Even for a GPS unit, a lat/long is not quite enough to display a position precisely. It is necessary to state the geodetic datum to which the position applies.
Earth not an exact sphere ●
The Earth is not a perfect sphere but is slightly attened at the poles [illustrated by the squashed orange]. Its shape is not constant either; it changes slowly due to the movement of tectonic plates and other geological processes. To cater for this, positions are referred to a mathematical model of the Earth's surface, called a geodetic datum, in order to calculate a specic latitude and longitude for a particular point.
different chart makers use different models ●
There are many different geodetic datums in existence, some produced in the past when the Earth's shape was not known as precisely as it is today, and others produced to emphasise regional features at the expense of global ones: all
geodetic datums contain some compromises. Every chart is drawn to a particular geodetic datum, which will be identied somewhere on the title block. Point out the datum on one or more charts.
All new charts are drawn to a datum called WGS84 [actually UKHO charts are being drawn to ETRS89, which is compatible with WGS84.] However there are still many charts in use drawn to different datums.
be set to use any of a large range of alternatives. For the last 20 years or so, charts drawn to older datums have included instructions for converting positions between WGS84 and the chart datum, in a block entitled Satellite Derived
Positions. Point out a block on a chart.
Avoid the need to make conversions by using the latest charts wherever possible
signicant differences ●
[The distorted coloured triangles on the orange on the VA illustrate how points move depending on the exact shape chosen to represent the Earth’s surface }. The errors introduced by using the wrong datum are signicant. The diagram shows how the apparent position of what is actually the same point on the
Earth's surface varies using three commonly encountered datums. WGS84 is the modern standard; OSGB36 was the previous standard for Admiralty charts (UKHO is converting all its older charts but had not nished the process in 2009); and ED50 is the previous standard used for many European charts.
Limitations
[Mention some in-car GPS navigator stories if you have them egs, cars driving into a river which was not shown on the map; people entering the wrong postcode and ending up miles from their intended destination, having failed to notice where they were going .]
must use correct datum ●
Using the wrong datum could easily put you over 100m from where you think you are, which could lead the boat into danger, and could also result in you failing to nd your dive site.
garbage in garbage out ●
GPS units rarely validate the positions entered into them. If you make a keying error, or enter a position that is otherwise incorrect, the GPS will happily accept it and process it.
large errors if satellites obscured ●
GPS accuracy declines if the unit can only see the minimum number of satellites. Out at sea in an open boat this is never a problem, but close up against high ground or harbour walls it can be. GPS units will not work properly if the aerial is indoors: this could be a problem if you are trying to use a por table unit inside a large boat.
relies on accurate chart ●
Provided the above points are addressed, a GPS unit will provide an accurate lat/ long referenced to the chosen datum. For many purposes, the lat/long will then be displayed on a chart, either manually or electronically. How well the resulting picture mirrors the boat's actual position relative to its surroundings depends on how accurate the chart is. Although modern charts are based on recent and accurate surveys for areas that are heavily frequented by shipping, some of the areas that divers operate in, close in amongst rocks for example, are areas that most chart users avoid and they may not have been surveyed for many years. Charts can have signicant errors in such places, and an electronic display may show you in safe water when you are not. In these areas it is prudent to navigate using the echo sounder and the mark one eyeball.
can become inoperable ●
By denition GPS navigation relies on having the equipment available and
working. GPS units can and do become inoperable for many reasons. Some risks can be mitigated by carrying a spare unit, perhaps a cheap handheld device, but others are system wide. Prudent navigators should always be able to navigate their boats to safety using traditional methods. A good dive boat navigator will
also be able to nd a selection of dive sites this way as well, though it is likely that the loss of GPS and/or the echo sounder will rule out most offshore sites.
electrical faults ○
Loss of power supply, aerial problems, wiring faults, internal faults, corrosion
damage, theft ○
Water ingress, hit by dive equipment or falling bodies, clumsy handling, vandalism, theft
military operations ○
GPS is ultimately a military system. Sometimes military exercises may involve jamming of signals locally, or reduction of accuracy for civilian users.