paalkr wrote:
Hi!
This issue is quite important to us, does anybody feel like contracting the
task of fixing this? Please contact me by email if interested,
pal.kristensen at statkart.no
(For reference, the original post is available at
http://n2.nabble.com/Layer-scale-problem-with-Mercator-EPSG-900913-td2575585.html)
You get this kind of effect because MapServer bases its scale
calculation on the units of the output projection, and not on the
location of your map on the sphere... it basically trusts the fact that
if your projection says "meter" then we can trust the units to really be
meters on the ground and base the scale on that. The problem in your
example is that for the area of your map, one unit/meter is not the same
size in UTM33 and in Mercator... at the equator one unit of the Mercator
projection is about one meter on the ground, but as you move up North,
this is no longer true, and one unit covers more like 2, 3 or more
meters on the ground.
So I'd be tempted to say that this is not a MapServer problem, it's a
poor projection choice problem, since your projection is lying to you on
the size of a unit/meter for the area of the world you are looking at.
It seems that to work around this we'd need an option in MapServer to
enable the concept of "true scale", based on the center of the map on
the sphere and not on the size of the projection units at the equator
(or wherever the projection's units are true), basically bypassing the
projection's definition of what a meter is for your part of the world.
Note that this would work around the current issue but would come with
its own side-effects: if you draw tiles using "true scale" in Mercator
or EPSG:4326 in OpenLayers, then the "true scale" of each tile will not
be the same, and the tiles in the North part of the map may not show the
same layers as the ones at the equator... so "true scale" could be
useful for single tile mapping only and not for tiled mapping interfaces.
All that being said, I played a few years ago with a similar idea to
have a more meaningful scale when you use lat,lon coordinates in
degrees, basically basing the scale on the latitude of the center of the
current map view, but I abandoned the idea when I saw the side-effects
(layers turning on and off) as you pan North/South. There is still code
in MapServer's msInchesPerUnit() function (mapproject.c) that is
currently #ifdef'd that would work for UNITS DD if you enable it.
I'd suggest you try enabling this code (recompile with
-DENABLE_VARIABLE_INCHES_PER_DEGREE and then retest with EPSG:4326. If
that fixes the issue for the EPSG:4326 case (I think that will), then we
could talk about making this an official option in MapServer, and making
it work for projected coordinate systems in addition to DD.
Daniel
-
Daniel Morissette
http://www.mapgears.com/
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