[Gdal-dev] gdal_translate for netCDF to AAIGrid

Michael Toews mwtoews at sfu.ca
Tue Oct 24 14:43:12 EDT 2006

Hi all,
'gdal_translate' on a netCDF file to an AAIGrid has odd behaviour using 
GDAL under Cygwin. My conversion:
$ gdal_translate -of AAIGrid -sds test.nc test.asc
$ mv test.asc1 test.asc
No errors were generated during the conversion.

The header of the AAIGrid output is:
ncols        86
nrows        225
xllcorner    747700.000000000000
yllcorner    5441461.627906977199
dx           98.837209302326
dy           99.555555555556
NODATA_value  -9999

However, these headers should read:
ncols         86
nrows         225
xllcorner     747700
yllcorner     5441200
cellsize      100
NODATA_value  -9999

The dx/dy in the output of the file was incorrectly calculated as:

which is both why they are different (dx != dy because ncols != nrows), 
and why they are not correct, since they are multiplied by the ratio of 
base0/base1 representations of their dimension lengths.

My netCDF file (attached) should has a cellsize of exactly 100, since my 
data are spaced every 100 m in the x- and y-dims. This netCDF file was 
produced using a custom python script, which uses Scientific.IO.NetCDF 
for python 2.3. The file was later averaged, using 'ncra', which is part 
of NCO (command viewed in 'history' global attribute). My netCDF file is 
not perfect; I've recently discovered that my 'ydim' is 5463700, 
5463600, ..., 5441300; or it is in reverse order (dy = -100). For netCDF 
files, this should not matter (although I'll admit that it is a bit 
sloppy); I've been able to use the file as normal with 'ncview' and NCO. 
(As a caution, I have also seen some netCDF files which order their 
latitudes from North to South, which is also in reverse order; I'm not 
sure if this is tested, and corrected in GDAL, but I would highly advise 
it for netCDF files).

I also get strange behaviour with 'gdalinfo'. Try:
$ gdalinfo test.nc
$ gdalinfo test.asc # using the correct header

Both the 'Size' and the 'Corner Coordinates' are pretty strange for the 
test.nc file, as it seems to think that it is 512x512.

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