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

[John Stevenson]

WolfgangZ wrote:
> 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
>
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Hi Wolfgang,

Thanks for the suggestion.  I just tried with GDAL_warp and got the same 
problem.  I tried linear, cubic and cubic spline interpolation.  The 30m 
grid spacing is very close to the 00:00:01 resolution of the data.  My 
final map ends up with a kind of grid pattern superimposed that is most 
visible on steeper slopes.  I am using tiles n33w117 and n34w117 and 
projecting to UTM (epsg:26911).

Does anyone else have suggestions on how to convert it to UTM without 
this aliasing issue?

Thanks

John

-- 


Dr John Stevenson
Postdoctoral Research Associate
School of Earth, Atmospheric and Environmental Sciences
Williamson Building (Room 2.42)
University of Manchester
Manchester M13 9PL, UK
tel. +44(0)161 306 6585; fax. +44(0)161 306 9361;
john.stevenson at manchester.ac.uk