Impact of forest canopy on quality and accuracy of GPS measurements

Abstract

Global Positioning System (GPS) technology has become an essential tool in the Earth observation field for georeferencing, classification and accuracy assessment activities. While it was developed to be employed primarily in the open, many users nowadays operate GPS receivers in less favourable conditions. We investigated the impact of varying types and degrees of forest cover overstorey on GPS data collection, accuracy and precision. While the occurrence of canopy overhead may degrade the positional precision by one order of magnitude, it is the presence of the foliage itself (leaf-on condition) that plays a major role in the signal reception and the positional accuracy. The relationship is inverse, with increasing relative canopy closure resulting in decreasing logging efficiency and accuracy. We also found that under larger forest canopy the Position Dilution of Precision (PDOP) values may improve signal reception without deteriorating accuracy. Moreover, the relationship between degree of canopy closure and the accuracy RMSE follows an exponential pattern with small increases in relative canopy closure leading to huge increases in positional error. Averaging proved to be a powerful way to diminish residual positional error under forest cover, 300 fixes being proposed as a good number for GPS data acquisition under mature canopy conditions. Ultimately, we judged that PDOP is not as good an indicator for positional accuracy under forest canopy as is universally acclaimed.