Effects of forest environment and survey protocol on GPS accuracy

The aim of the study is to test GPS equipment receivers commonly used for natural resource management, and to quantify recording rate and positioning quality under different conditions, the objective being to assist GPS users in their choices. Four factors were evaluated: (a) the type of receiver: three ranges of GPS equipment were compared; (b) forest cover effects (three covers were tested: open cover, coppice and deciduous high forest); (c) the effects of GPS survey components: the number of recordings (between 1 and 300), the Position Dilution of Precision (PDOP) thresholding (between 4 and 50), the time interval between recordings (between 1 and 15 seconds), and the differential correction effect; and (d) the season (winter and summer). A GPS survey was carried out and a database of 140,000 readings was established, from which a large number of random rover files were extracted for each combination of factors. It appears the only factor not to be significant is the seasonal effect. The type of equipment used and the forest cover effect both modify positioning accuracy by a factor of 2 or 3, as does the use of differential correction for Trimble receivers in open cover. Increasing the number of recordings and the time interval between recordings, and decreasing the PROP threshold, improve precision, with a different effect according to the GPS receiver and the forest cover. The effect is generally more pronounced under high forest cover. The combined effects of GPS survey components produce significant changes in accuracy at the expense of the time spent in acquiring data.