Maciek and other GRASS users,
These are great points to consider. My work leads me into the realm of terrain analysis, so this topic is near and dear to me. I think that it would be great if the pooled efforts of the GRASS community could be used to make GRASS the premier DEM creation / modification environment. Since the quality of a DEM is of the utmost importance when calculating primary and secondary terrain parameters, documentation of and confidence in the algorithms used for DEM creation should be a priority.
Currently v.surf.rst provides a very flexible means to produce elevation surfaces from point and contour data. The recent work by Tomas Cebecauer and others (See "Processing digital terrain models by regularized spline with tension: tuning interpolation parameters for different input datasets" from the proceedings to the 2002 GRASS conference.) shows how v.surf.rst can be used in a method similar to ANUDEM to enforce proper drainage networks, by adding a "terrain skeleton".
In addition, in reference to a recent message from David Finlayson regarding cross-validation of v.surf.rst derrived data, it might be a good idea for someone with some experience doing so to make a how-to document. I for one am still a bit baffled at how to properly use the cross-validation tools in v.surf.rst.
Of course all of this is easy for me to say, and much harder to implement. As my C skills are not the best, perhaps I can contribute to this aspect of future work in the form of documentation...
Any thoughts, ideas from other users?
--
Dylan Beaudette
Soils and Biogeochemistry Graduate Group
University of California at Davis
530.754.7341
On Aug 14, 2005, at 12:03 PM, Maciek Sieczka wrote:
Helena, Markus,
Since improving v.surf.rst is discussed, I would like to ad a few notes from
a simple user, who doesn't understand all the maths of DEM interpolation but
who would like to obtain decent results. Sorry for any naiveness. Please let
me know what you think and if any of my wishes you find worthy of adding to
v.surf.rst.
From: "Markus Neteler" <neteler@itc.it>
Here a FWD from Helena Mitasova:
On Tue, Aug 02, 2005 at 07:01:33PM -0400, Helena Mitasova wrote:
We are currently testing different interpolation methods including
topogrid (or whatever its current name is)
I gues we should call it ANUDEM - after Hutchinson. The TOPOGRID is an
ArcInfo program, which is an implementation of ANUDEM v. 4.5 or 4.6.3,
depending ArcInfo/ArcGIS version
http://support.esri.com/index.cfm?fa=knowledgebase.techarticles.articleShow&d=20779.
and v.surf.rst along with other methods and we
plan to implement modifications/improvements that we find beneficial.
If I may point 4 features I find very usefull in ANUDEM (and two other
tools) for creating DEMs from data extracted from topo maps. It would be
great if v.surf.rst could provide them.
ANUDEM is able to utilize following data besides elevation points and
contours:
1. watercourses, including their flow direction
I used it in TOPOGRID. At little effort - only digitise the watercourses in
the direction needed - it is possible to obtain a DEM where the water flows exactly the way you want it.
2. elevation discontinuity lines, called 'cliffs' in ANUDEM
This was introduced in version 5.1. I haven't had an occasion to use it,
since it is not available in TOPOGRID and I never had the original ANUDEM at
hand. I only read it's supported in new version
http://cres.anu.edu.au/outputs/anudem.php and think it is a great feature.
Often elevation contour lines and points alone simply can't express all the
complexity of terrain when gullies, scarps, embakments, walls, other
breaklines are involved. That's due to these are not parallel to elevation
isolines, thus cannot be presented as elevation isolines. Trying to
represent them as points, although in theory doable in *some* cases, would
require a lot of work to digitise points dense enough and to correctly
estimate each point elevation manually. Yet utilising elevation
discontinuity lines, digitised from topo maps, could greatly improve DEM
accuracy - at very little effort. Especially in areas of land slides,
erosion drived by river or flooding, land deformation due to mining, cliff
shoreline - to name those I can think of right now.
Such a functionality is also present in SURGE interpolation software.
http://www.geocities.com/miroslavdressler/surgemain.htm
But it's only freeware/shareware for Windows, not free software, is very
limited as to amount of data it can handle in one turn and the input data
format is non standard and pretty complex.
3. waterbodies
I used it in TOPOGRID. The elevation of waterbodies interpolated agreed very
well with their actual elevation as seen on topo maps and they are flat like
they should be. I bet many folks would find it usefull for accurate
elevation representation in lakelands, visualisation of areas in the
vicinity of water bodies etc.
The other feature, not supported in ANUDEM but practical I think, are
ridges. I found it supported in another DEM interpolation software,
CatchmentSIM (again freeware for Windows, sigh), as "Interpolation Training
Lines". The user can digitise them from topo maps and inlude during
interpolation to model the mountain ridges in his DEM as he wishes
http://www.toolkit.net.au/catchsim/.
Maciek
