LOCATIONAL ERRORS IN MAPS FROM ENVIRONMENTAL SURVEYS AND THEIR IMPLICATIONS FOR INFORMATION EXTRACTION

1. Information on all maps, including those which depict characters of the natural environment, is subject to locational error. 2. Locationa l errors on field-sketched maps are attributable to inaccuracies both in the base map itself and in the method of locating and recording fea tures onto the base map. 3. Two main sources of error associated with field sketch mapping were quantified using the River Corridor Survey m aps as an example. First, channel widths are routinely exaggerated on these maps to allow recording of in-channel features. Distortions of r iver bank length of the order of 25% were shown to arise from a four-f old exaggeration of the width of a sinuous channel. Second, discrepanc ies in feature location of the order of 20% were found along a 500 m r iver stretch, although errors could be reduced to approximately 2% if undistorted, large-scale base maps were used. 4. An abundance scale ap propriate to the inherent locational inaccuracy was devised to extract semi-quantitative information from River Corridor Surveys recorded on distorted base maps. Application of this scale to 29 River Corridor S urvey maps of a 14.5 km river corridor produced a useful representatio n of environmental gradients within the corridor. 5. With the increasi ng potential to integrate and analyse environmental information within large computer databases including Geographic Information Systems, it is essential that the level of accuracy is recognised and quantified so that the errors can be allowed for in the development of methods of extracting information from the maps. (C) 1996 Academic Press Limited