[OSM-dev] Slippy Map Elevation Overlay

OJ W ojwlists at googlemail.com
Thu Apr 30 21:43:50 BST 2009

It's computationally a bit more expensive on the server, but you can
have a tile server that makes the white areas of regular maps
transparent and overlays them on top of relief maps (i.e. opposite
order to what you tried).  It's trivial PHP/GD code, and uses less CPU
on the client-side than having semitransparent overlays (which can be
*very* slow), but only works when the road map doesn't contain
antialiased edges.

update: I found an example implementation, but it seems to be
overlaying something with antialiased roads :(


On Tue, Apr 28, 2009 at 10:30 PM, Graham Jones (Physics)
<grahamjones at physics.org> wrote:
> I have had a quick go at http://maps.webhop.net/srtm.html.  It sort of works
> - the high ground stands out, and you can see the shape of the ground, and
> you can switch the overlay off and on.  It does not work that well as an
> overlay though because it is not really transparent enough.  I find that
> sometimes the transparency works quite well and you can see the map
> underneath, and others it is practically opaque.

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