AD-SDI DATA STANDARD
GNSS FIELD DATA COLLECTION GUIDELINES
Version 1.0
September 2011
Prepared by
Abu Dhabi Spatial Data Infrastructure (AD-SDI) Abu Dhabi Systems and Information Centre (ADSIC)
Abu Dhabi, UAE
REVISION HISTORY
Revision # Reason Effective Date
1 Original Draft March 2010
2 Enhanced May 2010
3 Incorporated ADM comments, added 2 diagrams, and enhanced mostly Section 5.
July 2010
4 Incorporated AAM comments; the more general term GNSS is used instead of GPS.
September 2011
DISCUSSION HISTORY
Discussion # With Date Summary
1 ADM 4 July 2010 ADM provided feedback on the
document.
2 AAM 18 August 2011 AAM provided feedback on the document
Table of Contents
1 Introduction ... 5
2 Scope, purpose, and application ... 5
2.1. Scope... 5
2.2. Purpose ... 5
2.3. Application ... 5
3 Terms and definitions ... 6
4 Symbols, abbreviated terms, and notations ... 11
5 Guidelines for GNSS Usage ... 12
5.1. GNSS Applications ... 12
5.2. GNSS Receiver Grades ... 12
5.3. GNSS Accuracy... 14
5.4. DGPS and RTK Corrections ... 14
5.5. Reference Station Network ... 15
5.6. GNSS Equipment Setup ... 16
5.7. GNSS Metadata ... 19
5.8. Accuracy Classification ... 20
5.9. Collection Method Codes ... 20
5.10. Tips for Collecting GNSS Data ... 21
5.11. Using Offsets in GNSS Data Collection ... 22
5.12. Accuracy Specifications ... 23
6 GRS Network ... 24
6.1. Positioning Services ... 24
6.2. Other Services ... 26
6.3. Prerequisites for accessing GRS Network ... 26
6.4. Accessing GRS Network ... 27
List of Tables
Table 1: Comparison of GNSS Receivers by Grade ... 13
Table 2: GNSS Receiver Settings ... 16
Table 3: GNSS Metadata Elements ... 19
Table 4: Accuracy Classification ... 20
Table 5: GNSS Data Collection Method Codes ... 21
Table 6: Comparison of Positioning Services ... 25
1 Introduction
This AD-SDI Data Standards document is to support spatial data collection by AD-SDI stakeholders and external source data collectors. The standard will help facilitate the accuracy, validity, consistency, comprehensiveness, and relevancy of AD-SDI’s spatial data.
This document provides standards and guidelines for collecting spatial data from the field using Global Navigation Satellite System (GNSS) handheld devices. The procedures and standards outlined in this document must be applied to spatial data collection work performed by all AD-SDI stakeholders using mapping-grade GNSS receivers.
2 Scope, purpose, and application
2.1. Scope
This document is developed in response to the growing need for a set of guidelines to help guide the consistent and accurate acquisition of field data using GNSS receivers. In recent years, GNSS technology has greatly extended the capacity of individuals to capture geospatial data representing real world features. However, there is a common misconception that the technology is foolproof and that, with little or no training, anyone can pick up a GNSS receiver and start capturing high quality, accurate and useful data. The lack of a proper understanding of the technology has often resulted in inconsistent data of dubious quality and accuracy. This document is intended to serve as a guide for any AD-SDI stakeholder wishing to collect geospatial data with GNSS or contract out GNSS data collection services using mapping-grade GNSS receivers. It will also help both the novice and intermediate users in gaining a better understanding of the technology and to improve the quality and accuracy of the data they acquire.
2.2. Purpose
As the GNSS technology is continually evolving with the advancements in technologies and techniques, a large choice of solutions is available for field data collection. The guidelines provided in this document describe a few of the many methods for carrying out field data collection using mapping-grade GNSS receivers.
2.3. Application
GNSS receivers are grouped (Section 5.2) into three grades: Consumer grade, Mapping grade, and Survey grade. This document is applicable to the data collected mainly by the mapping grade GNSS receivers (and to the high-end consumer grade receivers), which are most often used by government entities, researchers, and other users who require more accurate and dependable coordinate fixes than a recreational (consumer grade) GNSS receiver can provide. It does not deal with survey grade GNSS receivers, which are the most accurate and the most expensive, and which are most often used by professional surveyors for high-accuracy measurements.
3 Terms and definitions
The following definitions clarify the subject of the Standard:
A-GPS A-GPS, or assisted GPS, is a system which can improve the startup performance of GPS navigation devices. It is used extensively with GPS-capable mobile phones. In a city, the radio signals from satellites may be weakened by tall buildings or by passing through walls or tree cover. In these very poor signal conditions, when first turned on, non-assisted GPS navigation devices may not be able to work out a position until a clear signal can be received continuously for up to 40 seconds (the time needed to download the GPS ephemeris). An Assisted GPS system addresses these problems by using location data available from the cellular network or by using proximity to cellular towers to calculate position.
Base Station A receiver that is set up on a known location specifically to collect data for differentially correcting Rover files. The base station calculates the error for each satellite and, through differential correction, improves the accuracy of GNSS positions collected at unknown locations by a roving GNSS receiver, known as Rover.
Channel GNSS receivers track the signals from satellites via “channels”, with the signals from one satellite occupying one channel on the receiver. Channel refers to a set of hardware in a receiver that detects, locks on and continuously tracks the signal from a single navigation satellite. The more channels a receiver has, the more likely that it will continue uninterrupted collection of data if the parameters (e.g., Positional DOP - PDOP) of one of the satellites fall out of optimal range. The more receiver channels available, the greater number of satellite signals a receiver can simultaneously lock-on and track.
CORS Continuously Operating Reference Station (CORS) is a GNSS reference station that provide carrier phase and code range measurements for real-time and post-processing differential correction. It is a fixed GNSS receiver site in continuous operation that transmits reference data on a 24-hour basis. Each CORS site provides GNSS carrier phase and code range measurements in support of 3-dimensional positioning activities.
Several CORSs operating together providing a range of GNSS data or correction sources constitute a CORS Network. See GRS Network.
Decimal Degrees Degrees of latitude and longitude expressed as a decimal rather than in degrees, minutes, and seconds.
Differential GPS (DGPS)
DGPS is an enhancement to GPS that uses a network of fixed, ground-based reference stations to broadcast the difference between the positions indicated by the satellite systems and the known fixed positions. These reference stations broadcast the difference between the measured satellite pseudoranges and actual (internally computed) pseudoranges, and rover receiver stations may correct their pseudoranges by the same amount. Differential corrections can be applied in either real-time or by post- processing. Since most of the errors in GPS are common to users in a wide area, the DGPS-corrected solution is significantly more accurate.
Dilution of Precision (DOP)
An indicator of satellite geometry for a constellation of satellites used to determine a position. Positions with a higher DOP value generally constitute poorer measurement
results than those with lower DOP. Factors determining the total GDOP for a set of satellites include, to name a few, Positional DOP (PDOP), Horizontal DOP (HDOP), Vertical DOP (VDOP), and Time DOP (TDOP).
EGNOS European Geostationary Navigation Overlay Service (EGNOS), maintained by the European community, is similar to the WAAS of USA.
Elevation Mask Elevation Mask is configured on the GNSS receiver to ignore satellites that are low in the sky. When a GNSS satellite is low on the horizon, the satellite signals must travel a greater distance through the atmosphere, resulting in lower signal strength and delayed reception by the GNSS receiver. Low-elevation satellites tend to yield noisy data.
Position data should be collected using only satellites that are at least 15 degrees above the horizon.
Estimated Position Error (EPE)
A measurement of horizontal and vertical position error, in meters, based on a variety of factors including DOP and satellite signal quality. EPE is the 50% Circular Error Probable (CEP). For example, if a GNSS position measurement is accurate to 10 meters CEP, then this means that there is a 50% probability that the measurement lies inside a circle with a radius of 10 meters. This also means that there is a 50% probability that the measurement lies outside the 10 meter radius circle!
External Antenna GNSS receivers have an internal antenna that is sufficient for general use in clear sky areas away from buildings. Most mapping grade receivers also have the option of an external antenna. The internal antenna is disabled when an external antenna is used so that signals are not received from both antennas. An external antenna is recommended when collecting data in urban or wooded areas where the sky is partially obscured and when acquiring data with a vehicle.
FKP For GNSS applications the RTK performance can be increased by using area correction parameters (in German: Flächen−Korrektur−Parameter FKP) information from reference station networks. The FKPs provide information about the distance dependent error components. For example, the German AdV organization, responsible for the operation of the SAPOS reference station networks, introduced FKPs as its standard technique to provide network information to any RTK rover.
GNSS (Global Navigation Satellite System)
A Global Navigation Satellite System is a network of satellites that transmit ranging signals used for positioning and navigation anywhere around the globe; on land, in the air or at sea. The US Global Positioning System (GPS), the Russian GLObal NAvigation Satellite System (GLONASS) and the European GALILEO system are examples of GNSS.
GRS Network GNSS Reference Station (GRS) Network operated by the Emirate of Abu Dhabi is a CORS (Continuously Operating Reference Station) network, consisting of 23 reference stations, strategically located throughout the Emirate of Abu Dhabi. Position information is fed back to a central server where it is broadcast over the internet to anyone with a login. A user can set up an account and using an internet-enabled device, begin streaming the data to their RTK capable GNSS receiver. GRS Network allows a user to have access to RTK level correction anywhere in Abu Dhabi Emirate as long as they have cellular access.
Latitude Latitude describes position in terms of how many degrees it is North or South of the
Equator (0° latitude).
Longitude Longitude describes position in terms of how many degrees it is East or West of the Prime Meridian (0° longitude) called the Greenwich Meridian.
Navigation The process of travelling from one place to another and knowing where you are in relation to your desired course.
NMEA 0183 A standard data communication protocol used by GNSS receivers and other types of navigation and marine electronics from National Marine Electronics Association.
NRTK Network Real Time Kinematic (NRTK) – GNSS network real-time kinematic technology based on a reference station network approach is a proven technology widely used today all over the world. Compared with traditional single baseline RTK technology, network RTK removes a significant amount of spatially correlated error due to the troposphere, ionosphere, and satellite orbit errors, and thus allows RTK positioning using reference station networks with inter-station distances of 40 kilometers or more while providing the performance of short baseline positioning.
NTRIP Network Transportation of RTCM Internet Protocol (NTRIP) is an RTCM standard designed for disseminating GNSS correction data (e.g. in the RTCM-104 format) or other kinds of GNSS streaming data to stationary or mobile users over the Internet, allowing simultaneous PC, Laptop, PDA, or receiver connections to a broadcasting host. NTRIP supports wireless Internet access through Mobile IP Networks like GSM, GPRS, or EDGE.
Position A unique location based on geographic co-ordinate system.
Position Dilution of Precision (PDOP)
PDOP is a value that indicates the quality of the geometric orientation of satellites being used to compute a position. The closer the PDOP value is to one, the better the geometry of the satellites and the likely position accuracy. If the satellites are close together, the geometry is not the best and the PDOP value may be greater than 6. The PDOP shall be less than six.
Real-time kinematic (RTK)
RTK satellite navigation is a technique used in land survey and in hydrographic survey based on the use of carrier phase measurements of the GNSS signals where a single Reference Station provides the real-time corrections of even centimeter level of accuracy. The reference station re-broadcasts the phase of the carrier that it measured, and the rover units compare their own phase measurements with the ones received from the reference station. This allows the rover units to calculate their relative position to millimeters, although their absolute position is accurate only to the same accuracy as the position of the reference station.
Reference Station A reference station is a point or site where surface stability, or tidal current constants, have been determined through accurate observations, and which is then used as a standard for the comparison of simultaneous observations at one or more subordinate stations. A reference station that transmits reference data on a 24-hour basis is known as a Continuous Operating Reference Station (CORS).
RINEX (Receiver INdependent EXchange format) A set of standard definitions and formats to promote the free exchange of GNSS data and facilitate the use of data from any GNSS receiver with any software package. The format includes definitions for three fundamental GNSS observables: time, phase, and range.
Route The direction to follow to reach a destination, expressed in an angular way in relation to the North.
Rover Any mobile GNSS receiver collecting data during a field session. The receiver's position can be computed relative to another, stationary GNSS receiver, known as Base Station.
RTCM Radio Technical Commission for Maritime Services (RTCM) is a format for using Differential GNSS broadcasts over radio to provide real-time correction of the error in the calculated positions. RTCM-104 is a serial protocol used for broadcasting pseudo- range corrections from differential-GNSS reference stations.
SBAS Satellite Based Augmentation Systems (SBAS), is the generic naming for services such as WAAS, EGNOS, and MSAS (Multi-Functional Satellite Augmentation System) of Japan.
Time to First Fix (TTFF)
Time to first fix (TTFF) is a specification detailing the time required for a GNSS receiver to acquire satellite signals and navigation data, and calculate a position solution, called a fix. Many receivers can use as many as twelve channels simultaneously, allowing quicker fixes.
Track A previous path of travel that has been stored in a GNSS receiver.
WAAS The Wide Area Augmentation System (WAAS) is an air navigation aid developed by the Federal Aviation Administration of USA to augment GPS, with the goal of improving its accuracy, integrity, and availability. Essentially, WAAS is intended to enable aircraft to rely on GNSS for all phases of flight, including precision approaches to any airport within its coverage area. In simple terms, WAAS is a satellite based DGPS.
Waypoint A waypoint marks an exact position fix so it can be recalled for future use. It usually indicates a town/village, a house, a health centre, a change of direction, an obstacle on a road etc. These positions can be put in memory. The instrument will then be able to guide you to these points, and to give you warning at the moment you reach them.
Factors that affect GNSS signals
There are a number of potential error sources that affect the GNSS signal. The diagram below illustrates the six main sources of errors. The accompanying table lists the error budgets and the cumulative error value.
Ionosphere: Before GNSS signals reach the antenna on the earth, they pass through a zone of charged particles called the ionosphere, which changes the speed of the signal. If the reference and rover receivers are relatively close together, the effect of ionosphere tends to be minimal. For the lower range of GNSS precisions, the ionosphere is not a major consideration. However if the rover is working too far from the reference station, you may experience problems, particularly with initializing the RTK fixed solution.
Troposphere: Troposphere is essentially the weather zone of the atmosphere, and droplets of water vapor in it can affect the speed of the GNSS signals. The vertical component (elevation) of the GNSS position is particularly sensitive to the troposphere.
Multipath is the reflection of signals similar to the phenomenon of ghosting on the television screen. GNSS signals may be reflected by surfaces near the antennae, causing error in the travel time and therefore error in the GNSS positions.
4 Symbols, abbreviated terms, and notations
The following symbols, abbreviations, and notations are applicable to this document.
Symbols, abbreviations, and notations applicable to multiple parts are listed in the Base Document.
DMA Department of Municipal Affairs RPS Real-time Positioning Service
HRPS High-precision Real-time Positioning Service GPPS Geodetic Post-processing Positioning Service
HGPS High-precision Geodetic Post-processing Positioning Service FKP Flächen-Korrektur Parameter
MAC Master Auxiliary Concept VRS Virtual Reference Station
5 Guidelines for GNSS Usage
This section provides an overview of GNSS data collection applications, description of GNSS receiver grades, accuracies obtainable from GNSS receivers using different techniques, and covers the different aspects of GNSS field data collection including equipment setup, metadata recoding, accuracy requirements, using GNSS with other measurement methods, etc.
5.1. GNSS Applications
Recent advances in GNSS and mobile technology facilitates rapid field data collection and transfer. Once the location and attribute data have been collected in the field, all the data can be imported into a GIS for mapping and analysis. As GNSS units are usable everywhere at any time, the system has a variety of applications. The only limitation is the fact that it is impossible to receive the signal inside buildings or underwater. The GNSS technology is a very adequate tool for several data collection applications in a variety of domains.
Following are some examples:
• Mapping of community facilities like school, post office, health center, bank, bus stop, police station, etc
• Collecting real-time data for incident reporting such as accidents, damages, disaster, emergency, etc
• Door to door data collection for census, social economic survey, utility services provisioning, etc
• Data collection for environment protection and nature conservation purposes
• Utility management and recording
• Position of drinking water sources and its services zone
• Mapping and tracking infectious diseases at community levels
Apart from mapping point features, GNSS units can also be used for mapping linear and areal features. Some examples are:
• Road / track network mapping in remote regions
• Mapping of parks, green areas, and data collection for tree cadastre, etc
• Mapping of natural resources boundaries
• Mapping of protected areas, population spread, habitats, polluted areas for nature conservation and protection
5.2. GNSS Receiver Grades
GNSS receivers and associated software can be used to obtain positions with accuracies from millimeters to tens of meters. The mapping grade receivers that are the focus of this
document are generally able to obtain positions with accuracies ranging from half meters to ten meters. The GNSS receivers may be broadly classified into the following three grades, depending on their differential and post-processing capabilities and the positional accuracy they can achieve.
Table 1: Comparison of GNSS Receivers by Grade
Type of Receiver Consumer Grade Mapping Grade Survey Grade Primary Use Navigation, hiking,
camping, fishing, hunting
GIS data collection, navigation
GIS data collection, surveying, vertical measurement DGPS Capability Real-time in some
receivers
Real-time in some receivers
Real-time in all receivers
Post-processing Capability
No Post-processing in all
receivers
Post-processing in all receivers
Horizontal Accuracy 10-20 m (no correction) 5 m (real-time correction only)
5-20 m (no correction) 0.5-5 m (real-time or post- processing correction)
< 2 cm (real-time correction)
< 1 cm (post-processing) Vertical Accuracy Not used to collect vertical
data
2-15 m
(2 to 3 times less accurate than horizontal)
< 2 cm (real-time correction)
< 1 cm (post-processing) Type of Spatial
Features Collected
Points only Points, lines and areas Points, lines and areas (primarily point data!) Option to Load
Custom Coordinate Systems, Projections, Datums / Ellipsoids
Some receivers All receivers All receivers
Support of Waypoints for Navigation
All receivers All receivers Not practical for navigation
Training Requirements Minimal Moderate Advanced
Some consumer grade GNSS receivers, that provide the capability to load background map data of roads and points of interest for display, are characterized using the term “mapping receivers”. This does not mean that the receiver is suitable for spatial data collection for GIS.
It is important to note that GNSS technology is dynamic and developing very fast with new capabilities evolving continuously. Because of the advances in technology, functionality currently available in higher-grade receivers is migrating to lower grade ones blurring the boundaries between the grades defined here.
5.3. GNSS Accuracy
The accuracy of a position depends on the grade of GNSS receivers. The number of satellites visible by the receiver can improve the accuracy. Some factors such as tall buildings, electronic interference or dense foliage can block signal reception and cause position errors.
GNSS units typically will not work indoors, underwater or underground.
Users can obtain higher accuracy by using an additional receiver fixed at a known location nearby. Depending on the number of GNSS receivers used for position determination the mode of positioning may be characterized as Stand-alone or Relative.
Standalone Positioning: This mode of positioning (also called “Absolute Positioning”) relies upon a single GNSS receiver. Standalone GNSS positioning relies on the receiver range measurements and is limited to an accuracy of a few meters.
Range measurements are affected by errors from sources such as satellite orbit, satellite clock, atmospheric delays, etc.
Relative Positioning: The Range measurements errors can be removed if a static receiver with a known position (reference station) transmits information about these errors to a user receiver (rover). The positioning technique based on this principle is also called “Differential Positioning”.
5.4. DGPS and RTK Corrections
In the relative positioning mode, the corrections received by the rover may be computed using two different techniques, namely DGPS or RTK as explained below:
DGPS Corrections: Differential correction works by comparing satellite signals received by the receiver (rover) with those received by a reference (base) station, which is fixed over a surveyed point of high positional accuracy. Base station correction values are calculated and then applied to the rover data to increase their accuracy to 5 meters or better, depending on the GNSS receiver grade. Differential correction removes certain types of errors from GNSS data. This can occur as you are collecting data in the field (real-time) or back in the office (post-processing). In
standard DGPS technology, only corrections to C/A code pseudo-ranges are being transmitted, which brings rover positional errors down to values about 1m. DGPS is widely used by navigation users, in particular for urban transportation and in offshore areas.
RTK Corrections: High-precision navigation / surveying applications use RTK technology based on the use of carrier phase measurements of the GNSS signals where a single reference station provides the real-time corrections, providing up to centimeter-level accuracy. The base station re-broadcasts the phase of the carrier that it measured, and the rover units compare their own phase measurements with the ones received from the base station. This allows the rover units to calculate their relative position to millimeters, although their absolute position is accurate only to the same accuracy as the position of the base station.
In traditional DGPS/RTK, errors due to atmospheric effects are not completely removed.
These errors increase with the baseline and for baselines of more than 15-20 km, ambiguity fixing is less reliable during periods of high ionospheric activity.
5.5. Reference Station Network
Various organizations around the world have set up the means to provide GNSS users with reference station data or differential corrections enabling users to acquire higher quality positions without the need for a second GNSS unit. Correction services vary greatly and can be utilized for single frequency pseudo-range, single frequency carrier phase or dual frequency carrier phase measurements. Data can be delivered in real-time by radio, telephone, internet or satellite broadcast, or reference station data files can be downloaded for post-processing at a later stage.
Network RTK (NRTK) technology uses information from a network of reference stations to better predict the variations of ionosphere delays and orbit errors. FKP corrections compensate satellite orbit and ionospheric errors as a function of baseline length and effectively fix ambiguities for longer baselines than with a standard RTK. In the NRTK Reference Station Network facility, a central data server collects the raw observations from a number of CORS, and sends corrections to the user's GNSS receiver (rover) after carrying out an integrated processing. The rover combines these corrections with its local carrier phase observations, to obtain a high accuracy real time positioning solution. The architecture of NRTK system is illustrated in the following figure.
In the Emirate of Abu Dhabi, the GRS Network (see Section 6 for details) provides the RTK correction values. This real-time correction should be utilized for all GNSS data collection work undertaken in Abu Dhabi.
(Ref: http://www.cors.com.au/applications/surveying)
5.6. GNSS Equipment Setup
The mapping grade GNSS receivers must be:
• Capable of producing and storing data in a format compatible with standard base station data used to perform differential corrections (RINEX etc.)
• Capable of storing attribute data about features collected
• Capable of storing time and coordinates of features collected
• Capable of exporting features collected to a format that can be used by a GIS
Table 2: GNSS Receiver Settings
Parameters Standard
Horizontal Datum WGS84
Vertical Datum Height Above Ellipsoid (HAE) or Mean Sea Level (MSL); if MSL is used indicate the Geoid Model
Coordinate System UTM
Position Mode 3D mode, which requires signals from a minimum of four satellites to determine a 3D location (a fix)
Correction Real-time correction using Abu Dhabi GRS Network Unit of Measure Meters
Feature Types Point, Line, Area (polygon)
Logging Intervals Point: 1 second. Line /Area: walking 5 seconds; driving 1 second.
Minimum no. of positions 4 for a point feature
Almanac1 Let the almanac update itself once the unit is turned on outside.
Antenna Heights2 1 – 2 meters Elevation Mask3 15 degrees
PDOP Mask4 6.0 or less
Position Mode5 Manual 3D SNR Mask6 6.0 or greater
Notes:
1. An almanac provides information for satellite acquisition in the field and for mission planning in the office. Typically, almanacs are considered current for up to thirty days. In the field, almanacs are acquired every 12 minutes when using a GNSS receiver. For mission planning you shall acquire a fresh almanac. Almanacs can be acquired from a GNSS receiver, GNSS software, base station file, or the Internet.
2. Antenna heights will vary depending on the specific application. Typically, the antenna height shall be set at 1.0 - 2 meters. The antenna height is directly related to vertical and horizontal accuracies. It is imperative to properly set the antenna height when collecting vertical GNSS data.
3. The elevation mask shall be set to 15 degrees since position data should be collected using only satellites that are at least 15 degrees above the horizon. There are GNSS projects that may require a higher or lower degree of mask, but these instances shall be reviewed and documented in the GNSS metadata.
4. Position Dilution of Precision (PDOP) can be monitored by setting the receiver to collect data with a PDOP of 6.0 or less. This will insure that the satellite vehicles are adequately
distributed. Collecting fixed positions during periods when the PDOP is higher than 6.0 could result in less accurate data.
5. The position mode setting on the GNSS receiver shall be set to manual 2D or 3D. This will provide positions collected with a minimum of three or four satellites. The 3D mode will allow for horizontal and vertical data collection
6. Signal to noise ratio (SNR) refers to the strength of the code given off by the satellite vehicle. If the SNR falls below 6.0 the code is considered weak. The SNR shall be set to accept positions when the SNR is 6.0 or higher.
Minimum number of satellites required: At least four common satellites - the same four satellites - must be tracked at both the reference receiver and rover for either DGPS or RTK solutions. Also to achieve centimeter -level accuracy, a fifth satellite is required for on-the fly RTK initialization. This extra satellite adds a check on the internal calculation. Any additional satellites beyond five provide even more checks, which is always useful.
5.7. GNSS Metadata
Metadata shall be recorded for all GNSS features. The metadata addressed here refers to metadata specific to the GNSS receiver and is different from the metadata for spatial data features. GNSS metadata includes: accuracy of receiver, base station used for differential correction, coordinate system, dates of collection, datum, differential correction applied, elevation setting, horizontal and vertical accuracy, minimum number of positions collected, PDOP, SNR, type of receiver, and units of measure. The metadata shall be supplied with the feature data layer. Some GNSS projects may warrant receiver settings or collection methods outside of the standard presented here. When this occurs it shall be precisely documented in the metadata.
Table 3: GNSS Metadata Elements
Requirements Example
Type of receiver Manufacturer / Type : Trimble- GeoXT
Number of receiver channels 12
Number of available satellites Minimum of 4 satellites Horizontal accuracy of receiver (manufacturer’s
specifications for predicted error)
1-5 meters+ 2ppm (root mean square)
Vertical accuracy as stated by manufacturer 3-5 meters+ 2ppm (root mean square)
Coordinate system UTM
Datum WGS 84
Date of collection (YY/MM/DD) 10/09/07
Time of Collection (HH:MM:SS) 11:37:52
GNSS methodology utilized RTK
Altitude reference Height above Ellipsoid (HAE)
Minimum number of positions for a point feature 4
PDOP Mask Maximum of 6
SNR Mask Minimum of 6
Units of Measure Meters
5.8. Accuracy Classification
Accuracy standards specify the relative accuracy of positions. Relative accuracy differs from absolute accuracy in that it is a measure relative to a known position, e.g., a point whose coordinates are accurately established through survey grade GNSS.
The accuracy table below provides a common reference for classifying different GNSS measurements by relative accuracy. This table can be used for specifying the accuracy requirements before a data collection project as well as for classifying the collected data as per the accuracy achieved and for using the data appropriately.
Table 4: Accuracy Classification
Accuracy Code Accuracy Class Class Range 1 5 decimeter 0.201 - 0.50 meters 2 1 meter 0.51 - 1.00 meters 3 2 meter 1.01 - 2.00 meters 4 5 meter 2.01 - 5.00 meters 5 10 meter 5.01 - 10.00 meters 6 20 meter 10.01 – 20.00 meters
To understand the term accuracy one must look at an associated term namely, precision.
Whereas, accuracy is connected to the quality of a result, precision is connected to the quality of the process used to obtain the result. For example, a measuring tape that has been stretched may measure the length of an object consistently too long making the measured value of low accuracy (an incorrect absolute measurement) but if the same value was returned each time then the process of measuring the object can be defined as one of high precision.
Likewise, the measurement of a point by a GNSS receiver reporting the same coordinate values repeatedly may be of high precision, but the resulting accuracy may be low if the coordinate value differs very much from the absolute coordinate value of the point.
5.9. Collection Method Codes
Different grades of GNSS receivers and different data capture procedures can be employed for field data collection. The following GNSS data "collection method" codes define how data is captured using a GNSS receiver. Including the "GPSCODE" field and the respective collection method choices in a data dictionary provides insight into the accuracy of the resulting data. If the same approach is used for each field data collection effort, e.g., same GNSS receiver and the same correction method, then a default code can be set to avoid
having to enter the code for each feature collected. However, if different methods are used to collect different features then this code should be used to record the collection method.
Table 5: GNSS Data Collection Method Codes
Data Collection Method Code
GNSS data collection method
DC01 Survey grade receiver stationary during data collection DC02 Survey grade receiver moves during data collection
DC03 Mapping grade receiver with real-time differential correction DC04 Mapping grade receiver with post-processing differential correction DC05 Consumer grade receiver with real-time differential correction DC06 Mapping or Consumer grade receiver with no differential correction DC07 GNSS receiver grade and/or differential correction procedures unknown
5.10. Tips for Collecting GNSS Data
GNSS data shall be collected in one of three geometries (point, line or area). Each geometric shape requires a specific collection method. A point feature shall be collected at one-second intervals with a minimum of 4 measurements per point.
Line and area features shall be collected, depending on the shape and accessibility of the feature to be mapped, at five-second intervals if walking, and one-second intervals if driving.
An area (polygon feature) shall be collected by closing the feature prior to returning to the first point acquired.
For mapping of linear and areal features the preferred method is to measure the vertex points of the linear / aerial features. If this is not possible, then walk along the linear feature / boundary of areal feature and continuously measure points, or do a combination of vertex points and continuous measurements.
The following are general tips for capturing data and recording Waypoints with a GNSS receiver:
• Ensure that the GNSS unit is properly configured. The first time a GNSS unit is used in a new location, it will need up to a few minutes to orient itself.
• Once a target has been located for data capture, make sure that a clear a view of the sky is available
• Do not collect coordinates of stationary targets while traveling in a car or motorbike.
Alight from car, or dismount from motorbike and go close to the target.
• Record your position as a Waypoint
• Fill out the data capture form completely. Ensure that the corresponding Waypoint number is recorded on the form.
• Always check the accuracy level of GNSS unit before taking a reading; less than 15 meters accuracy level is preferable
• Depending on the memory capacity of the GNSS unit, periodically save Track logs.
Once the unit’s memory is full, it will begin to overwrite the earliest tracks.
5.11. Using Offsets in GNSS Data Collection
There are times when GNSS receiver has its limitations in mapping. Typically, a GNSS receiver is inherently designed to compute the locations of its antenna. This means that in order to capture the location of a feature, it is necessary to occupy that location. This is not always practical or safe, when measuring the location of features such as:
• a building where GNSS satellite signal is blocked by obstructions such as trees or high-rise buildings
• a building that is not accessible because of a surrounding wall
• a monitoring station in the middle of a stream
• a manhole in the middle of the street, etc
By integrating a laser rangefinder and an electronic compass (that provides distance, inclination and azimuth) with the GNSS receiver, it is possible to eliminate the need to physically occupy the feature.
The ability to apply offsets to features by recording both a GNSS position and an associated distance and direction to the object of interest is very useful in challenging situations. For example, suppose you wish to record the position of a building (say, a mosque) which is surrounded by a wall. You can use the offset capability to stand and record the position of the building from outside the surrounding wall and then record the offset to the building using a laser range finder. Benefits of using offsets in GNSS data collection include:
• Making data collection easier in hostile environments
• Decreasing safety liabilities in dangerous situations
• Reducing travel time to sites and objects
• Allowing you to record offsets and attributes of several features from one location
• Eliminating extra manpower needed to collect data
• Saving time, money and resources
5.12. Accuracy Specifications
The horizontal and vertical accuracy specifications for spatial data are provided in the AD- SDI Spatial Reference System Standard document. The accuracy specifications shall apply for the data collection requirements of point data (community facilities, environmental sampling locations, point pollution sources, well locations, wellheads…), line data (utility lines, roads, streams…), and polygon data (green area boundaries, environmentally protected area boundaries…).
The vertical (elevation) spatial data is only required in cases where elevations will be derived from the location, such as the reference point for measuring ground water elevations or derivation of contour lines. Elevations shall be generated as orthometric heights (relative to mean sea level) determined using the Abu Dhabi geoid conversion model and must be in meters, and referenced to the Ras Ghumays vertical datum of Abu Dhabi.
6 GRS Network
The GNSS Reference Station (GRS) Network operated by the Emirate of Abu Dhabi is a CORS (Continuously Operating Reference Station) network, consisting of 23 reference stations that have been surveyed to a high level of accuracy. GRS Network provides continuously transmitted precision location data referred to as “corrections” to a roving GNSS receiver that is operated by the user, offering Real Time Kinematic GNSS services around the clock for all customers. It also provides additional services such as a constant overview of file availability and data quality, as well as RINEX file upload for automatic coordinate computation in post processing mode for remote users.
6.1. Positioning Services
The GRS Network System of Abu Dhabi provides different types of positioning services to the users. The GRS Network control center collects satellite observations from the GRS Network, performs calculations and sends RTK corrections to the rover. GRS Network system provides different types of positioning services. A summary of these services is provided in the following table, showing the different characteristics of each service in the first column.
It may be noted that only the first two positioning services, namely RPS and HRPS, are applicable to the mapping grade GNSS receivers considered in this document. The last two services are meant to be used with survey grade GNSS receivers for advanced surveys.
The difference between RPS and HRPS is that RPS is a single-station RTK whereas HRPS is a network RTK. Network RTK uses correction information from a number of reference stations covering a large area, whereas conventional corrections from a single station are valid up to a limited distance from a reference station.
For GPPS and HGPS, positioning function is provided in real time and for kinematic, dynamic or rapid static applications. The user streams GNSS data, associated metadata, and a desired epoch date to an IP port, and receives back a stream of epoch-date geodetic coordinates (latitude, longitude, and ellipsoidal height) and orthometric heights. The difference between GPPS and HGPS is the rate at which data and positions flow.
Table 6: Comparison of Positioning Services
Service Long Name
=>
Real-time
Positioning Service
High-precision Real- time Positioning Service
Geodetic Post- processing
Positioning Service
High-precision Geodetic Post- processing
Positioning Service
Short Name RPS HRPS GPPS HGPS
Service type Differential GNSS service (uses a single reference station)
Network RTK service (FKP, VRS, MAC)
Carrier phase and code range measurements of physical CORS
Carrier phase and code range measurements of physical CORS Accuracy* 0.5 to 3 m
(50 to 300 cm)
3 - 5 cm (horizontal), 5cm (vertical)
1 cm < 1 cm
Method Real-time Real-time Post-processing Post-processing Data Format RTCM 2.0 RTCM 2.3 and
RTCM 3.1
RINEX 2.11 RINEX 2.11
Data Rate 1 second 1 second ≥ 1 second < 1 second
Broadcastin g Technique
Internet / NTRIP Mobile radio (GSM) internet / NTRIP
Internet / HTTP Internet / HTTP
User Equipment
DGPS capable single frequency receiver, modem for data communication
RTK capable geodetic GNSS receiver, modem for data communication
Geodetic GNSS receiver, internet access (FTP, email)
Geodetic GNSS receiver, internet access (FTP, email)
Fields of application
GIS, Vehicle Navigation, Fleet Management, authorities and organizations involved in security duties, etc.
Engineering
Surveying, GIS with higher accuracy requirements, Real estate cadastre, supply and disposal, register of utility services, ...
Aerial
photogrammetric survey, laser scanning, geodetic survey,
Reference systems of the Abu Dhabi Emirate survey, analysis of geodynamic processes, ...
Hypothetical Subscription
Unit 1500 per year Unit 1 per minute Unit 2 per minute Unit 8 per minute
* The Accuracy for the different positioning services may be adapted by the municipalities as per the evolution of the GRS network infrastructure
The last row above titled the Hypothetical Subscription is provided to give an idea on the
“relative value” of different types of positioning services showing typical subscription plans.
At present, these services are provided free of cost to the users.
6.2. Other Services
The following additional services related to positioning are planned to be provided as they would be useful to the AD-SDI community:
• The GRS Network positioning service can apply various mathematical models at the server without having to make the models accessible to the end users. An accurate geoid model shall be applied to convert the GNSS-computed ellipsoid heights of coordinates and provide the orthometric height values.
• Since the user community has lots of data in the Nahrwan 1967 datum a Coordinate Conversion Service, to convert coordinate values from Nahrwan 1967 datum to WGS84 (ITRF2000.0) datum, shall be provided.
These services shall be available as online services, similar to the way the positioning services are provided to the users.
6.3. Prerequisites for accessing GRS Network
To obtain access to the RTK correction data from the GRS Network, the user needs to obtain a username and password.
The GNSS Receiver should be “Network Capable”. However, it may be noted that GNSS receivers with RTK capability, which come under the Survey Grade category and is 20-30 times more expensive than DGPS receivers, are not necessary to work with the GRS Network.
A Modem to connect to the internet is required. A modem, either built into the GNSS Receiver or built into the Controller, is the best.
A Data Collector with GNSS controller software is required.
GRS Network data streams are broadcast using NTRIP. NTRIP is an application-level protocol for streaming GNSS data over the internet. NTRIP is an RTCM standard designed for use of the internet to disseminate differential correction data or other kinds of GNSS streaming data to stationary or mobile users.
To receive GRS Network data streams you will need to use an NTRIP client program. These are widely available. A list of NTRIP clients, both free and commercial, can be found on the BKG Germany’s GNSS Data Center website (http://igs.bkg.bund.de/ntrip/download).
A common way to connect to these data streams is by using an internet-enabled mobile phone to transmit data via Bluetooth to an NTRIP client embedded in a GNSS receiver. Note that while the data streams are provided free of charge from GRS Network, data charges may apply from the telecom provider.
6.4. Accessing GRS Network
In order to make use of the GRS Network facility, users are requested to contact the respective Municipalities.
