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

[Hamish]

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.


v.surf.rst has issues with contour line input, as it gives detail where many 
input points exist. For a line this means lots of points clustered along the
line and the surface gets streched to that. You can generate a surface then
take a random sample (ie evenly distributed in space) from that then rerun
*.surf.* on the random map for something smoother. Or cull out many of the
points when you converting lines to points.

it's not really ideal...


Hamish



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