Positional Accuracy of Biological Research Data in GIS – A Case Study in the Swiss National Park

Stephan Imfeld 1, Ruedi Haller 2 and Patrick Laube 3
1 Department of Geography, University of Zurich
Winterthurerstr.190, CH-8057 Zurich, Switzerland
Tel.: + 41 44 635 5253; Fax: + 41 44 635 6848 imfeld@geo.unizh.ch
2 Swiss National Park
Chasa dal Parc, CH-7530 Zernez, Switzerland
Tel.: + 41 81 856 1282; Fax: + 41 81 856 1740 rhaller@nationalpark.ch
3 School of Geography and Environmental Science
University of Auckland
Private Bag 92019
Auckland, New Zealand
Tel: +64 9 3737599; Fax: +64 9 3737434 p.laube@auckland.ac.nz 

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. 

Keywords: spatial accuracy, error analysis, uncertainty, environmental data, model

In: Caetano, M. and Painho, M. (eds). Proceedings of the 7th International Symposium on Spatial Accuracy Assessment in Natural Resources and Environmental Sciences, 5 – 7 July 2006, Lisboa, Instituto Geográfico Português

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