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<html xmlns="http://www.w3.org/1999/xhtml"><head><title></title></head><body><div>I’m having an issue generating a contour shape file with gdal_contour from a source file that has been merged with gdal_merge.py
</div><div>I can generate contours from a single unmerged source tif without problems, so it seems something gdal_merge.py is doing is causing an issue.
</div><div><br></div><div>This is my process:
</div><div><br></div><div>I’ve downloaded SRTM data for the UK from
</div><div><a href="http://srtm.csi.cgiar.org/SELECTION/inputCoord.asp">http://srtm.csi.cgiar.org/SELECTION/inputCoord.asp</a><br></div><div><br></div><div>After downloading the tifs I’m using gdal_merge.py to combine them all into a single file like:
</div><div>gdal_merge.py srtm_*.tif -o uk.tif<br>
</div><div><br></div><div>I’m then re-projecting it to Google Web Mercator with:
</div><div>gdalwarp -s_srs EPSG:4326 -t_srs EPSG:3785 -r bilinear uk.tif uk-3785.tif<br>
</div><div><br></div><div>Finally I’m generating the contours with:
</div><div>gdal_contour -a elev -snodata 32767 -i uk-3785.tif uk-contour.shp<br>
</div><div><br></div><div>Generating the contours from a single reprojected tile works fine (takes about 80secs), however even trying it on two tiles that have been merged with gdal_merge.py it seems to process for a very long time (the .shp file quickly rises to a few gigs but then doesn’t change size) until after an age it errors with:
</div><div><div>ERROR 1: Failed to write shape object. File size cannot reach 4294964860 + 2696.
</div></div><div><br></div><div>It’s strange that a tif combined from two tiles fails when it reaches a ~4GB file size and yet doing just one of them individually only results in a 90MB .shp file.
</div><div><br></div><div>As a side note I’m also generating hill and slope shading images (for use in TileMill).
</div><div>You can see my full script for that here: <a href="https://gist.github.com/subblue/9705999">https://gist.github.com/subblue/9705999</a>
</div><div><br></div><div>I’m using gdal v1.10.1 that has been installed via brew on OSX 10.9.2
</div><div><br></div><div>Are there any tricks to generating contours from merged files or is it just going to be a case of scripting the generation of lots of .shp contour files from each of the elevation tiles?
</div><div><br></div><div>Thanks for any help, I’m still on the steep bit of the learning curve with all of this :)
</div><div><br></div><div>Tom
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</div><div style="font-size: 12px; line-height: normal; orphans: 2; widows: 2; margin: 0px;"><b>Tom Beddard</b></div><div style="font-size: 12px; line-height: normal; orphans: 2; widows: 2; margin: 0px; color: rgb(20, 79, 174);"><span style="text-decoration: underline;"><a href="http://www.subblue.com/" style="color: rgb(0, 128, 255);">www.subblue.com</a></span></div><div style="font-size: 12px; line-height: normal; orphans: 2; widows: 2; margin: 0px; color: rgb(20, 79, 174);"><span style="text-decoration: underline;"><a href="http://twitter.com/subblue" style="color: rgb(0, 128, 255);">twitter.com/subblue</a></span></div></div></body></html>