Using a GPS device

The development of satellite navigation systems has proved to be of enormous value to forest surveys (Kleinn 2003), enabling sampling locations to be located and relocated relatively easily. Most GPS units use the NAVSTAR-GPS system operated by the United States Ministry of Defense. The Russian government also operates a satellite navigation system called GLONASS. In Europe, a new programme called Galileo is currently being deployed and is expected to significantly improve coverage and precision when it becomes operational in 2008.

GPS receivers can now be obtained relatively cheaply, and are reasonably user-friendly. Although models made by different manufacturers vary in the details of their operation, typically they can be used to determine a location, to navigate from one location to another, and to store both individual locations and tracks. GPS is particularly useful where available maps are of poor quality, or where there are few features that can be identified from a map. As different models vary in their accuracy, the type of GPS unit used in collecting a set of information should be reported (Johnston 1998).

One of the main problems with use of GPS in forest survey is that it can sometimes be difficult to obtain an accurate location fix from underneath a forest canopy, which can prevent communication between the GPS unit and the satellites. To solve this problem, it may be necessary to locate the nearest canopy gap in order to obtain a measurement. The second main problem is the degree of accuracy obtained with a typical hand-held GPS unit, which is typically 5-10 m at best (Longley et al. 2005), especially under forest canopies (Figure 3.3). This may not be of sufficient accuracy to relocate a sampling unit. Sometimes a more accurate fix can be obtained if the GPS unit is left for several minutes, to increase the chances of detecting a relatively large number of satellites. Taking averages of repeated

Fig. 3.3 A differential GPS system. The antenna and GPS receiver are illustrated here. A reference station is also required (not illustrated), which consists of an additional GPS receiver and antenna. Such systems can provide location data with an accuracy of a few centimetres. However, accurate measurements may be difficult to obtain under a forest canopy. This instrument is manufactured by Leica Geosystems AG of Switzerland. (Photo by Harry Manley.)

Fig. 3.3 A differential GPS system. The antenna and GPS receiver are illustrated here. A reference station is also required (not illustrated), which consists of an additional GPS receiver and antenna. Such systems can provide location data with an accuracy of a few centimetres. However, accurate measurements may be difficult to obtain under a forest canopy. This instrument is manufactured by Leica Geosystems AG of Switzerland. (Photo by Harry Manley.)

readings can also increase accuracy (Johnston 1998). More expensive differential GPS systems are available that can provide a degree of accuracy of1 m or better, but these still function less well under a forest canopy. Measurements of altitude made by GPS units are often prone to a high degree of error, and it may be preferable to use a conventional altimeter instead.

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