Averaging absolute GPS positionings made underneath different forest canopies: a splendid example of bad timing in research

Dealing with data collection of natural resources for management planning purposes, there is an interest in capturing the spatiality, i.e. the geographical location of the features of interest. The Global Positioning System (GPS) provides a tool, both for navigation and positioning, although not without limitations. Several sources of errors have impact on the positional accuracy. Differential GPS increases the accuracy but might in certain applications be unavailable. Navigating in difficult terrain (e.g. mountain areas), where traditional maps give little or no guidance, highlights the benefits of GPS. By applying a not-so-advanced algorithm, arithmetical averaging, using absolute (non-differential) GPS measurements, it is possible to improve the positional accuracy. Initial horizontal mean errors are decreased by 50 % after approx. 10 minutes of active logging of singular GPS-positions and averaging these. Improvement of the accuracy continues thereafter, although substantial improvements require long time of observation. The impact of the canopy was marginal, however assumed to cause longer time of logging in dense forests before a certain number of positions are obtained, compared to measurements made under a clear sky. However, it must be pointed out that the results are of somewhat doubtful value, since the conditions were drastically changed when the intentional distortion of the GPS-signals (the Selective Availability, SA) was stopped. In the current situation, the different structure of the remaining errors leads to a limited use of averaging. This study was carried out just before the SA was stopped, which happened quite unexpectedly in May 2000.