Handheld GPSr devices Good handheld GPSr instruments have limitations but: You will find that they are sufficiently accurate to create maps that reflect landscape and archaeological features. You will see that it is possible to reflect earthworks down to least 4 or 5 metres across. The maps generated will readily identify the features on the ground. When familiar with the 2 stage mapping process, you can produce a map in an hour or two, depending on the complexity of the site. The final map will be suitable for publication.
GPSr Accuracy Measuring a fixed point repetitively with a Trimble type instrument, demonstrates how much the measured co-ordinate for a fixed location varies with time. Professional instruments use ‘correction factors’ for the general location and time of day, to correct each primary reading. Handheld GPSr co-ordinates will always be relatively inaccurate in comparison.
GPSr Accuracy Instruments like the Garmin units being used today can be configured to enable WAAS / EGNOS correction. (EGNOS is a European system currently not in use.) WAAS is a Federal Aviation Authority system which uses ground stations to constantly monitor accuracy and send correction factors to the satellites every 5 seconds. WAAS enabled GPSr’s then use these factors to correct the primary data. This correction is not as accurate as the local UK correction factors used by professional instruments, but does theoretically improve data quality.
GPSr Accuracy Indicator 1.The accuracy as indicated on the handheld GPSr, and the accuracy of the corrected Trimble data are in the order of 4m and 1cm respectively. 2.This value is an indication of the level of uncertainty of a waypoint’s measured position relative to the BNG. 3.It is not an index of how well a GPSr can measure one waypoint relative to the next, when waypoints are measured contemporaneously. 4.A good GPSr’s can measure the position of one waypoint relative to the next with adequate accuracy for generating maps.
5. This allows maps to be drawn with sufficiently accuracy for the maps to be useful, whilst acknowledging that there will be up to a 4m overall error relative to the BNG. 6.Features mapped from both sets of data will look very similar with the exception of the feature co-ordinates which may have up to a 4m offset. GPSr Accuracy Indicator
GPS Receivers (GPSr) Buying Considerations 1.My GPS is Garmin GPSmap 60CSx which has been superseded by the GPSmap 62s. 2.Buy a ‘High-Sensitivity’ model which usually has an obvious aerial like the 60CSx. 3.Do not buy a GPSr that records waypoints using multi- function ‘joy-stick’ or similar control buttons. 4.Buy a model which has specific Mark and Enter buttons for recording GPS Waypoints – you will pressing them hundreds of times. 5.Buy a model that has WAAS correction. 6.Must have a USB socket. 7.From a mapping point of view, there is no need to have any other map installed other than the basic map the unit comes with. 8.Preferably has an external aerial socket.
External Aerial NOTE: 1.How the GPS’s co-ordinates have changed as it stabilises from tuning it on. 2.How it stabilises more as the aerial has been attached. 3.How the altitude has changed to reflect the aerial position. 4.The GPS uses a circle to indicate the inaccuracy relative to the BNG.
The Software and OS Maps The freeware used include: –TrackMaker –IrfanView –Google Earth –Easy GPS –Ordnance Survey Open Data –CutePDF writer and associated file converter –(GPS Utility) Commercial software –Serif’s DrawPlus DP8, X2, X3 (or X4) See the SWAAG GPS Technology web pages for the download links
Waypoints uploaded from GPSr and into TrackMaker
GPSr Accuracy Indicator. It is beneficial if your GPSr Map page can display data fields. The 60CSx can display up to 4 fields. The data displayed for each field can be selected. Displaying Accuracy and Elevation is recommended (if your GPSr can measure altitude). Note Elevation is metric and Accuracy Imperial Units.
OSGB36 Datum f.o. = false origin of the BNG, chosen so all co-ordinates are positive numbers. 1 st letter denotes the 500km x 500km square. 2 nd letter denotes the 100 x 100km square within the 500km x 500km square. NZ 146 124 100m x 100m. NZ 14610 12456 1m x1m square. 414641 512456 1m x 1m square. (British Uniform Grid: TrackMaker)
GPSr Measurement Recommendations Always use freshly charged or new batteries. Turn your GPSr on 5+ minutes before using it. Set the GPSr to indicate feet rather than metres, so that the current waypoint accuracy in feet is displayed. This gives you a more sensitive indication on how the GPSr is performing whilst taking measurements. Display the GPSr map page zoomed in to the maximum setting (20 feet usually), with data fields for Accuracy and Altitude displayed. Have an indicated accuracy figure as low as possible and preferably below 20ft and ideally below 15 feet. Consider using an external aerial when the GPSr indicates an accuracy in excess of 20 feet. Continued ….
GPSr Measurement Recommendations Make at least 10+ waypoints around the boundary of small features. Always holding the GPSr upright at chest height. Before taking each waypoint measurement, watch the map indicator arrow on the GPSr unit (at maximum zoom in) until it has settled down. Usually about 5-10 seconds. In-between waypoint measurements keep the GPSr in the same position. If you drop your arms down or put the GPSr in your pocket, you must let it stabilise again at chest height before taking your next measurement. Draw a plan view in the log so this can be compared with the waypoint data plot. Continued ….
GPSr Measurement Recommendations Take your time. Hurried measurements will lead to inaccuracy. More measurements is always better than fewer. Remember proximity to trees, buildings, hillsides can degrade your readings. Consider using external aerial when Accuracy > 20ft. Log what you are measuring. Cross-reference any images taken in the log with waypoint numbers
Just before we have coffee and go the Hagg Farm