Hello list,
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?
Thanks,
Bob
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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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