This approach is preferable when you are going to trace only 5-7 % of the entire raster information. If that is the case, raster preparing for digitalization can be reduced to creating of color sets. Each of these sets will be regarded in future as a single color typical of a certain kind of objects. To create a set, specify several points of an object.

• advantages - simplicity and clearness, minimum of preparatory operations.
• misadvantages - fuzzy tracing of raster lines and boundaries caused by their dithering or erosion. Inevitable time waste for editing of the resultant vector objects.

If more than 10 % of the entire raster information should be input, it is better to use the method of black-and-white thematic layer separation. Authomatic digitalization becomes possible sometimes after this procedure that reduces raster tracing time from several days to several hours. At semi-automatic digitalization, the initial and obtained layers are being used together. In its outward appearance, the process is identical to the first method, i.e. you see the color raster on your screen, but the programm is tracing lines of underlaying black-and-white thematic layers.

• advantages - accessibility of preliminary filtering of the black-and-white rasters, removal of regular noice (grid, rivers, roads). As a result, you will not have to edit most vector lines obtained by semiautomatic or authomatic tracing of selected areas or the whole raster layer. Decrease of total processing time and increase of end data quality.
• misadvantages - time spending for separation and preparing of thematic raster layers. One should know the technique of these operations.

Final choice of the method is up to you of cource, but we consider the second technique to be preferable today. It allows REALLY EFFECTIVE applying of authomatic raster to vector conversion thanks to raster preparation (filtering, regular noice removal, specifying of the areas to be excluded from tracing). Without this stage, results of automatic tracing are absolutely unfit or demand a lot of editing.