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The GPS in Forestry Applications 
Bento, Joâo Manuel* 
Aranha, José T. M.* 
Gonçalves, José Alberto** 
Walford, Nigel*** 
* Forestry Dept., Universidade de Trâs-os-Montes e Alto Douro - Portugal 
** Mathematics Dept., Universidade de Trâs-os-Montes e Alto Douro - Portugal 
*** School of Geography, Kingston University - England 
Summary 
With the increase of NAVSAT Satellite Constellation, since the late 1970s, Global 
Positioning System (GPS), have been used in several forestry applications, including 
wildfire mapping, harvest site measurement, tracking of unmade roads and footpath or 
locating sampling points . GPS technology is a useful tool in field studies, but the nature of 
the stand canopy can introduce some additional difficulties during the field work. 
Key words: GPS, Forest Stands; Positioning 
Introduction 
The development of Global Positioning System (GPS) started towards the end of the 
1960s, as secret military programme, and the first satellites launches took place at the end 
of the 1970s. In recent years these systems have become partially available for civilian 
proposes. The greater availability of GPS has resulted in a range of new applications areas 
being developed. 
According to Greer (1993), a GPS may be considered to have three parts: 
- Constellation of satellites, there are currently 24 active satellites; 
- Ground Control Stations, 4 stations; 
- Users. 
The 24 satellites of the satellite constellation are distributed into six orbital planes, 
with an inclination of 55 degrees from Equator. These satellites describe near circular orbits 
with a radius of 20 000 Km and a 12 hours orbit period. They carry an extremely precise 
atomic clocks, which sends constant radio signals to the Earth. With the aid of a portable 
GPS receiver, users can receive these signals and, by means of specialist software, these 
signals can be processed in order to calculate position on earth's surface. The satellite