[mapserver-users] mismatches tiling images produced by mapserver

[Brian Hill]

I am still wringing the bugs out of a new 3.3.012 installation, and am 
hoping someone on the list will have experience with this one.

Our application needs to tile images produced by the mapserver.  I.e., 
if an image is produced with extent,

(lon - delta_lon / 2, lat), (lon + delta_lon / 2, lat),

then it should fit very neatly to the left of an image with extent,

(lon + delta_lon / 2, lat), (lon + 3 * delta_lon / 2, lat).

Here is an example of four tiles which should fit together:

http://skip.nextbus.com/tile-skip.html

The .html file hits a perl wrapper for the map server four times.  As 
you can see by scrolling to the center of the image, the four panes do 
not fit together neatly.  The projection is Mercator, so tiles bounded 
by lines of constant latitude and constant longitude should fit.

If there are rounding errors or some other problem down in the C 
libraries that is partly or completely understood and could explain 
this, please let me know.

Wading directly into the sources is a bit daunting, especially given 
that any of three layers could be involved (perl/mapscript, 
mapserver-3.3.012 or proj-4.4.4), but it is critical for our application 
that the tiling come out neatly, so with some pointers, I am motivated 
and might be able to track it down.  It is possible the bug is in the 
perl file that makes all the mapscript calls, but I think it is lower 
down, and therefore that fixing it might be of general interest.

Brian Hill
Sr. Software Engineer
NextBus Information Systems