[GRASSLIST:8187] Re: [GRASS5] v.surf.rst - serious errors in DEM

[Maciek Sieczka]

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