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.