Extracting area objects from raster image data

An algorithm for extracting the spatial objects contained in raster image data is presented. Input to the algorithm is a Siemens computer-aided design (SICAD)-Hygris raster file representing a classification of the plane. Output of the algorithm is a SICAD geographic information system (GIS) external file containing the topologically complete definitions of all area objects and their constituents, such as arcs and nodes, as implied by the classification. The algorithm is based on a one-pass plane sweep that processes the input raster data in a strictly sequential order. Processing takes place only if certain color changes, also called events, occur. Each event triggers a well-defined sequence of simple actions