TEST METHOD: Using a GPS in field surveys
Revised: 30 October 2006 (initial release)
GPS (Global Positioning System) devices make field survey mapping relatively simple, providing the data collected can be related back to aerial photography and mapping. Aerial imagery and assorted paper maps are related to the data collected by the GPS through a process of collecting reference points. The following procedure outlines how this is done.
TASK SAFETY REQUIREMENTS
Delta dress code applies.
SPECIFIC JOB STEPS
1. Before you go out on site, collect the following:
An aerial photograph of the site (preferably laminated, or at least several photocopies) - these can be downloaded free of charge (for areas in SA) from Naturemaps or purchased from Mapland. Other States of Australia have different methods of delivering aerial photography. However most have at least the facility to identify the area of photography you require online, and then order it. Some States allow you to download georeferenced aerial photographs. Naturemaps provides georeferenced aerial photography. An explanation of recording reference points using Naturemaps or using paper mapping is provided below.
A topographic map or orthotopo map that includes the site - these are particularly important if the aerial photography you have obtained has not been georeferenced as these will provide you with the reference points.
2. Selecting likely reference points on the aerial photography is the next step. Choose locations that will be easy to find on the ground and mark them onto the copy of the aerial photograph you will be taking out into the field. The locations should be also locatable on the paper maps if you are not using Naturemaps. Choose 5 or 6 locations.
Good reference points if your photography is on Naturemaps: the middle of road interesections, railway crossings, very visible corners of fencelines, road/creek crossings, corners of buildings, corners of transmission pylons, rocky outcrops or other small geographical features and large isolated trees. Be aware that tempoaray features such as trees may have been felled since the photography was flown.
Good reference points if you will be referencing your photography using paper mapping products: the middle of road interesections, railway crossings and creek/road crossings or geographical features like waterfalls are the best items to use if your only reference map is a topographic map. If you are using an orthotopo map as a reference the aerial photography on the map may offer you a wider range of choices. Remember, though, that temporary features such as trees may have been felled since the map was printed.
3. Record the locations in a table: Number the locations you have selected and record the easting and northing of each. On a topographic or orthotopo map use the printed map grid and a ruler to determine the eastings and northings of each location. If you are obtaining your photography from Naturemaps, use the Information Tool while you are online to provide the co-ordinates for each reference location. Also record a written description of each location in the table.
4. Record the map datum you are using: Naturemaps uses GDA94 (equivalent to WGS84). This datum is used on modern topographic mapping, such as that found in the CFS mapbooks. GDA94 is approximately 150m offset (in the Adelaide metropolitan area) from the datums used in most of the older orthophoto mapping. Older datums include AGD66 and AGD84. These two older datums are only about 7m different to each other and may be used interchangeably for larger scale projects. Record both the datum itself, and which mapgrid the area is located within (eg Adelaide is in mapgrid 54S)
5. Extra reference points: If you know that a benchmark exists at a site (either from discovering one on AtlasSA or in the SurveyGEM software), record its details to take along with you. You will not be able to see it on the aerial photograph, but may be able to estimate its location. Ensure the benchmark coordinates use the same datum as your other refernce locations. If they are in different datums, convert the benchmark to match the other reference points. Confirming your GPS reading to a benchmark is a good extra check.
6. Set your GPS to the same datum as all your reference data. Ensure you have spare batteries before you visit the site.Take your reference data and the marked aerial with you.
7. Go to your first reference location OR a benchmark location and turn on the GPS, making sure it has a clear view of the sky. Wait for the GPS to obtain sufficient satellites to get a good reading. The first reading at a site may take quite a long time, as the GPS becomes more accurate the longer you stay in position. Record the GPS location reading next to your record of the reference location and check to see how much difference is apparent. Is the difference of the same magnitude as the 'accuracy reading' on the GPS display?
If it is, and the accuracy is acceptable, move on to record all your other refernece points.
If the readings are similar but the accuracy is lower than you require, wait for the GPS to obtain more satellites.
If the difference between the reading and your record for the location is much larger than the accuracy reading for the GPS, give the GPS longer. If the accuracy remains low, check your datums on the GPS and the reference data are the same.
If the difference between the reading and your record for the location is much larger than the accuracy reading for the GPS, but the datums are matching, move on to the remaining reference points. The error will hopefully be consistent, and as long as you record the readings at each location, and the error is consistent, the data can be corrected back at the office.
8. Once all your reference data has been recorded, you can start recording the survey data you have come to collect. Remember to write a full description of what you are observing at each location you record.
9. If you need to survey over several days, revisit one of your reference sites before starting any subsequent day's work, to confirm you are obtaining acceptable readings from the GPS.
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