Environmental mapping made simple

In environmental applications such as forestry, the capture of the polygonal structuring of the data is expensive in terms of hardware, software and labour due largely to difficulties in constructing a topologically complete map. This paper will show that, for many applications, topological structuring and data entry methods may be made quite simple, The example used is a paper map of forest polygons. The map must usually have the relevant polygons digitized manually, but instead of drawing individual boundaries to be connected later the operator is instructed to draw around the interior of each polygon in turn, generating a set of ‘fringe’ points labelled with the relevant polygon’s number. If the map has no other information on it then it may be scanned instead, and saved as an image. Simple image processing techniques may be used to generate fringe points as in the manual process. Polygon boundaries are generated by first creating the Voronoi diagram of these fringe points. These triangle edges are examined in a single-pass ‘visibility order’ to extract the Voronoi cell edges only between fringe points with differing labels. These edges are connected with no topological errors. They may be exported to a GIS as arcs, or preserved internally for polygon labelling and other processing. Both Voronoi cells and extracted arcs are saved internally using the QuadArc, a modification of the Quad-Edge data structure, which is guaranteed to manage connected graphs on the manifold using just two operators. In an objectoriented language the data structure management is only a few lines of code, giving a simple and elegant approach to topology management. 252 GIS Technologies and their Environmental Applications