GPS Approach

The Global Positioning System (GPS) can be used to estimate land areas, either by walking along the boundary for small projects or by traveling in a vehicle for areas larger than that for ground physical measurements described earlier (Section 8.2.1). GPS is being extensively used in all research in land-based projects such as ecological studies and agricultural development.

GPS is defined as the constellation of satellites that circle the Earth in a very precise orbit and transmit signal information to the Earth. GPS receivers take this information and use triangulation to calculate the user's exact location. Essentially, the GPS receiver compares the time a signal was transmitted by a satellite with the time it was received. The time difference tells the GPS receiver how far it is from the satellite (www.navcen.uscg.gov). Any reading of a position is accompanied by information on which satellite is used in the calculation and the accuracy of the reading.

Single unit GPS is what is referred to as a handheld GPS, and uses several satellites to pinpoint a location. A more precise GPS system is to use a reference station on the ground, which can be a permanent unit of a portable unit. This is referred to as differential GPS. The method described here is based on a single unit, handheld GPS, which gives an accuracy of ±10 m. This accuracy is adequate for most land area estimations.

GPS data of the boundary of a land area can be stored in a geographic information system and facilitate future verifications (Greenhouse 2002). The positions can also easily be applied manually to any map that shows the latitudes and longitudes.

Materials required include GPS, batteries, a tagging device, notebook and a local map showing latitudes and longitude for verification.

Preparation for fieldwork Inform landowners and land users of the objective of the project and involve key informants who can provide information on the current and past land-use patterns.

8.2.2.1 Steps for GPS

Step 1: Set up the GPS according to the manufacturer's instructions. Determine the geodetical datum, coordinate system and units required and enter them into the GPS. Use the same parameters as the end product, usually the base map used for the project. If there is no information on any of these parameters, a basic set-up is World Geodetic System (WGS84) for geodetical datum, Universal Transverse Mercator (UTM) or Plane for coordinate system and metres/feet for unit. Most GPS have these parameters prelisted and can therefore be easily selected.

Use a local map that gives latitudes and longitudes.

Step 2: At the starting point of GPS readings, let the GPS obtain an average reading over 5 min to obtain the most accurate reading. Usually a minimum of four satellites are required to get a reasonably accurate location. Step 3: Check your position on the map. If there are differences, check the datum settings and try again. If the accuracy is not obtained, move to a new site and try for a new reading. Dense crown cover and hills can lower the accuracy. Step 4: For periodic monitoring, tag the starting point and all points on the boundary whenever the boundary changes direction. Tagging can be done with metal clips that are added to trees or other permanent features. Step 5: Make notes of error estimates given by the GPS (usually given on a relative scale or in metres).

If the GPS data are stored digitally it can usually be downloaded and used in GIS programs. If these possibilities are not available, the readings can be recorded manually. The land area can be calculated and then used in estimation and monitoring procedures. Accuracy of the positions given by a GPS is ±10 m, which for many planned projects is sufficiently good for land estimation. A handheld GPS unit can be bought starting from less than US$100 and can go up to a few thousand dollars.

8.2.2.2 Merits and Demerits of GPS Technique

Merits

• Supplies data even while on the move, that is, the data are not restricted to a few fixed locations

• Compatible with GIS

• Suitable for projects covering a small geographical area

• Suitable particularly for project proposal development phase

Demerits

• In dense forests or mountainous regions, signals from the satellites can be obstructed, lowering the accuracy of data

• Difficult if there are multiple land units that are far apart

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