[GRASS-user] ogr2ogr and ortho/rgb display

[Dylan Beaudette]

Hi Bob

Looking over some notes, it is pretty simple: note example is not with 
spearfish, but close enough.

1. convert raster DEM in source location (i.e. lat-lon) to vector points, 
leave out creation of att table with the -z flag:

 r.to.vect -z in=elev out=elev_pts feature=point

2. change to target location (i.e. AEA proj) might want to read-up on the -z 
flag for v.proj .... I am not an expert on this.

v.proj in=elev_pts location=latlong_location mapset=PERMANENT dbase=...
 out=proj_pts 

3. re-interpolate DEM in target location with appropriate region settings, and 
interpolator. i.e setup resolution and use either v.surf.rst or perhaps 
v.surf.nbathy (?) 

I'll cook up some good examples of this, with stats posted this week - along 
with comparisons to field observation (n= 270).

cheers,

Dylan

On Monday 08 January 2007 09:56, Moskovitz, Bob wrote:
> Maciek,
>
> Can you demonstrate this method using Spearfish to the Grass newbies out
> there?
>
> Thanks in advance!
>
> Bob Moskovitz
> Seismic Hazards Zonation Program
> California Geological Survey
> http://gmw.consrv.ca.gov/shmp
>
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> -----Original Message-----
> From: grassuser-bounces at grass.itc.it
> [mailto:grassuser-bounces at grass.itc.it]On Behalf Of Maciej Sieczka Sent:
> Saturday, January 06, 2007 2:50 AM
> To: Daniel Farnan
> Cc: grassuser at grass.itc.it
> Subject: Re: [GRASS-user] ogr2ogr and ortho/rgb display
>
> Dylan Beaudette wrote:
> > be sure to project the DEM data with gdalwarp, and then import with
> > r.in.gdal . Note that DEM projection requires some planning if you will
> > be performing any flow-related analysis. If you are looking for a
> > 'smooth' DEM then I would recommend looking into the "-rcs' parameter to
> > gdalwarp .
>
> Daniel,
>
> Please note that reprojecting a floating point grid will usually yield
> "stairs" distortion in the output, depending on how big is the skew
> between the input and output projection. That's propably what Dylan
> meant. The "stairs" distortion will be the bigger, the less smoothing
> involved in the reprojection.
>
> Although aggressive smoothing like "gdalwarp -rcs" will reduce most of
> the distortion, it will also filter out the details from the input DEM
> and alter the original value range (min will go higher, max will go
> lower). To avoid both shortcomings ("stairs" and over-smoothing) I
> usually transform the grid into vector points, reproject the points and
> interpolate a grid from them, with a suitable algorithm (there are few
> in GRASS; for regularly spaced points input RST or IDW do fine).
>
> Maciek
>
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-- 
Dylan Beaudette
Soils and Biogeochemistry Graduate Group
University of California at Davis
530.754.7341