laout

Smbols and abbreviations

Latitude & longitude

Eercises

Availability

Charts can be purchased from Admiralty Chart Agents and leading yacht chandlers. Similar charts are published in other countries so charts should be available for almost all sea areas of the world.

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Charts concentrate on the seabed, coastal topography, and particular features such as potential hazards to navigation. What are hazards for large ships are often the sites that are of interest to divers using smaller boats. For example, large ships need to avoid wrecks and reefs at certain depths, but these may provide a good dive site.

Paper or digital

Paper charts come in a variety of formats. The most common is a large (typically about 1 metre by 2/3rds meter) sheet rolled, or at and folded in half. Char ts for leisure use may come folded like a map, or be split into a number of smaller, separate sheets (typically about A3 size). Some charts are available printed on waterproof and/or plastic paper, so that they can be used in rugged environments such as a RIB.

Show examples of various types of chart. Point out that paper char ts are still valuable even in the digital age.

There are two different types of digital chart. A raster chart is basically a digital photograph of a paper chart. An electronic chart system which uses raster charts can overlay the boat's course and position on the chart, but will not be able to recognise any of the information printed on it. They are relatively cheap, but need a reasonably large screen (laptop and upwards) to be used effectively.

Most electronic navigation systems use vector charts. A vector chart consists of a database which holds all the elements of the chart such as spot depths, wrecks, lights, boundaries and so on as separate data items. This means that in addition to overlaying boat position information, the software can process all the information on the chart. A major advantage for small boat users, who use GPS plotter units with small screens, is improved display scaleability. Data items that would otherwise clutter the screen can be omitted or reduced in size when zooming in so that the screen remains readable. Depending on the manufacturer, digital charts may be supplied as downloads, on CD/ DVD or on memory cards (which themselves come in a number of different types eg, SD, MMC, etc). The le formats used by different digital chart vendors vary, and each manufacturer may sell a range of charts containing different levels of information. All this means that not all digital charts can be used in all electronic navigation devices. It is important to get up to date advice before buying.

Main brands

[BSAC instruction in chartwork assumes use of British Admiralty Charts. Overseas, instructors should tailor the following list to use any other charts which are in common use in their part of the world. This information is correct in January 2010.]

Admiralty ●

Published by the UK Hydrographic Ofce (UKHO) for the Royal Navy, they are amongst the best and most detailed in the world and are available to anyone who goes to sea. They include seabed information of interest to divers. Admiralty Charts cover all coastal waters of the UK and many other coastlines of the world. UKHO products include paper charts in standard, leisure, and Tough Chart

formats (waterproof, but only available for some areas egs, S Coast, N Wales, Clyde), as well as raster (ARCS) and vector (AVCS) digital versions.

Imray, Stanford ●

Both produce paper charts, principally for yachtsmen. Small boat versions available.

C-Map ●

Digital charts for a wide variety of GPS units and electronic navigation systems.

Navionics ●

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Digital charts for a wide variety of GPS units including Eagle, Furuno, Lowrance, and Humminbird.

BlueChart ●

Digital charts for Garmin systems.

Chart agents, chandlers, websites

Most chandlers carry stocks of paper charts covering their local area, and digital charts which are compatible with the brands of equipment they sell.

Charts can also be purchased online.

Admiralt charts

The full worldwide range of Admiralty charts can be purchased from Admiralty Chart Agents.

catalogues ●

The full list of Admiralty charts is published on the UKHO website. Paper

catalogues showing various subsets of the full range of charts and publications are also available. NP109, the NW Europe Catalogue (pictured on VA) shows all the UK charts.

Other manufacturers also provide catalogues of the charts they produce.

Notices to Mariners on web ●

Over time the information shown on charts becomes out of date as buoys are moved, new wrecks occur, channels are dredged or silt up, and so on. Changes to charts are published on the UKHO website as they occur. The updates are collectively known as Notices to Mariners and can be viewed free of charge, so that charts can be kept up to date.

Most vector chart suppliers provide an update service for their products.

Scale

Show examples of large and small scale charts for your local area.

Various scales (1:2,500 - 1:20,000,000)

small scale - large area - little detail ●

Small scale charts cover large areas in little detail. For example, a chart of the entire North Sea would be a small scale chart. Small scale big number eg 1:20000000.

large scale - small area - more detail – best for divers ●

Large scale charts cover small areas in great detail. For identifying dive sites and navigating in their vicinity, use large scale char ts where available. Large scale small number eg 1:2500.

Chart laout

Use this VA to introduce charts in general and to go over the main topics and points found on them.

Chart layout includes various pieces of information which apply to the whole of the chart. The various topics do not occur in a xed position, but are usually printed on land mass parts of the chart so they do not obscure soundings and seabed information. Where space is available they are printed together, but the nature of the area covered by the chart sometimes means that this is not possible.

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Title and number

The chart title gives a full description of the area covered, the date of survey, denes units and lists cautions, etc. Admiralty Charts are printed in metric units, so depths of water and heights of land are quoted in metres. The chart number is printed at the top left and bottom right corners of the chart. On standard charts, the title and number are also printed on the back.

Scale

The scale is printed near the title.

Positions/datum

Charts are drawn to a particular geodetic datum, to which all positions are referred. Older charts may also contain a block entitled “Satellite-derived Positions”. The signicance of these items is covered later in the course. The relevant information is printed in the title block.

Cautions

A caution is a hazard which is not described by any particular chart symbol. Usually the hazard applies generally across a substantial area of the chart. Examples include marine farms, military operations, areas which become exceptionally dangerous in bad weather, and depths/channels which change frequently.

Source data

There is normally a small chartlet which gives details of the hydrographic surveys which provided the data used to populate the various areas of the chart. This is useful for assessing the reliability of chart data when looking for new dive sites. This topic is discussed more fully in the lesson Finding Dive Sites.

Corrections

A record of all the Notices to Mariners corrections that had been applied at the time of sale is shown at the bottom left of the char t.

Tidal levels

This is a table showing the mean heights of the tide at various points on the chart.

Tidal streams

The Tidal Streams table is used in conjunction with tidal diamonds. It is usually found close to the chart title. Its function is explained in the Tides lesson.

Compass rose

Point it out. Its use is covered in Measurement basics.

Smbols and abbreviations

Road or Ordnance Survey maps give a coded representation of how the land would look, with its features of towns and roads drawn to a scale. Charts are similar but concentrate on the seabed, coastal topography, particular features, such as potential hazards to navigation.

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Over 1000 different smbols and abbreviations

Charts contain a huge amount of information. They can be used to pick interesting dive sites; to see what they are, how deep they are, what the seabed is like, and so on. But the main purpose of charts is to assist mariners in the safe navigation of the seas. This requires an understanding of everything which appears on the chart, not just those things needed to nd dive sites. It is unrealistic to expect mariners to remember every symbol, but you do need to know the main ones, and to know how to nd out about ones you are unfamiliar with.

Chart 5011

The UKHO publish a book entitled Symbols and Abbreviations used on Admiralty Charts. It is normally known as Chart 5011 or simply 5011 for short. Anyone navigating a vessel at sea should have access to a copy and should make themself familiar with its contents.

Show a copy.

For this course

The following VAs show the main categories of symbols that diver navigators need to be familiar with. They include common examples of symbols within each category, but not the entire set.

colours ●

depths ●

Depths at CD are covered in this lesson but actual depths (ie, including tides) are covered in theTides lesson.

buoys ● wrecks ● rocks ● landmarks ● nature of seabed ●

tides and currents ●

The symbols relating to tides and currents are covered in the Tides lesson.

For each of the following VAs 7-14, get the students to nd examples of the relevant symbols on the local char ts, show them the relevant pages in 5011, and if practicable get them to use 5011 to nd the meaning of additional symbols in the same category that appear on the chart but not on the VA. [Include any additional symbols that are important locally (such as trafc separation schemes) either here, or in the Voyage Planning Exercise].

Colours

Dr land is shown as a buff colour

Areas that cover and uncover with the tide are green

Sea is mainl white, with shallow areas either shades of

solid blue or edged in blue

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Depths

Explain that the depths shown on a chart are at chart datum, which is approximately the lowest astronomical tide, which means that the actual depth will almost always be deeper than shown. Calculating the actual depth taking into account the rise and fall of the tide is covered in detail later, in the Tides lesson.

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● ₆ sounding 14.6m ● 1₅ drying height 1.5m

----10---- depth contour (isobath) 10m ●

Isobaths full a similar function to height contours on a land map, except that whereas the latter has contours spaced at regular intervals (eg every 10m on a 1:25000 OS map), isobaths are spaced more closely near the surface than in deeper water, and the actual spacing varies from chart to chart.

merging contours = wall ○

Mention, and if possible demonstrate on a local chart, how isobaths coming close together or merging can indicate an underwater cliff which might make a good wall dive. The VA shows the excellent vertical wall at Calve Island in the Sound of Mull.

Buoys

Colour and shape, light characteristics

Point out that this course covers daylight navigation and so lights are not covered. However many buoys are lit, including those students may be using in daylight, which is why the examples on the VA include lit buoys, recognisable by the purple blob. Stu-dents do not need to know what the light characteristics mean, but can look them up in 5011 later if they are interested.

Meaning

lateral - marks edge of channel ●

Lateral marks (port, starboard, preferred channel) are generally used to mark well dened channels.

cardinal - marks area to avoid ●

Cardinal marks indicate safe water to the named side of the mark.

others ●

Other marks are used to show a variety of things such as isolated dangers, safe water in mid-channel and at landfall, and sewage outfalls.

Buoage - lateral

[Note that the VAs deal with IALA area A (red to port) which includes the UK and most of the world except the Americas. In IALA area B lateral buoyage is the other way around.].

red = port green = starboard ●

going in direction of rising tide ●

going in direction of falling tide ●

direction of buoyage symbol used where direction not obvious ●

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Cardinal

[Although the course deals with daylight navigation only the light characteristics of cardinal marks are shown because they are always the same, and thus easy to identify even without a chart. Lateral mark characteristics, apart from their colour, vary from buoy to buoy.]

Wrecks

Explain how the presence of a wreck symbol on the chart does not necessarily mean there is actually a diveable wreck there. Point out the difference in certainty between symbols that indicate specic details such as swept by wire or exact soundings, and those that do not. Draw attention to symbols such as PA and PD in 5011.

Rocks

Rock symbols often indicate the presence of reefs and pinnacles which may yield good dive sites.

Landmarks

Landmarks are important for visual position xing.

The rst six examples are man made objects. The last two illustrate natural features, which can also be useful for position xing, and in remote areas may be all that is available.

Nature of seabed

For navigators, the nature of the seabed is of interest mainly to assess the suitability of a potential anchorage. For divers it can suggest what types of marine life might be encountered, and give clues as to the likely visibility.

Abbreviations for material

R = rock ● G = gravel ● S = sand ● Sh = shells ● Sn = shingle ●

People often get Sh and Sn confused.

M = mud, etc ●

Qualifing terms

bk = broken ● f = ne ● m = medium, etc ●

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Used in combination

Material abbreviations are separated by a full stop eg M.S as shown on the VA. Qualifying terms may be included, as in the following example.

fS.M.Sh? ●

Ask what this combination would signify.

ne sand.mud.shells ●

Grid reference sstem

Chart makers face the problem of representing the seas of the world on a at piece of paper. As the world is a sphere (for purists it is an oblate spheroid, slightly attened at the poles), transferring its shape to a at surface will always means distortion. However, chart makers have allowed for this by drawing char ts as 'projections' using mathematical formulae.

As maps use a grid reference to be able to locate a position, so do charts and these grid lines are referred to as the parallels of latitude and meridians of longitude.

Parallels of latitude

Meridians of longitude

Latitude

Imagine the inside of the world scooped out except for the equator plane and a rod joining the two poles. This rod passes through the centre of the equator plane and,

therefore, the centre of the Earth.

Equator to Pole angle = 90°

The angle measured at the centre of the earth to one of other of the poles will be no more than 90° to the equatorial plane.

Parallels of latitude

Now imagine a pencil, held at a 50° angle from the equatorial plane at the centre of the earth. Rotating the pencil around this centre point, not changing the angle, would mark a circle on the earth's surface parallel to the equator circle and always at 50°; this is a parallel of latitude.

Equator divides earth into north & south

The Equator divides the earth into two, the northern and southern hemispheres.

Longitude

To complete the earth's grid system, the meridians of longitude need to be put in place.

Looking down on equator plan from North Pole

Imagine standing on the North Pole and looking down through a transparent earth onto the equatorial plane. Radiating lines, like spokes of a wheel can be drawn from the centre to the edge of the equator plane.

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equator, divide the world vertically, rather like segments of an orange.

Greenwich Meridian

The base line of 0° is an internationally agreed meridian - the Greenwich meridian, which passes through the UK and this divides the world into two. One hemisphere is west of Greenwich and the other hemisphere to the east. Longitude measurements are from 0° - 180° west, and 0° - 180° east.

Latitude & longitude

Describe the notations used for expressing positions.

Lat & long angles measured in

degrees° minutes.decimal’ (hemisphere) ●

egs, 55° 31.55', 07° 06.8', 005° 59'. It is usual to include leading zeros.

hemisphere = N, S, E, W ●

It is important, when using the Earth's grid of latitude and longitude - the abbreviation is lat and long - to give complete information to dene a position. Just saying 50° and 30° could mean north of the equator and either east or west; or it could mean south of the equator and either east or west. The latitude position should always be sufxed with either N or S and the longitude position always sufxed with either W or E. Then the correct position is given.

A position

latitude = 51° 43.250´ N ● longitude = 002° 59.950´ W ●

1 degree = 1° = 60 minutes = 60´

Each minute can be subdivided with decimals to an

precision required

50°30Ń 50°30.2Ń 50°30.21Ń 50°30.207Ń 50°30.20669Ń ●

However there is no point expressing a position to a greater precision than the accuracy with which it was determined warrants. Three places of decimals in minutes of latitude correspond to a distance of about 2 metres.

Mapping programs sometimes use decimal degrees

50°30´N = 50.(30/60) = 50.5° ●

This notation expresses each coordinate with a single signed number (south and west are negative, east and north are positive), which is more efcient for computation in software. It is easy to convert between the different notations, as in the above example: divide minutes or seconds by 60 to get decimal degrees or minutes respectively. Multiply the decimal part of degrees or minutes by 60 to get minutes or seconds.

Degrees/minutes/seconds becoming obsolete

51° 43.250´ = 51° 43´ 15˝ ●

In the past, positions were often expressed using seconds instead of decimals of minutes. One minute contains 60 seconds (1'=60"). So 50°30.2’ = 50°30’ 12". This notation may still be encountered in old guidebooks and articles. You will not

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Latitude & longitude

The latitude and longitude of any point on a chart can be read using scales along its borders. Demonstrate the procedure by stepping through the examples on the VA.

Summary

Availability

Scale

Chart laout

Smbols and abbreviations

Latitude & longitude

Eercises

Examples of questions and answers for the Chart Exercises practical can be found after the black slide VA23 at the end of this lesson. These exercises are based on Admiralty Charts 2172 Harbours and Anchorages on the South Coast of England and 2611 Poole Harbour and Approaches. See Appendix A Tailoring the course.

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MEASUREMENT BASICS

Lesson Objectives

This lesson covers the basic techniques and equipment for measuring angles, distance and speed on charts and at sea.

Achievement Targets

At the end of this lesson students should: understand how to use a compass rose ●

be able to plot & measure course directions on a chart ●

appreciate the difference between true and magnetic bearings ●

be able to measure distances from a chart ●

know how to convert speed, time and distance ●

appreciate the difference between travel over the ground and travel through the ●

water

Additional visual aids

Local charts

Parallel rule, Portland plotter and dividers

Hand bearing compass, several types if possible.

Measurement Basics

Outline

Direction

bearings ● magnetic variation ● compass rose ● measuring at sea ●

Chartwork instruments

drawing on charts ● bearings ● distance ●

Speed time and distance

relationships ●

measuring at sea ●

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Direction

There are a few basic terms which students need to be familiar with. Star t to get across the concept that a boat moves through or over the water, which itself may be moving relative to the ground.

Nautical direction terms

heading - direction in which the boat is pointing ●

Heading is the direction through the water (ignoring effects of wind, and assuming the boat is moving).

course - direction in which the boat is (or should be) steered ●

Course is normally a direction over the ground.

track - direction in which the boat is moving ●

If the track equals course, the boat is on course. The track and the heading may be different if the water is moving relative to the ground.

bearing - direction of an object ●

Bearing is the term used to describe the direction of one object from another, illustrated in the VA by the relative positions of the boat and the yellow blob (representing an island). Bearing is also used as a generic term for all direction measurements, including heading, course and track.

Direction epressed as an angle

Direction is an angular measurement relative to true north. A bearing describes the direction of an object from you in relation to north, measured in degrees true or degrees magnetic.

Bearings

Bearings in degrees

For navigation, bearings are measured in degrees relative to north, which is dened as 0°. Bearings increase in a clockwise direction, and reset to 0° after a full circle. A bearing in degrees may be expressed to any level of precision required, although a whole number of degrees is more than adequate for small boat navigation.

relative to North = 0° ● clockwise ● 360° = 0° ● precise ●

Cardinal points

Cardinal points (N, S, E, W) and their subdivisions (NE, SSW, etc) are often used to express approximate directions. When bearings expressed as cardinal points are intended to be taken as precise, the prex 'due' is normally used.

often used for approximate direction ●

‘due’ N, S etc if precise ●

Understand both

Students need to be comfortable using both notations, and should be able to convert between them easily.

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Magnetic North

Charts aligned to North Pole

North on a chart refers to geographic (true) north: the top of the globe where all the meridians of longitude converge.

Compass points to Magnetic Pole

A compass needle points not to geographic north, but to the magnetic north pole, which is hundreds of miles away from geographic north and also moves about slowly. [Technically a compass does not actually point to the magnetic north pole; it aligns with the local magnetic lines of force, which emanate from the magnetic north pole. ]

angle between true and magnetic is ‘variation’ ●

different in different places ●

changes slowly with time ●

currently about 3-5°W in UK ●

Variation not signicant for us

'Us' means UK divers. Variation is less than the accuracy with which a small boat can be steered using a magnetic compass, so it can be disregarded for small boat navigation. Any errors are swept up by the methods described later in the course.

However in other parts of the world variation can be substantial eg, at the south tip of New Zealand it approaches 25°E. See Course Manual appendices for a char t of global magnetic variation and notes on how to work out and use variation. [ Appendix E in this manual].

Distinguish between

chart measurements - °T ● compass measurements - °M ●

Even though variation is small, it is good practice when recording bearings to state whether they are relative to true or magnetic North, in case the readings are later used for something where precision is important.

Measuring bearings at sea

Fied compass

shows boat’s heading ●

To take a bearing of an object you would need to point the boat at it.

smallest graduation 5° ●

on the model shown, which is a common one. This is commensurate with the accuracy to be expected from these compasses.

deviation ●

error from nearby ferrous metal, magnets, electrics ○

The loudspeaker magnets in radios and mobile phones can have a large effect on a compass. On large vessels, the compass can be isolated from most magnetic inuences, and a deviation card can be carried, which shows the corrections to be applied to bearings on different headings.

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Bearings corrected for variation and deviation are shown as °C, whereas bearings corrected for variation alone are shown as °M.

not usually considered on small boats ○

Although a xed compass should always be mounted as far away as possible from extraneous magnetic inuences, in the cramped and chaotic environment of a small dive boat it is impractical to ensure a constant magnetic environment. For this reason deviation is normally ignored, and any compass errors are compensated by other methods when steering.

Hand bearing compass

A hand bearing compass is impractical for steering, but is a convenient and relatively accurate tool for measuring the bearing of objects.

used to measure bearing of objects ●

smallest graduation 1° ●

can distance from sources of magnetism – less deviation ●

Demonstrate a selection of hand bearing compasses.

Record as °M

Drawing on a chart

Charts are expensive and relatively fragile, so it is important to treat them carefully.

Paper charts

soft pencil (2B) ●

Anything harder than 2B is likely to leave an imprint on the paper and be difcult to erase. Erasing removes material from the paper, so the less rubbing that is required the better.

draw gently ●

plastic eraser for corrections ●

Laminated charts

Charts taken to sea in an open boat must be weatherproof. Plastic and/or waterproof charts are available, or you can make copies from paper or electronic originals and laminate them. Copyright restrictions may apply.

indelible marker pen ●

clean with solvent ●

Eercise

draw a line from slipway/harbour to rst waypoint on chart ●

Specify the rst waypoint (after the launch point) (see Appendix A Tailoring the course)

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Measuring bearings on charts

Demonstrate the use of each instrument in turn, then get the students to have a go.

Parallel rules

line up on along direction of travel ●

walk rules to centre of nearest compass rose ●

read bearing from true scale ●

Portland plotter

line up along direction of travel ●

twist circular scale to line up grid lines with grids on chart, north pointer to ●

North

read the bearing from plotter ●

Record as °T

Eercise

measure the bearing of the line just drawn, enter on voyage plan ●

Reciprocal bearings

Explain the concept of a reciprocal bearing by working through the example on the VA.

Bearing

direction of island from boat ●

Reciprocal bearing

direction of boat from island ●

add 180° (or subtract 180° if answer > 360°) ●

90°→270° ○

350°→170° ○

your bearing from object ●

This can be important in some circumstances: for example the preferred method of stating your position at sea in a distress call (if an accurate lat/long is not available) is bearing and distancefrom a landmark.

your way home ●

For an out and back trip, planning your return passage is simply a matter of working out the reciprocal bearing of each leg of your outward voyage.

Add reciprocal to voage plan

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Measuring distance on charts

Distances at sea are measured in Nautical Miles. The nautical mile is based on an angular measurement of ‘one minute of arc’. Imagine two straight lines with an angle of one minute between them projected outwards from the centre of the earth. At the surface of the earth the lines would be one nautical mile apart. The equivalent distances in metres and statute miles are 1852 and 1.15 respectively.

The border on the sides (not top or bottom) of a chart includes a latitude scale. It is always subdivided into units of 1 minute of arc, and on larger scale char ts it is further subdivided into tenths of a minute. Each subdivision then represents one tenth of a nautical mile so on large scale charts, it is possible to work accurately to one twentieth of a nautical mile or better. Dividers or drawing compass are used to compare a distance between two points shown or plotted on the chart with the adjacent latitude scale, thus determining the actual distance between them.

1 Nautical Mile (nm or Nm or NM or M) =

[Several different abbreviations are used for nautical miles. There is no single 'correct' abbreviation. The International Hydrographic Organization uses M while the International Civil Aviation Organization prefers NM. The abbreviations nm, though conicting with the SI symbol for the nanometre, and Nm are also widely used. 'nm' is used throughout this course.]

1´ latitude ● 1852m ● 1.15 statute miles ● 1/10nm = 1 cable (ca) ●

Short distances are often described in cables in nautical publications.

Measuring length of line

Demonstrate the procedure.

span using dividers ●

or set dividers to one nm & walk length of line ●

Convert to distance

read off distance in nm from latitude scale ●

latitude scale widens towards poles so use scale at same latitude as line ●

The latitude scale widens because of the way the Mercator projection works. On large scale charts the effect is negligible.

Eercise: measure line on chart and add to voage plan

Speed, time and distance

Speed

Speed at sea is measured in nautical miles per hour. 1 nautical mile per hour = 1 knot.

1 knot (kn) = 1nm / hour = 1.15mph ●

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Explain the relationship between distance, speed and time, and run through the options available for calculating one parameter when the other two are known. Numerate students will nd this topic trivial, but other students may take some time.

Distance (nm) = Speed (kn) x Time (hr) ●

Speed = Distance / Time ●

Time = Distance / Speed ●

for Time (min) use Speed (kn) /60 ○

or Time (min) = Time (hr) x 60 ○

or use triangle, if easier ●

cover required item, read the others ○

or use look-up tables ●

Look-up tables can be found in most nautical almanacs. Tables have the advantage of being easy to use at sea, whatever your level of numeracy.

nautical almanac ○

easy to use at sea ○

Eamples and eercise

Run through the examples. If necessary, set additional ones and coach the students who struggle.

Eamples

how long to cover 0.5nm at 15kn? ●

how far after 30min at 12kn? ●

how long for 7nm at 21kn? ●

Eercise

complete details for 1

● st_{leg on voyage plan}

Add the estimated speed for the leg from the slipway to waypoint 1 and calculate the elapsed time.

Measuring distance & speed at sea

For divers, GPS has superseded traditional methods of speed and distance measurement.

GPS

distance travelled ●

A GPS unit obtains position xes by using satellite signals, and calculates the distance travelled between xes.

calculates speed ●

A GPS unit does not measure speed directly, but it can calculate it knowing the distance travelled in a given time. Typically, the speed displayed on a GPS is averaged over a few seconds.

Ground track v water track

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course, heading, and track. Click through the diagram, explaining how the distance in 1 hour through water is different from the distance in 1 hour over ground when a tide is moving across the boat's heading.

Summary

Direction

bearings ● magnetic variation ● compass rose ● measuring at sea ●

Chartwork instruments

drawing ● bearings ● distance ●

Speed time and distance

relationships ●

measuring at sea ●

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TIDES

Lesson Objectives

This lesson covers basic information about the nature and causes of tides and explains how to obtain tidal information from tidal stream atlases, charts and tide tables. It explains the hazards that tidal streams create for diver navigators.

Achievement Targets

be able to extract tidal information from tide tables ●

be able to calculate the depth of water at any place on a chart at any time ●

be capable of using tidal ow information from charts and tidal stream atlases to ●

estimate the rate and direction of tidal currents

know how to determine the time and duration of slack water ●

know how to choose days when the tides are good for diving ●

understand the importance of knowing the depth of water at launch and dive sites ●

appreciate the implications of tidal currents for diver navigators ●

Additional visual aids

local charts

tidal stream atlas

Tides

Outline

Tidal terms

Depth and tides

Tide tables

times and heights of HW/LW ●

springs and neaps ●

depth of water for launch, recovery, diving and navigation ●

Tidal diamonds & atlases

slack time and duration ●

rates for navigation ●

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Tidal terms

The blue diagram on the VA illustrates the way the height of the tide changes with time. Explain the following terms:

High water (normally abbreviated as HW)

Low water (normally abbreviated as LW)

Tidal range Ebb

Flood

Tidal cycle - the time between consecutive high waters is approximately 12.5 hours. The time between HW and LW, and between LW and HW is usually half a tidal cycle, but this may vary due to local effects.

[In some parts of the world, chiey around the Indian and Pacic Ocean coasts, the tidal regime is more complex, and varies from the above pattern during some or all of each lunar month. Instructors in such areas should adjust the material in this lesson accordingly.]

State- is a generic term used to describe the position in the tidal cycle: HW, LW, ebbing, ooding, half-tide, etc.

Depth and tides

Knowing the maximum depth on a site is important for planning when considering the level of divers, diving gas and decompression requirements. Navigators need to know that there is sufcient depth of water for safe passage, and they need to know if and when the boat will dry out when moored or at anchor.

The tides are caused by the moon and, to a lesser extent, the sun. The moon's gravitational pull on the ocean creates a bulge of high water with a balancing bulge on the opposite side of the Earth. [Students may ask why there is a balancing bulge. The maths is difcult, but qualitatively it is because as the ocean nearest to the moon bulges towards it, the gravitational pull of the bulge itself on the ocean at the opposite side of the Earth is reduced because it is further away.] The sun's gravity also has an

effect on the oceans, but although the sun is a more massive body than the moon, it is much farther away and so has a weaker effect. As the sea surface rises and falls with the pull of the moon, it is the Ear th's crust spinning with its skin of seawater that gives an observer standing on one piece of land the impression that the tide is rising and falling. On a perfect sphere, the water would remain almost stationary. But, as the world is not a perfect sphere and with the shape of the land and seabed affecting the way the water distributes itself, the rise and fall of the tides is accompanied by horizontal water movement in many areas of the world.

Spring tides

greatest tidal range ●

If the moon and sun are in line, which occurs at the new and full moon, both their gravitational pulls combine causing a big rise and fall of the sea level. These are known as spring tides.

Lower LW ○

Higher HW ○

A large tidal range can be expected on spring tides, giving a high HW level and a low LW level.

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Neap tides

Least tidal range ●

If the moon and sun are at right angles to each other, the sun's small gravitational pull conicts with that of the moon and reduces its 'pull' effect on the sea. The resulting rise and fall of the sea is known as neap tides.

Higher LW ○

Lower HW ○

A smaller tidal range can be expected on neap tides, giving a lower HW level and higher LW level when compared to spring tides.

Abbreviations

MHWS MLWS MHWN MLWN ●

Published information about tides often needs to distinguish between springs and neaps, usually referring to mean levels. The following abbreviations are used: Mean High Water Springs, Mean Low Water Springs, Mean High Water Neaps, Mean Low Water Neaps.

Check that the students can put them in the correct order of height.

Point out that since these are means; actual tides may be higher or lower.

Tide tables

Tide tables are used to nd out tidal heights and times. You can nd them in almanacs, where they are likely to cover the entire UK coast for a calendar year. You can also buy cheap booklets from chandlers and marinas, covering the local area for a year. Tide tables covering the next few days are published in some newspapers, and tidal information may also be found on the internet although some of it is of unknown accuracy.

Some electronic navigation systems also display tidal information.

Tides are predictable

calculated for standard ports ●

Tide tables are computed and published for selected locations around the country, known as standard ports.

differences for secondary ports ●

Almanacs and local tide tables usually contain a list of corrections which can be used to nd tide times at other 'secondary' ports adjacent to each standard port. For example, HW at Menai Bridge occurs 28 minutes before HW at Liverpool, a standard port. The correction or 'tidal difference' is -0028. [Some almanacs also publish corrections for tidal heights at secondary ports, but these are outside the scope of the course]. Use the tables for the nearest available port, unless directed otherwise [eg tidal diamonds, below ].

Similarly, using the Time Differences table in the Course Manual [and in Appendix C of this manual], you can nd HW times at any standard port in the British Isles from HW Dover, which is normally quoted in national newspapers. For example, the tidal difference of Whitby on Dover is +0500, ie 5 hours after Dover.

Tide times calculated in this way can vary from the actual predictions for the port in question by a few minutes.

usually Universal Time (UT), add 1hr for British Summer Time (BST) ●

Tide tables are usually published in UT (formerly known as GMT). However, this is not always the case, so it is important to check.

gives HW & LW times ●

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Tide tables give the time and height on every day in the year, of HW and LW -heights are given in metres.

Spring & neap tides

Looking at the tide tables will show when spring and neap tides occur. Some tide tables include small moon symbols against the date to indicate spring tides. Otherwise, springs and neaps can be recognised by looking at the heights of the tides and applying the classications from the previous VA:

Springs ● Higher HW ○ Lower LW ○

The highest high water and lowest low water heights indicate spring tides

Neaps ● Lower HW ○ Higher LW ○

The lowest high water and highest low water heights indicate neap tides

Tide table eercises

Go through the exercises on the VA with the students.

Eercises

we need a neap tide to dive a wreck. Which of the three weekends would you ●

recommend and why?

29th – smallest tide

if we had to dive at LW on 21st July on which tide would you plan the dive? ●

11:00 – dark at 23:19

[Unless a night dive is required...]

what will be the states of the tide at 14:00 and at 21:00 on 22nd July? ●

14:00 ood 21:00 ebb

what will be the tidal range on the morning of 22nd July? ●

5.6m

are the times shown BST or UT? ●

The information given does not specify: you would need to check, and add an hour if the table is in UT.

Calculating depth

This VA covers how to calculate depth at HW and LW.

Charted depths

Remind the students of the material covered in Chart basics showing how depths are indicated on charts.

depth marked ●

Depths are marked on a chart as depth 'soundings' in metres.

lowest predictable level ●

The soundings show the depth at chart datum, the lowest astronomical tide. [Tide levels can vary from astronomical predictions due to weather conditions. In

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extreme weather tides may be a metre or so higher or lower than predicted ].

does not include the water above chart datum - tides ●

However, the depth at chart datum does not include the water that will usually be above it due to the effects of tides and different tidal ranges. To nd out the expected depth for a dive on a particular day, requires the charted depth and tidal variation to be combined.

For example, the charted depth of the seabed is 24m.

Looking at the tide tables for the day, they show 0800 LW as 1.5m, 1405 HW as 5.5m. The difference between LW and HW, the tidal range is 4m.

From the tide table information at LW there will be 1.5m of water over chart datum. So, at low water, the actual depth will be 25.5m.

At HW, there will be 5.5m of water over chart datum. So, the actual depth will be 29.5m.

This shows the importance of planning. For example, If the dive was planned just on the chart datum, it would seem ideal as a site for a new Sport Diver's rst dive to build their depth experience with a planned maximum depth of 24m to the seabed. But diving the seabed at LW would break the dive plan by 1.5m and, at HW, by 5.5m A second example of why depth planning is important is that of Nitrox divers. They will be carrying a pre-mixed gas with a MOD selected to be compatible with the planned depth. Getting out to a site and nding, from the echo sounder check, that the site is deeper because the effect of tides has not been included, could make a dive manager extremely unpopular.

Depth planning is also of obvious importance to navigators: they can check that there will be sufcient water to pass safely over submerged hazards; and for harbours and slipways which are only useable at certain states of the tide, they can check that it will be possible to get in and out at the required times.

Rule of Twelfths

So far we have shown how to calculate the depth at HW and LW. This VA shows how to calculate the depth at intermediate times using the Rule of Twelfths, an easily applied rule of thumb which gives an approximation to the rise and fall of the tide without needing extra documentation and is therefore easy to use in any situation including small open boats. Go through the example on the VA.

Tide tables indicates that LW is at 0630 0.5m, HW is at 1245 5.3m. The tidal range is therefore 4.8m.

To use the Rule of Twelfths, divide the tidal range, 4.8m, by 12. One twelfth of the tidal range = 0.4m.

The planned dive shows 24m at chart datum (CD) and the dive time is planned for 1030.

Low water is 0.5m above chart datum = 24.5m. High water is 5.3m above chart datum = 29.3m.

The Rule of Twelfths is applied as follows:

In the rst hour from LW the tide rises by one 12th: 24.5m + 0.4m = 24.9m In the second hour, the tide rises by two more 12ths: 24.9m + 0.8m = 25.7m In the third hour, the tide rises by three more 12ths: 25.7m + 1.2m = 26.9m In the fourth hour, the tide rises by three more 12ths: 26.9m + 1.2m = 28.1m

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In the fth hour, the tide rises by two more 12ths: 28.1m + 0.8m = 28.9m In the sixth hour, the tide rises by one more 12th: 28.9m + 0.4m = 29.3m HW Planning the dive at 10.30 shows the anticipated depth is 28.1m.

Put another way, at 10.30, in the fourth hour there is a total of 9/12 of the tidal range to add to LW 24.5m+9 x 0.4m (3.6m) = 28.1m.

The same procedure can be used to determine the depth between HW and LW.

[In certain areas, such as around the Solent, and wherever accuracy is critical, it may be better to use tidal curves. Tidal curves are not taught on this course, but details may be found in Lesson AT1 of the BSAC Advanced Diver course, and in nautical almanacs. There are now several proprietary software packages (eg Belleld Tidal Plotter, available on the BSAC website), and some websites (eg Admiralty EasyTide ), which generate tide tables and calculate tidal heights at any state of the tide. They are likely to make the manual use of tidal curves a thing of the past.]

Depth at an time

Using a chart and a tide table we can calculate the depth at HW and at LW. By using the rise of tide worked out with the Rule of Twelfths we can calculate the depth at any time. Step through the VA, explaining each point.

CD + LW ●

Tide tables give the height of the tide above CD at LW.

CD + LW + rise of tide ●

Adding the rise of the tide from the Rule of Twelfths gives the height of the tide at any time.

CD + HW - fall of tide ●

We could also calculate the height of the tide at any time by nding the height of the tide at HW from tide tables then subtracting the fall of the tide worked out with the Rule of Twelfths. Either method gives the same result. It requires slightly less arithmetic to apply the Rule of Twelfths if you use whichever of HW or LW is nearest to the time for which you wish to know the depth.

height of tide ●

The height of the tide above CD is calculated using information from tide tables. To nd out the actual depth we need to refer to the chart and nd the depth symbol nearest to the point required.

Actual depth = height of tide above CD...

plus charted depth ●

Depths are below CD, so simply add the charted depth to the height of tide above CD to get the actual depth.

minus drying height ●

Drying heights are above CD, so subtract the drying height from the height of tide above CD to get the actual depth.

drying height > height of tide = above water ●

If the drying height is greater than the height of the tide it means that the point is above water at the time in question. Subtract the height of the tide from the drying height to nd out by how much.

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Depth calculation eercises

Direct the students to do the rst four exercises using the tide table extract on the VA. Students should assume that all times are BST, for the purposes of the exercise (see Appendix ATailoring the course).

Go through the answers.

Set the nal question, which involves updating the voyage plan with local information, from the relevant tide tables. If appropriate, demonstrate how using a secondary port can give improved accuracy. Students must specify BST or GMT on the voyage plan.

Tidal ow

In tidal waters, charts show the speed and direction of currents that can be expected in the general area at hourly intervals throughout the rise and fall of the tide.

Tidal ow information is of interest for several reasons: many dive sites can only be dived when the tidal ow has stopped; dive managers need to know the strength and direction of the current if a drift dive is to be planned safely; the strength and direction of the current may affect the navigation of the boat; and some areas may become dangerous at certain states of the tide.

Calculated slack water times are notoriously imprecise, and actual slack may occur before or after the calculated time. Divers are advised to get to sites early.

Time of least water movement

slack water ●

Tidal currents usually ow in one direction for a few hours and then reverse and ow in the opposite direction. The point when the rate of tidal ow is minimal is known as slack water. It usually occurs close to the time of HW or LW, but local geographical features can bring about slack water as much as halfway between HW and LW. Slack water is not a momentary point in time, but generally represents a period during which there is negligible water ow. For diving, ‘slack’ generally refers to the period when the current is half a knot or less. [There is no precise denition, and the tolerable amount of current may also depend on the capability of the divers and the nature of the dive]. The length of this period is known as the duration of slack.

longer slack at neaps ●

The bigger the tidal range, the more water there is to move about in the same time. Hence tidal streams are stronger on springs and weaker on neaps, and slack water is longer on neaps and shorter (or non-existent) on springs.

Tidal diamonds

The diamond shape with the letter A on the chartlet is known as a tidal diamond (proper title "position of tabulated tidal data with designation"). It is a pointer to a table elsewhere on the chart which lists the tidal ow at the position of each tidal diamond. The nearest tidal diamond to your point of interest is usually the best, but this may not necessarily always be the case, especially around the entrances to harbours / river estuaries.

Table on chart

The tidal stream information is given in 4 columns.

hours before and after HW ●

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port, not necessarily the nearest standard port, which may be some distance away, so high water times may be considerably different), so the time of HW needs to be conrmed using tide tables for the day of the dive.

direction of ow ●

The next column indicates the direction of ow of the tidal stream as a bearing relative to true north. The direction given is that which the tide is owing to.

speed on spring tides ●

The next column indicates the speed (aka rate) of the ow in knots on springs.

speed on neap tides ●

The last column indicates the rate in knots on neaps.

The greatest tidal ranges occur on springs so the rates are faster than on neaps, when the least tidal ranges occur. The rates shown refer to mean spring and mean neap tides.

Using the table on the VA, show how to determine slack by locating the periods when the current is least, and the periods either side when the current is 0.5kn or less. Use the Sp or Np columns as appropriate. Point out how the ow changes direction during this period.

On springs, the least water movement is 5 hours before HW at 0.2kn, or with a bit of current, 0.5kn, 2 hours after HW. On neaps tides there is a slack 5 hours before HW and quite a large slack window from HW to 3 hours after.

Away from mean spring and neap tides, rates will be faster or slower from those shown in the table. They can be interpolated or extrapolated. For example, on a day half way between springs and neaps, take a rate half way between the gures given. Demonstrate this idea at HW, where the spring rate is 1.3 and neap rate is 0.6, making half way 0.95: say 1 knot.

Tidal stream atlas

Charts are not the only source of tidal stream information. Tidal stream atlases are often a useful source, not least because they can convey much more information in a single glance than detailed examination of the tidal streams tables on a chart, especially in conned waters.

Set of diagrams showing currents in area

Admiralty publications, and in almanacs ●

A set of UKHO publications covers the whole UK coastline. Small scale chartlets cover large areas where there are few local variations, and inset or special

chartlets cover smaller areas with strong and localised tidal streams. The example of Portland Bill on the VA is one such local area.

12 or 13 diagrams cover tidal cycle ●

time relative to HW Standard Port ●

arrows indicate strength & direction ●

numbers show speed in 1/10kn ●

2.5kn neaps, 5.0kn springs ●