[Gdal-dev] DEM projection conversion
jdenisgiguere at fastmail.fm
Mon Jun 7 21:07:37 EDT 2004
Frank Warmerdam a écrit :
> Jean-Denis Giguere wrote:
>> Hi list,
>> I have USGS-DEM data and I would like to convert them in rasters in
>> Lambert projection. I want to use these rasters with MapServer. I have
>> tried to convert them in several format (pcidsk, hfa and bt) with
>> gdalwarp, but the output rasters are always envelopped and I have bad
>> result when I use gdaltindex after.
>> This is a sample execution of gdalwarp, maybye I don't use correct
>> $ gdalwarp -of BT \
>> -s_srs '+proj=latlong +datum=NAD83' \
>> -t_srs '+init=epsg:42304' \
>> some_dem_file some_dem_file.bt
>> Any idea to solve this problem will be greatly appreciated.
> can you be more specific about what you mean by "enveloped"? Is the
> issue that you have triangles of "nodata" in the output product? This is
> to be expected. Are you unclear about how to use this data in MapServer
> in such a way that the triangles of nodata are not rendered?
> First, you should ensure the nodata areas are produced with a value that
> is distict from valid values. You might do this something like:
> gdalwarp -of BT -dstnodata -100 -wt Int16 \
> -s_srs '+proj=latlong +datum=NAD83' \
> -t_srs '+init=epsg:42304' \
> some_dem_file some_dem_file.bt
> This forces "nodata areas", that is areas with no value written to them to
> be initialized to -100. The "-wt Int16" forces use of the Int16 type in
> the warp ... and hopefully results in the output file being of type Int16
> though I think that is implicit with BT format.
> Ensuring the "nodata" value isn't used in MapServer is a bit tricky though.
> If this was just 8bit greyscale data, you could just use the OFFSITE
> in the raster layer in MapServer. However, if you are allowing
> MapServer to
> autoscale your DEM data, the offsite stuff likely won't work quite like you
> expect since the value it is looking for will be the post-scaling value.
> If this is your issue, we will need to look at enhancements to MapServer or
> perhaps preprocessing your data to 8bit in gdal_translate in such a way
> your nodata value remains distinct.
> Best regards,
Thank you very much Frank,
I will try to precise a little bit what is my problem. It's not really
problematic to see region with no data displayed. However, your advice
will consederably improve output graphical quality. My problem is when I
use gdaltindex to create tile for my rasters. I have contiguous rasters
in latlong but, the converted rasters are larger and overlap.
| | | |/+--+
+-+-+ become +-|//|
| | | |/+--+
Result, some no-data area hide data.
Hope this clarify a little bit,
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