[GRASS-user] wierd artifacts in DEM from seamless.usgs.gov

[Maciek Sieczka]

Dylan,

On Sat, 20 May 2006 18:19:24 -0700
Dylan Beaudette <dylan.beaudette at gmail.com> wrote:

> Thanks to everyone for the helpful advice.
> 
> After a couple hours of working with the data here is what I found
> out:

<snip>

> Vertical artifacts appear in all of the projected images

Hmm, I never had vertical artifacts using -rcs... Maybe that just was
my "luck" with partricular datasets (reprojecting 90m SRTM tiles into
Transverse Mercator, SW Poland).

> except forthat of r.proj using the cubic convolution sampling
> method.

... and except for r.to.vect->v.proj->v.surf.* approach, right?
(artifacts at the borders of v.surf.rst output don't count here)


> (how exactly does r.proj work?).

When sorting out 2 bugs in r.proj months ago (they were fixed), I
learned that it is wrong to compare gdalwarp and r.proj output directly.

I'll quote some Frank Warmerdam wisdom here:

> In order to properly compare r.proj and gdalwarp results, you will
> need to ensure that they both are outputing to a file with exactly
> the same extents. You can force gdalwarp to create a file with
> specific extents using the -te switch.

Also:

> I would suggest using the -et 0.0 switch with gdalwarp. Without
> that, I believe that gdalwarp uses a linear approximation for the
> reprojection as long as the error does not appear to exceed some
> fraction of a pixel.  With -et 0.0, gdalwarp will do a full
> reprojection operation for each pixel.

Please see
http://thread.gmane.org/gmane.comp.gis.gdal.devel/3916/focus=3933

> The best _looking_ tangential curvature
> raster was derived from gdalwarp with the -tps and -rvc (cubic
> resampling) flags. However, this is probably due to the inherent
> smoothing that goes along with this set of options- as Maciek pointed
> out.

-tps is a coordinate reprojection method rather than a resampling
method like -rn, -rb, -rc or -rcs. -tps influences only how x,y coords
are tranformed, but has no impact on the particular raster value beyond
this.

> converting raster -> points -> v.proj -> v.surf.rst seemd to work ok
> (no artifacts in  the shaded relief besides edge effects), albeit
> very time consuming.

But this is the safest way and yields best result.

Maciek

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