[GRASS-user] Re: SRTM data aliases in UTM

[WolfgangZ]

John Stevenson schrieb:
> Hi,
> 
> I have imported some tiles of 1 arcsecond SRTM data into GRASS 6.3 using 
> r.in.srtm into a lat-long region.  Whenever I import them to a UTM 
> region, res=30, I get aliasing problems.  This results in a grid of 
> lines of jumps in elevation running through my data.
> 
> This happens when I use r.proj.  I have also tried generating a text 
> file of UTM coordinates via r.stats -1g, and cs2cs.  Importing this as a 
> raster or vector, and interpolating with r.surf.rst or v.surf.rst also 
> give aliasing errors.
> 
> What is the best way round this?  Importing at a higher resolution?  How 
> much higher?  My ultimate aim is to compare the SRTM with LiDAR data, so 
> if possible I would like to use an exact interpolation where possible.
> 
> Cheers
> 
> John
> 

You have to check what interpolation methods are used. If you don't 
specify anything, Grass might use "nearest", but I'm only using Grass 
rarely.
For myself I use gdalwarp with cubic or cubic spline resampling for that 
task.

http://gdal.org/gdalwarp.html

Regards
Wolfgang