I found some void filled data files for New Zealand on the viewfinderpanoramas 
site.  The provided files appeared to cover the area in which I'd observed the 
artifacts.  To use them I simply downloaded the corrected .hgt files and 
overwrote the corresponding .TIFF files in the SRTM3 dataset with the 
replacement TIFFs.  I then re-ran gdal_merge.py to create a unified DEM file 
which the hillshade images were generated from.  Unfortunately I'm still seeing 
the artifacts.

Cheers
Adam
________________________________________
From: [email protected] 
[[email protected]] On Behalf Of Lars Lingner 
[[email protected]]
Sent: 05 October 2009 22:44
To: [email protected]
Subject: Re: [Mapnik-users] Artifacts in Hillshade Raster

Adam Ratcliffe schrieb:
> Hi,
>
> I've been generating hillshade images using the SRTM3 data, these are looking 
> good but I've noticed some artifacts around the area of the Southern Alps of 
> New Zealand.  These can be seen in this sample image: 
> http://test.geosmart.co.nz/images/relief-artifacts.png
>
> Being a little bit new to this I'm neither sure of why the artifacts exist 
> and how to remove them in the processing of the data.  So far I've 
> experimented with some of the options to gdalwarp, on the Hiking/Biking maps 
> wiki page it was suggested to pass "-wt Float32 -ot Float32" to the gdalwarp 
> call - I observed no difference with these options.  I also tried setting the 
> error threshold '-et' option to 0 which had no effect.
>
> Any suggestions on how to tackle this problem are much appreciated.
>


I'm doing also my first steps with generating hillshades. The wiki page
(HikingBikingMaps) helped a lot and I got my shades with the known
artifacts.

As I understand, these came from the SRTM3 data itself and can only be
fixed with more precise data. Now comes the SRTM30 [1] dataset in play.

The viewfinderpanorama website [2] lists also some alternative sources
for filling the gaps, but I just cant find a way how to process all the
data.

Does anyone have tried to combine/merge the datasets to fix the
artifacts? I would like to know the way how to do it. There is also the
SRTM4 dataset from CGIAR-CSI (with fixed gaps) but this is not an option
right now.

Lars

[1] http://dds.cr.usgs.gov/srtm/version2_1/SRTM30/
[2] http://www.viewfinderpanoramas.org/dem3.html

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