[GRASS5] v.surf.rst - serious errors in DEM

Jaro Hofierka hofierka at geomodel.sk
Wed Sep 7 14:11:04 EDT 2005


Sorry, I did not follow this discussion, but I am curious why you have 
not interpolated this area with a sharp fault line using separated 
interpolations on 2 areas divided by fault line that can be later 
combined in 1 area. I remember that Simox Cox was doing similar tasks 
with faults quite successfully perhaps 10 years ago.
I believe that Surfer is doing the same but "behind the curtain".

I am sorry if I missed somethong in your previous discussions.

Jaro

Maciek Sieczka wrote:
> Helena,
> Thank you for your interest in my problem. Below I'm trying to answer and
> explain more.
> 
> All,
> If you have any remarks regarding my problems with v.surf.rst, please join
> the discussion.
> 
> Helena Mitasova wrote:
> 
> (regarding
> http://www.biol.uni.wroc.pl/sieczka/udostepnione/vsurfrst/vsurfrst.png and
> http://www.biol.uni.wroc.pl/sieczka/udostepnione/vsurfrst/vsurfrst_exag.png
> - MS)
> 
>> I have never seen anything like that - check the elevations on your
>> faults - it behaves as if the elevation in the highest row of the fault
>> was lower than the elevation in the second row
> 
> 
> No, it isn't the case. The elevation of all points in the highest row of 
> the
> fault is higher than in rows below. Otherwords, the elevation of each point
> in the fault decreases down the fault. It is hard to describe spatial data
> in words, do you mind if I send you a sample for testing with v.surf.rst?
> 
>> and the tension was too low
> 
> 
> So I tried higher tension. But even as much as tension=170 is v.surf.rst
> still warns me about overshoots. And the "ditch" artifact visible on
> http://www.biol.uni.wroc.pl/sieczka/udostepnione/vsurfrst/vsurfrst_exag.png
> is *only reduced*, it still remains - see
> http://www.biol.uni.wroc.pl/sieczka/udostepnione/vsurfrst/vsurfrst_exag_tension170.png. 
> 
> 
> I've tried even more tension, but the higher the tension, the more distinct
> the artificial "pikes" on the hilltops get...
> 
> So I tried less tension and more smoothing to get rid of "pikes", but the
> results is that the "ditch" betwen contour line and the fault remains, as
> well as the "pike" on the hilltop:
> http://www.biol.uni.wroc.pl/sieczka/udostepnione/vsurfrst/vsurfrst_exag_tension80_smooth1.png. 
> 
> 
> More smoothing along with tension 80 still doesn't get rid of "pike" or the
> "ditch", only an artificial wave along the contour line gets more visible:
> http://www.biol.uni.wroc.pl/sieczka/udostepnione/vsurfrst/vsurfrst_exag_tension80_smooth10.png 
> 
> 
>> Where is the fault line?
> 
> 
> Here:
> http://www.biol.uni.wroc.pl/sieczka/udostepnione/vsurfrst/fault_line.png
> 
> On
> http://www.biol.uni.wroc.pl/sieczka/udostepnione/vsurfrst/surfer8_mincurv_exag.png 
> 
> you can see a 2m res DEM interpolated in Surfer8 using minimum curvature,
> with a Surfer's "blanking file" indicating this fault line (that's why the
> fault is so sharp on this picture). The vector points you also see on this
> screendump are points which were sampled over this Surfer's DEM, so I could
> include elevation faults in v.surf.rst. The results you saw above...
> 
>> And why are you interpolating 1m resolution DEM from data that are 10-20m
>> appart?
> 
> 
> Is it forbidden? And seriously - for visualization with a topo map, 
> which is
> 1m, and for experiments with modelling drainage network, which is consisted
> mainly of land reclamation ditches or other narrow watercourses, 1-5 m.
> Resolution 10-20 m  wouldn't cover such details.
> 
> Besides, e.g. 5m doesn't improve anything:
> http://www.biol.uni.wroc.pl/sieczka/udostepnione/vsurfrst/vsurfrst_exag_res5.png 
> 
> 
> I tried also 10m, but although the "ditch" artifact cannot be distinguished
> anymore, I believe that a coarser resolution doesn't improve the v.surf.rst
> interpolation, it only hides it's deficiencies. And I can't be sure if the
> same error wouldn't pop up in the area where generalisation to 10m isn't
> enough to "hide" the error. Coarser resolution is not a asolution.
> 
>> Did you try different parameters? You may want to read a little more 
>> about
>> rst - there is plenty on my web site
> 
> 
> I read, tried different settings but to no avail. Why does v.surf.rst
> produce pike on the hilltop, no matter what settings? I can't get rid of
> pikes unless I go for very low tension and high smoothing, like:
> http://www.biol.uni.wroc.pl/sieczka/udostepnione/vsurfrst/vsurfrst_exag_tension15_smooth10.png 
> 
> 
> but then the ditch between the contour line and fault remains - because of
> tension to low this time, dead end. And besides, with high smoothing and 
> low
> tension the overall DEM accuracy when compared to input data seriously
> suffers.
> 
> See the Surfer's DEM for comparison: there is a smooth gradient from the
> hilltop towards the contour line below, and then into the fault boundary.
> This looks more reasonable than an outstanding "pike" on the hilltop and a
> "ditch" between the fault and contour line. I know this is a different
> algorithm, but could v.surf.rst behave similarily? Or do I have to settle
> for that v.surf.rst is completely no good for non-homogenic data - in spite
> of it is claimed to be?
> 
> Maciek
> 
> P.S.
> 
> I could be told by you "If you are so happy with results from Surfer, go 
> use
> it and give v.surf.rst and Grass a brake". To avoid it I'll say something
> more first:
> 
> 1. I want Grass to be usefull for interpolating DEM from contour lines 
> and elevation points. On both Grass lists and some publications users 
> are kept being said v.surf.rst is capable of doing it in a reliable way. 
> Altough I've been trying to, I can't see how though. I'm not that dumb 
> you know - if it would be possible for a regular inteligent human being, 
> I would be able to do it. I'm still ready to admit I'm wrong and that 
> v.surf.rst is indeed as good for interpolating elevation data from topo 
> maps as it is advertised. Please convince me. From my experience so far, 
> I can only admit that v.surf.rst is good for interpolating homogenous 
> data - say for resampling SRTM DEMs, and completely no good for 
> non-homogenous data.
> 
> 2. I want to use free software as much as I can. Thus I wanted Surfer only
> to help me in obtaining additional data for faults, which I could then feed
> back to v.surf.rst together with contour lines and elevation points - to
> produce a complete DEM having used as much free software as possible.
> 
> 3. Minimum curvature algorithm implementation in Surfer8 is the only tool
> available to me which is capable of processing my contour lines and
> elevation points *together with fault lines* in a reasonable time and 
> fairly
> convenient way. Yet it has several other deficiencies I don't want to
> elaborate on here now not to go OT. Anyway, I'll say it once more, Surfer's
> minimum curvature DEM interpolator I want to use only for interpolating
> decent faults across my DEM, sampling these faults next and patching them
> into the input for v.surf.rst for final DEM generating. Or is v.surf.rst 
> not
> able to handle such a combination, because as Michelle wrote:
> 
>> I think the problem is that your data are clusterized, and the resolution
>> for interpolation is to hight with respect to the distribution of your
>> data. You can try to add more isolines.
> 
> 
> (Quick answer to Michele here - I don't have any more isolines on my 
> topo map to input. And I expect DEM interpolator to be able to 
> interpolate something reasonable from those data I provide, as they are 
> a reasonable input and not ditches and pikes. Quality data in != garbage 
> out.)
> 
> Maciek
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