Contour vectorization

[David Osborne]

I would like to be able to take a colour scanned image of a map and 
extract the contours as vectors, to be able to prepare a DEM of the 
scanned area. From what I've read of the online GRASS manual pages, it 
seems that r.thin and r.line might help me get started... am I on the 
right track? Has anyone tried this already who could give a methodology 
for achieving what I want? The end result should be a DEM raster I can 
use with ERDAS Imagine.

Is there any other software outside of GRASS I should be considering? 
I've heard of GSNAKE but I'm not sure that does what I want. I'm really 
looking for interactive line-following software, where the operator 
indicates a line segment by clicking on the endpoints and the software 
follows the line between them, generating either a co-ordinate stream 
or a vector as it goes.

Any help greatly appreciated...

David

-- 
David Osborne                         david.osborne at nottingham.ac.uk
School of Geography             phone/voicemail: +44 (0)115 951 3397
University of Nottingham                    fax: +44 (0)115 951 3358
Nottingham NG7 2RD, UK     http://www.geog.nottingham.ac.uk/~osborne