[OSM-dev] [Tilesathome] Projection problem with tilesgen.pl at very low zoom levels

[Martijn van Oosterhout]

On Jan 21, 2008 11:58 AM, 80n <80n80n at gmail.com> wrote:
> I've been working on a captions layer for low zoom levels (z4 - z7).
>
> The results are nice, but there seems to be a projection problem.  Items are
> being displaced vertically.
>
> Everything is spot-on for z8-z12, but when I try to repeat the process for
> z4-z7 then the error is apparent.
>
> I'm guessing at the moment that the problem has always been there, but is
> insignificant at z8, whereas at lower zoom levels it starts to become quite
> noticeable.

Sounds a lot like:
http://wiki.openstreetmap.org/index.php/Osmarender_bug

The error grows by the square of the zoom, so at lower zoom levels it
gets worse, much worse.

> Is there anyone who is familiar with the mercator projection code in
> tilesgen.pl / tahproject.pm that might be able to shed some light on this
> problem?

My guess is that the mercator preprocessor needs to be invoked to
project each point individually, and not just the four corners with
linear interpolation.

Note: I think the new lowzoom process has similar issues:
http://www.informationfreeway.org/?lat=-33.92&lon=22.52&zoom=11&layers=B000F000F

Have a nice day,
-- 
Martijn van Oosterhout <kleptog at gmail.com> http://svana.org/kleptog/