Global Positioning System (GPS)
A GPS receiver determines its geographical position (X, Y, Z) on the earth’s surface. In the context of smallholder agriculture, this may be used to determine location of a farm field (either as a single point or as a string of boundary points) or that of a ground control point. The receiver measures the distances to a number of satellites with accurately known positions via time-stamped signals that these satellites emit (Misra and Enge, 2006). From the distances and known locations of the satellites, the position of the receiver is calculated. Open area positioning works best. It is advisable not to use a GPS in areas of high canopy cover (or high buildings), as these may block or weaken the signal, or cause multipath errors, between the GPS receiver and satellites.
Two types of GPS device can be used. These are the kinematic (differential) or handheld GPS (Figure 6.2).
As the name suggests, a handheld GPS device is small and is handled with ease (fits into one’s pocket). However, the positional accuracy of these devices is often inadequate (between 3 and 8 m), and may be good only for some purposes (to determine an approximate point location of a field) but not be so good for other applications (in cadastral surveys, where position has legal value).
A kinematic GPS on the other hand, consists of two receivers. Each receiver is mounted on a tripod or stand for stability. The reference receiver must be positioned on a monument/pillar with known coordinates, while the other is installed at the location for which coordinates are required. This system has a positional accuracy expressed in millimetres because the errors at the two stations cancel out. Thus, in instances where high positional accuracy is required (e.g. Ground Control Points, cadastral, etc.), kinematic GPSs should be used.