Positional accuracy of biological research data in GIS – A case study in the Swiss National Park

Abstract

Original field research data requires information about the positional accuracy of objects located in the field, expecially when analysed in the context of GIS. We present the results of a case study assessing the spatial accuracy of vegetation sampling data. The positional accuracy of a research grid consisting of adjacent squares of 20mx20m set up using a large scale orthophoto (1:2000) used for vegetation studies was assessed using surveying techniques. To study the absolute positional accuracy of the setup, the exact locations of a large part of these squares were determined using surveying techniques. The mean positional error was 5.2m (span 0.9-9.1m) for pegs located in the corners of the squares. The size of the individual squares ranged from 64% to 133% of the planned size of 400m2. The average (horizontal) distance of the true locations (n=335) was exactly as planned (20.0±2.1m). The minimum distance was 14.8m and the maximum distance was 25.4m. The mean horizontal angle in the corners of the plots was 89.9±3.3° (span 77.4-102.3°) (n=615). Overall, 67.4% of the whole area was in accordance to the GIS database, 32.6% was falsely attributed to wrong sampling squares. The influence on vegetation classification statistics was small (maximum of 0.58%). Even with the aid of relatively sophisticated instruments such as orthophotos, the positional accuracy in the original study was low, resulting in differences in plot area of over 200%. Nevertheless, the influence on the results of a single study are moderate. By contrast, these errors are of high concern in areas of intense interdisciplinary research such as national parks. It is thus recommended that for a focus research area as the site under investigation, surveying techniques should be implemented to enable long-term research and to minimize the risk of incorrect research results due to inaccurate spatial data