Some digitisation questions

[Mike Thomas]

Hi all.

I want to digitise some scanned topographic maps, and I'm trying to work out
the smartest way of doing the job.  I hope someone can help with these
questions:

1. Is there a module in Grass which will generate a raster from another
raster file by extracting a particular colour (or colours similar to it,
because scanned image colours vary slightly)?  For example, I would like to
extract streams, contours and other similar features into separate raster
files.

2. If Grass can't do this, can GIMP?

3. If I digitise some topographic contours (topgraphic, geochemistry etc),
is there a way of converting each line into the value it represents in a
raster image, which can then be used to interpolate a full set of heights
over the raster?

Cheers

Mike Thomas.