Scanning maps into GRASS

[Grass package]

I've given the scanning of maps some thought, and concluded that, for the
time being at least, the resultant digital maps would in most cases not be
useful as other than backdrop cell maps, because:

1) the map original is a cartographic product not intended for scanning - it
contains lots of different information types (texts, 'vectors', crosshatching,
alphanumeric labels, etc) which are only partly separable after scanning;

2) scanning an original and then georeferencing it (i.rectify) may produce 
something that looks like a good map layer, but in fact you don't have
very much control over the exact positioning of the 'pixels'.

If you do want to go ahead, the following may be useful:
- when scanning, try using 'posterising' settings to control the number of
colors in the scan, and use 'descreening' settings to remove most of the
printing screens in the original;
- do as much of the cleaning (removing texts, for instance) as possible 
using an image editing program - GIS isn't suited for this;
- scan at a resolution at least twice as high as the one you want to end
up with, because georeferencing will bugger up thin map features;
- do separate scans for the separate information types in the original,
making sure you don't move the map or change the scanning area in between -
that way you can georeference them all in one go.

Hope this helps,

Martijn van Leusen