Hi Adam,

On Thu, 1 Oct 2009, Adam Ratcliffe wrote:
With further experimentation it seems that 32767 does seem to be the correct value for the -srcnodata option. I've also found that if I omit the -dstnodata option from the gdalwarp command the black holes do not appear, presumably because the void data is then not included in the reprojected image.

On 1/10/2009, at 3:56 PM, Adam Ratcliffe wrote:
I've created a hillshade image using the NASA SRTM3 data following the notes at: http://wiki.openstreetmap.org/wiki/HikingBikingMaps. Looks great except for the occasional 'black hole' in the image, I expect this is due to data voids not being rendered correctly.

What's got me puzzled about this is that the wiki page suggests using a value of 32767 for the -srcnodata and -dstnodata options to gdalwarp, howver in the NASA docs (http://dds.cr.usgs.gov/srtm/version2_1/Documentation/Quickstart.pdf) it says data voids are assigned the value -32768.

Should I be using the NASA value for -srcnodata and -dstnodata instead?

Thanks for working this out! Maybe you can correct the wiki page?

I wrote that Wiki page, and I just remember that at that time (a year ago) the interpretation was unclear for me. I remember having some problems and experimenting; but in my region, there are no voids, so I might not have found the right solution at that time. And I remember encountering "black holes" later on in other regions (thinking there is no way around having them, without setting them explicitly to some arbitrary value), as well.

I will try out your suggestions in a few days myself (and might update the page afterwards, if you have not done so then) ...

Thanks,
Holger
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