[GRASS-user] Artefacts when interpolating DTM from Contour Lines with v.surf.rst

[Andreas Neumann]

Markus and others,

I would like to say thank you for your regarding DTM interpolation with 
v.surf.rst.

It worked fine for me using the following approach: I interpolated it 
first with v.surf.rst, using a small tension value to get a smooth DTM. 
This version contained the artefacts. Then I did r.random to generate 3d 
points from the first result and finally did another v.surf.rst with the 
default tension/smoothing values. This removed all the rectangular 
artefacts.

My project was probably a stress test. I worked on a map of the 
Kilimanjaro region with rather dense contours (from an older map) and 
had to generate a 2m DTM for an area of 40 x 50km. For the full region 
it interpolated for several hours (around 12 hours) but it finally 
worked well. The result will be used in a new topographic map for 
mountaineers in the Kilimanjaro region.

I can say that the results of v.surf.rst are much better compared to 
what I can achieve with ESRI software. The TIN approach generated ugly 
triangular artefacts and the topogrid method had ugly artefacts 
perpendicular to the contour lines. And I couldn't fine-tune the 
topogrid algorithm to get better results.

For v.surf.rst it took me a while to figure out the correct parameters. 
There are a lot of settings that influence the results, but finally it 
worked out fine.

Thank you very much for your help!

Andreas

Markus Neteler wrote:
> Andreas,
>
> here some older post from Helena (I have taken out non-relevant 
> comments, and it is now v.surf.rst):
>
> On Thu, Jan 10, 2002 at 11:01:13AM -0500, Helena Mitasova wrote:
>   
>> substantially reduce the number of
>> points on contours - they are oversampled (choose dmin at least 1.5 times
>> resolution and dmax a BIG number so that the program does not add
>> additional points to contours). If you still get the segments - run
>> r.random on your new DEM in the area where you have the visible segments
>> to generate few additional points and add them to your contours and
>> reinterpolate. And use smoothing at least 0.1 (you can use variable
>> smoothing with s.surf.rst - set it zero for the peaks and higher value for
>> the flat areas).
>>     
> ...
>   
>> It is not too difficult to fix the visible segments problem - the errors
>> is very small so it is more cosmetics (but a very unpleasant one) - so
>> those segments just need to be smoothed out withing the program - we used
>> to have it with older segmentation that we used for the MG93 paper).
>>     
> ...
>   
>> Helena 
>>     
>
> You also find this addressed in the new GRASS book (3rd edition).
>
> Markus
>   


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