The U. S. Geological Survey has a requirement for the collection of large amounts of digital map data from existing graphic map separates. Prototype production techniques have been developed to capture line data from the map separates using a raster-scanning input device. After minimal editing in raster form, the data are converted to 'unstructured' vector form. In order to be processed further, the line data must be topologically structured (nodes identified and start-node, end-node, area-left, and area-right tags associated with each line-segment chain). This paper describes the techniques which have been developed at the U. S. Geological Survey to topologically structure the raster-scanned line map data.