Hi Damian,
Okay Damian, good to know. So it sounds like there is a problem in
mapnik whereby reprojection (or some relatively expensive check) still
happens even if you specify the srs with exactly the same. Could you
please file a ticket on this, ideally with your test case and a small
shapefile?
It is great that optimizations like Robert's are finding their way
into Mapnik and it will be good to track this one down.
Regarding the output if you strip all srs parameters... yes, mapnik
should render the tiles looking fine without an srs, because it
assumes both the map and the layers are == '+proj=latlong
+datum=WGS84', and the data is rendered cleanly without reprojection.
But what will be screwy it is you need to query those layers using
coordinates in mercator, or (like you have found) the calculation of
the scale, among other unanticipated artifacts of not rendering
projection-aware. The scale rules are likely screwy because mapnik has
not converted DD to meters internally, thinking it is still in wgs84.
Dane
On Dec 2, 2008, at 9:07 AM, Damiano Morosi wrote:
Hi,
another hint: if I remove all the srs from the mapfile, the tiles
are generated CORRECTLY while Dane said that the result should be
something screwy... The only thing that doesn't work here are the
scale based rules. I think that Mapnik ignore what the measure unit
is, so it cannot calculate the scale correctly. Maybe introducing a
unit="..." param for the map should be a good thing?
2008/12/1 Robert Coup <[EMAIL PROTECTED]>
I would assume Proj4 does a no-op if the source and destination SRSs
were the same, but maybe it doesn't. If it's reprojecting, it does
some reasonably complex calcuations on every vertex of every
geometry, which would explain the added time.
In https://trac.mapnik.org/ticket/138 I added a no-op around not
calculating extents for same-SRS map/layers, but maybe
proj_transform should do a similar no-op rather than pass it through
to Proj4?
Rob :)
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