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

[Maciek Sieczka]

On Thu, 18 May 2006 19:04:36 -0700
Dylan Beaudette <dylan.beaudette at gmail.com> wrote:

> Howdy,
> 
> downloaded a 1/3 arc-second (about 10m res) DEM from the
> seamless.usgs.gov site. 
> 
> projected the lat/lon raster to UTM via gdalwarp using the -tps option
> 
> import into grass, and compute shaded relief... and it looks like
> crap: http://169.237.35.250/~dylan/temp/seamless_dem_artifacts.png

Dylan,

When I need to reproject a DEM raster, first I convert it to points
(r.to.vect), then reproject the vector and re-interpolate DEM out of
it with v.surf.idw or v.surf.rst. This method yields lowest disturbance
in DEM look and feel, derivatives (curvature, slope, aspect are those I
tried) and preserves the min max best.

Due to high memory demand of Grass >= 5.7 vector engine you might run
into troubles though, then 5.4 and sites is your friend. Or transform
raster into x,y,z list (r.stats), reproject with cs2cs, import as vector
with v.in.ascii in points mode, interpolate this?

The only way you could avoid such stripping artifacts in the
reprojected DEM, without going trhough raster->vector->raster, is to use
gdalwarp's cubic spline resampling (-rcs), as other resampling methods
(bilinear, cubic) are not able to fully remove stripping artifacts. On
the other hand, the amount of smoothing involved by -rcs is
unacceptable for most applications besides rough visualisation IMHO.

You might like to search for messages several months ago by Frank
Warmerdam and Paul Kelly, either in Gdal list or one of the Grass lists,
in reply to my questions about DEM reprojection.

Maciek

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