Effect of forest canopy on the accuracy of a differential global positioning system (dGPS). The possibilities to use dGPS (differential global positioning system) for forest mapping are analyzed. The efficiency of dGPS in forest condition is linked to canopy interference, which intercepts signals sent by some of the satellites or reflect them (multipathing). Two types of equipment have been tested: a Trimble Svee Six coupled with a RDS differential receiver and a Trimble SK8 coupled with an OMNISTAR differential receiver. Recordings have been achieved according to two modes: static (recording on one spot during one minute) and cinematic (moving continuously along the stand limits). Two types of forest canopy were compared: light canopy made of young poplar plantation with few obstacles in the vicinity, and closed canopy of mature spruce stand edged with other mature stand. Cinematic mode efficiency is higher than static one (mean errors are respectively 3.1 m and 9.1 m). In the same way positioning is more accurate for the stand with light canopy than for the other one (mean errors are respectively 2.9 m and 9.4 m). Difference between recording modes are probably due to multipathing effect that filtering algorithm integrated in the GPS receivers are not able to remove in static mode. The efficiency of differential signal reception is also different for the two stands: 1.3/ and 20.5/ of missing differential signal respectively in the light and dense canopy conditions
