RE: [GRASS-user] ogr2ogr and ortho/rgb display

GRASS GRASS user
,
1

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

CONFIDENTIALITY NOTICE: This communication is intended only for the use of the individual or entity to which it is addressed. This message contains information from the State of California, California Geological Survey, which may be privileged, confidential and exempt from disclosure under applicable law, including the Electronic Communications Privacy Act. If the reader of this communication is not the intended recipient, you are hereby notified that any dissemination, distribution, or copying of this communication is strictly prohibited.

-----Original Message-----
From: grassuser-bounces@grass.itc.it [mailto:grassuser-bounces@grass.itc.it]On Behalf Of Maciej Sieczka
Sent: Saturday, January 06, 2007 2:50 AM
To: Daniel Farnan
Cc: grassuser@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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2

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

CONFIDENTIALITY NOTICE: This communication is intended only for the use of
the individual or entity to which it is addressed. This message contains
information from the State of California, California Geological Survey,
which may be privileged, confidential and exempt from disclosure under
applicable law, including the Electronic Communications Privacy Act. If the
reader of this communication is not the intended recipient, you are hereby
notified that any dissemination, distribution, or copying of this
communication is strictly prohibited.

-----Original Message-----
From: grassuser-bounces@grass.itc.it
[mailto:grassuser-bounces@grass.itc.it]On Behalf Of Maciej Sieczka Sent:
Saturday, January 06, 2007 2:50 AM
To: Daniel Farnan
Cc: grassuser@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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grassuser mailing list
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http://grass.itc.it/mailman/listinfo/grassuser

--
Dylan Beaudette
Soils and Biogeochemistry Graduate Group
University of California at Davis
530.754.7341