[GRASS-dev] r.relief

[Helena Mitasova]

In areas of low relief the integer DEMs have steps (you get alternating flat and steep areas, with steep
areas along contours which you may be seeing in your shaded relief map), so when you run
r.relief and you get a nice smooth map because the data were converted to float using bilinear interpolation
you do not realize that you have the steps (you don’t see them on the 2D elevation map).
Then you run some modeling or analysis using slope or flow routing running through these flats (which could be quite large) and you get various artificial patterns of the modeling results in these flat areas or you run some statistical analysis you will get serious bias in the histogram
see the slide #14 here
http://courses.ncsu.edu/mea582/common/GIS_anal_lecture/GIS_Anal_Ltopoanal.ppt

If you convert to float using nearest neighbor you may still get the flats depending on the resolution.
Integer DEMs are pretty much a pain to work with if you are doing more than just visualizing 
the terrain and the steps along 1m contours
are quite hard to get rid of in areas of low relief - you pretty much have to reinterpolate the DEM.
Integer shaded relief output alerts user that the data are integer and they may have a problem,
or if it is in mountains that the integers are not an issue.

So what may be a convenience for one user may cause problems for other users, so if we want to conversion it should be a flag (similar situation like the -a flag in r.watershed).

Helena



> On Apr 30, 2015, at 10:00 AM, Paulo van Breugel <p.vanbreugel at gmail.com> wrote:
> 
> Out of curiosity, what are those reasons? From a user perspective it might not be that obvious (well, it wasn't for me at least). If the current implementation is maintained, perhaps it would be useful to add a note to the manual page?
> 
> Paulo
> 
> 
> 
> On 30-04-15 15:15, Helena Mitasova wrote:
>> I think that user should do the conversion (in most cases it needs to be reinterpolated).
>> I think that the current behavior is right for many reasons,
>> 
>> Helena
>> 
>> 
>>> On Apr 30, 2015, at 8:25 AM, Paulo van Breugel <p.vanbreugel at gmail.com> wrote:
>>> 
>>> Hi,
>>> 
>>> When running r.relief (in GRASS7.1) on a integer DEM raster, the output is an integer map. Is this intended behavior? Would it be possible to have r.relief convert the layer to double precision / float automatically if the DEM is of CELL type?
>>> 
>>> Paulo
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>> Helena Mitasova
>> Professor at the Department of Marine,
>> Earth, and Atmospheric Sciences
>> and Center for Geospatial Analytics
>> North Carolina State University
>> Raleigh, NC 27695-8208
>> hmitaso at ncsu.edu
>> http://geospatial.ncsu.edu/osgeorel/
>> "All electronic mail messages in connection with State business which are sent to or received by this account are subject to the NC Public Records Law and may be disclosed to third parties.”
>> 
> 

Helena Mitasova
Professor at the Department of Marine, 
Earth, and Atmospheric Sciences
and Center for Geospatial Analytics
North Carolina State University
Raleigh, NC 27695-8208
hmitaso at ncsu.edu
http://geospatial.ncsu.edu/osgeorel/
"All electronic mail messages in connection with State business which are sent to or received by this account are subject to the NC Public Records Law and may be disclosed to third parties.”