[gdal-dev] How to publish null as nodata by gdalwarp?

[Even Rouault]

Le lundi 17 septembre 2012 07:12:24, OHTSUKA Ko-hei a écrit :
> Hello,
> 
> I have a GeoTiff DEM, which is written in WGS84 and describes nodata as
> null. (No data means sea area.)
> 
> I want to make shaded-color-relief tiff in spherical mercator from it, so I
> run:
> 
> gdalwarp -s_srs EPSG:4326 -t_srs EPSG:3785 -r bilinear wgs_dem.tif
> merc_dem.tif
> gdaldem color-relief -alpha -co ALPHA=YES merc_dem.tif
> ramp.txt merc_rel.tif
> gdaldem hillshade -compute_edges merc_dem.tif
> melc_shd.tiff
> hsv_merge.py merc_rel.tif melc_shd.tiff melc_shdrel.tiff
> 
> But, result file melc_shdrel.tiff not contains null data.
> null data are all changed into value 0, so they are not transparent,
> and colored as ground.
> 
> 
> I tried several change to command, consequently,
> 
> gdalwarp -s_srs EPSG:4326 -t_srs EPSG:3785 -srcnodata "" -r bilinear
> wgs_dem.tif merc_dem.tif
> 
> This command makes null data not to value 0 but to -9999.
> "gdaldem color-relief" detect this change and make sea area as transparent.
> But, "gdaldem hillshade" not detect this, so sea shore shown as huge
> cliff, and sea area became
> gray.
> 
> I tried '--dstnodata' or '-dstalpha' option, but they makes any good
> change.
> 
> 
> Is there any ways to get destination data which describes null as
> nodata in gdalwarp?

Try adding -dstnodata 0 to your gdalwarp commandline.
The option -srcnodata "" is likely interpreted as equivalent to -srcnodata 0, 
which should not be necessary at all, as gdalwarp will autodetect it

> 
> Or, making "gdaldem hillshade" to detect -9999 as nodata?

In your ramp.txt, make sure that you have a line like :
"nv 0 0 0 0"

Note: the important 0 is the 4th one, which means that the alpha component for 
the nodata value (nv) will be 0.

A quick review of hsv_merge.py shows that it should preserve the alpha band of 
the colored raster.

But to debug this more easily, make sure to check that the result of each 
intermediate step is OK.

> 
> 
> Regards,
> 
> OHTSUKA Ko-hei
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