[OSM-dev] Problem with Postgresql / Mapnik / SRTM Contours

Keith Sharp kms at passback.co.uk
Mon Nov 24 21:28:40 GMT 2008


On Sun, 2008-11-23 at 22:51 +0000, Andy Allan wrote:
> On Sun, Nov 23, 2008 at 6:35 PM, Keith Sharp <kms at passback.co.uk> wrote:
> 
> > Anyone have any thoughts as to what is causing this?
> 
> Yep, it's a boundary effect at the edge of each SRTM tile. What you
> are seeing is the end of the contour straightening out after it gets
> to the last datapoint in the SRTM data. There's a corresponding start
> of the same level contour in the next file over, and because the edge
> points are shared and overlapping by the SRTM tiles everything meets
> fine. So you get a kind of:
> \
>  \_
> 
> and
> 
> _
>  \
>   \
> 
> meeting at the same point. It's not impossible to detect and edit the
> polyline, or even see if there's a gdal_contour option to fix it, but
> I haven't spent any time doing so.

Thanks for the explanation.  I'm not enough of a whiz at GIS to fix this
myself, but I might try and find some giants whose shoulders I can stand
on!

Keith.

P.S. I had noticed that the Cycle Map had voids filled, there were a
number around the summit of Ben Nevis.  I mentioned it on IRC, just not
in my email.





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