SSMG-11 (C. G. Carlson and D. E. Clay)

The Earth Model--Calculating Field Size and Distances between Points Using GPS Coordinates

Summary:

An ever-increasing number of farmers have global positioning system (GPS) receivers on their combines. When not harvesting, GPS receivers are useful for more than locating one’s favorite fishing spot. They can be tools for determining the distance between two points or to accurately determine the acres in a field that is to be rented. The distance between two sampling points and the area of a field can be found using GPS coordinates and knowledge of the Earth Terrestrial Coordinate System. Because GPS latitude and longitude are in terrestrial coordinates, determining the distance in length measurements (feet, meters, yards, kilometers, and miles) rather than degrees, between two points is not trivial. The objective of this guideline is to provide a method that farmers, ranchers, or agricultural practitioners can use to calculate distances between points and to calculate the size of a field using Excel, a commonly available spreadsheet. A more detailed description for calculating distance and area is found in Carlson (1999), in which the mathematics and assumptions used to create the model used in this guideline are described. A Basic programming language computer program is also available in that paper.

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