[GRASSLIST:2877] visible segments in the DEM

[Steve Trick]

Dear grasslisters,

I am conducting a project looking at lines of sight in and around a 
river valley.  My problem regards generating a satisfactory DEM on 
which to base the analysis.  I have been using v.surf.rst to generate 
a surface from vectorised contours but I always get visible segments 
in the DEM, which translate as unnatural 'steps' on the surface 
model.  I have tried increasing the minimum number of points used for 
interpolation, npmin, but to get good results, a value of around 2000 
instead of the default 200, the calculation time gets into the realm 
of weeks rather than hours and this is impractical.

The problem mainly occurs in flat areas of the terrain, such as 
floodplains, where the contours are few and far between, and I guess 
the program can't get enough points for interpolation.  Can anybody 
suggest a way of getting a 'smoother' DEM, either by adjusting the 
parameters of v.surf.rst or using another interpolation program? I 
have tried r.surf.contour but find this is quite crude and it tends 
to truncate the tops of hills.  I have looked at some of the other 
grass modules but find not all of them are implemented in GRASS 
5.pre2 which I am using.

Any comments welcome.

Thanks in advance,

Steve


-----------------------------------
Steven Trick BA MSc
School of History and Archaeology
Cardiff University
PO Box 909
Cardiff
Wales
CF10 3XU
email: TrickSG1 at cf.ac.uk
mob: 07799897107
http://www.cardiff.ac.uk/hisar/people/st/