Hi Holger,

I've edited the wiki page, just removed the -dstnodata option to gdalwarp - 
that works for me!

Cheers
Adam
________________________________________
From: Holger Schöner [[email protected]]
Sent: 02 October 2009 22:09
To: Adam Ratcliffe
Cc: [email protected]
Subject: Re: [Mapnik-users] Black holes in hillshade image

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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