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 Заголовок сообщения: How many raster layers should be there in the project
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Many vector lines above a well scanned topographical map are barely visible. White areas in a good image are really white i.e. as bright as your monitor allows.

Thin and bright vector lines are the best for work, but they are poorly visible against the background of a bright image, let alone eye fatigue.

The way out is to create a special image to control exactness of line tracing. This image is a clone of the initial one but its brightness is 30-50% lower, and contrast - 10-20% lower. Junctions between raster objects become more graded and do not distract from vector lines.

Additionally. It is desirable to use a TrueColor (16M colors) image for extraction of thematic layers, but 256 colors are quite sufficient for tracing control. The file decreases significantly and your computer works faster.

As a result, the set of images at vectorizing of topographical maps includes:

    1.The source image – 16M colors, lossless compression, 400-800 DPI (100-700Mb), the image is registered (positioned) and clipped along the frame. Serves as a source of clones for extraction of thematic layers. The image is switched off during vectorizing.

    2.The «dust-cover» image - 256 colors, (20-100Mb) decreased brightness and contrast. The image is intended for check-up of tracing exactness and semantics.

    3.Thematic black-and-white images (700Kb – 1,5Mb):

      - black unthinned - for input of point objects and symbols;

      - black thin - for line and grid tracing;

      - relief thin;

      - swamp strokes;

      - hydrography thin;

      - vegetation fillings.

There can not be too many images. Remember: image preparing takes minutes and saves tens or even hundreds of working hours

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