Extracting the linear elements from the map image is a much more complicated process. The complete contour and lake contour image is extracted from the pixel maps using image processing methods. The margins to neighbouring map sheets are edited and the raster data is transformed into vector data. Each contour is then assigned its value. This part of the work, as well as the entire extraction process, is carried out both automatically and interactively.
Independent tests are carried out and if necessary, the basic model is corrected.
Before the final matrix computation, the adjacent zones on neighbouring sheets are included to ensure that the values along the map margins are accurate. The 25 m grid of the height matrix corresponds to a mm-grid overlaid on a NM25. The data file is a regular array of height values in a 250 m grid and it is the first height model covering all of Switzerland. It is simply the transforma- tion of a normal map into a digital form. The information is separated only according to the printed colours and not according to any thematic structure.