[gdal-dev] maintain resolution with gdalwarp

[Seb]

Hi,

Does one need to set a the -tr (resolution of output) argument to
gdalwarp to ensure the resolution of the input is maintained in the
output?  I have a GeoTIFF image in UTM (+proj=utm +no_defs +zone=20
+a=6378137 +rf=298.257222101 +towgs84=0.000,0.000,0.000) with square
raster cells that I want to project to latlong WGS84, *maintaining* the
same resolution.  My call is currently:

gdalwarp -s_srs ${GEOTIFFFILE}.prj -t_srs 'EPSG:4326' \
    -tps -r cubicspline ${GEOTIFFFILE}.tif ${GEOTIFFFILE}_geo.tif

However, the resulting image also has square cells, which I don't expect
if the resolution were to be maintained.  Is there some formula one can
use to calculate an optimal resolution for the -tr argument?

I realize this is probably more of a general GIS question, since I have
encountered this situation in the past and have just gone about it
through trial and error.


Cheers,

-- 
Seb