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Locational

Global Positioning System (GPS)

A GPS receiver measures the geographical position (X, Y, Z) of features on the earth’s surface. In the context of smallholder agriculture, this could be the location of a farm (either as a single point or boundaries) or that of a ground control point. The receiver measures the distances to a constellation of satellites with accurately known positions via the time-stamped signals these satellites emit (Misra and Enge, 2006). From the distances and known locations of the satellites the position of the receiver can be calculated.. As a result, it is advisable not to use GPSs in areas of high canopy cover, as trees may block or weaken the signal between the GPS receiver and satellites.

There are two types of GPS devices one could use. These are the kinematic (differential) or handheld GPS (Figure 6.2).

Figure 6.2. Example of handheld (extreme left) and kinematic GPS receiver (reference in the middle and rover to the far right).

As their name suggests, “handheld” GPS devices are small and can be easily handled (fits into one’s pocket). However, the positional accuracy of these devices is often low (between 3 and 8 m), which may be good for some purposes (e.g. determining approximate point location of a field) but may not be so good for others (e.g. cadastral surveys).

Kinematic GPSs on the other hand, consist of two receivers. Each receiver is setup on a tripod or stand/rod for stability. One receiver (often called reference) must be setup on a monument/pillar with known coordinates, while the other is installed at the location where coordinates is required. This system produces positional accuracies in the order of millimetres because errors at both stations cancel out. Thus, in instances where high positional accuracy is required (e.g. Ground Control Points, cadastral, etc.), kinematic GPSs must be used.