[GRASS-user] Creating dem using a contour map.

[Maciej Sieczka]

Hamish wrote:
> Bob Moskovitz wrote:

>> I have a 20 foot contour map shapefile that has over 47000 line strings. 
>> Many of these lines have gaps (see  
>> http://picasaweb.google.com/bobdebm/Problems/photo#5133539789447176242 )
>> in many of the contour lines.  I want to use v.surf.rst to make a dem.  Will
>> I have problems using this file? 

> v.to.rast + r.surf.contour  is the usual way for creating DEMs from contour
> lines.
> 
> r.surf.contour still needs some updating from the GRASS 4 days, ie the output
> is integer only and it treats 0 elevation as NULL. you can work around that by
> multiplying the input map by 1000 or 10000 and then shifting 0 values up one
> unit before processing and dividing by the same after.

Another issue is that r.surf.contour doesn't handle
disjointed contour lines well, which might be problematic
for you. Also, it interpolates "flat peaks" and "lakes" if
you don't supply elevation points at hilltops and
depressions in the input raster. You might consider
r.surf.nnbathy (with alg=nn or alg=ns) from the the GRASS
AddOns WIKI - unlike r.surf.contour it handles disjointed
contour lines fine, though it suffers from the latter issue
as well.

BTW - can anybody say if the algorithm used in
r.surf.contour is the same as that described in [1] ->
"ILWIS Operations" -> "Interpolation" -> "Contour
interpolation" -> "Algorithm"? The authours mention:

Gorte, B.G.H. and Koolhoven W., 1990. Interpolation between
isolines based on the Borgefors distance transform. ITC
Journal 1990-3, pp. 245-247. ITC, Enschede

in the references.

[1]http://52north.org/joomla/downloads/ilwis_help/ilwis.chm

Maciek