[GRASS-user] ogr2ogr and ortho/rgb display

[Maciej Sieczka]

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