Hi,
I am a fresh Mapnik user and bombed Artem Pavlenko directly with my
questions before I found this mailing list.
I am relatively experienced in using stuff like gdal, ogr, Mapserver,
Geoserver and Oracle Spatial, and I am beginning to do with PostGIS.
What I don't do is Linux and almost all programming.
So far I have managed to install Mapnik on Windows and make the
rundemo.py work. I can also generate a map in the script default
projection from OSM data if I have imported it into PostGIS in spherical
mercator projection with osm2pgsql.exe by using the -m switch. What I
want to do next is to generate maps in other projections and so that I
could give the extents of the map in the output projection.
Now I have a bunch of questions:
1. Have I understood right, that in the OSM.XML file, in the layer
object, the actual projection where the data are in database or
shapefile, is given? For example, this layer is originally in spherical
mercator, or Google mercator, or epsg:900913:
<Layer name="power" status="on" srs="+proj=merc +a=6378137 +b=6378137
+lat_ts=0.0 +lon_0=0.0 +x_0=0.0 +y_0=0 +k=1.0 +units=m [EMAIL PROTECTED]
+no_defs +over">
2. In the same layer object I can either let PostGIS to estimate the
geographical extents of the layer (="estimate_extent">true), or I can
give the extents myself by using the same units that the original data
source is using
<Parameter name="estimate_extent">false</Parameter>
<Parameter
name="extent">-20037508,-19929239,20037508,19929239</Parameter>
Is this right?
3. The output projection of the resulting map is defined in the
beginning of the OSM.XML file
<Map bgcolor="#b5d0d0" srs="+proj=merc +a=6378137 +b=6378137 +lat_ts=0.0
+lon_0=0.0 +x_0=0.0 +y_0=0 +k=1.0 +units=m [EMAIL PROTECTED] +no_defs
+over">
In this case the output is in spherical marcator, but I should be able
to change it to Finnish KKJ, Zone 3 (epsg:2393) by defining:
<Map bgcolor="#b5d0d0" srs="+proj=tmerc +lat_0=0 +lon_0=27 +k=1.000000
+x_0=3500000 +y_0=0 +ellps=intl
+towgs84=-90.7,-106.1,-119.2,4.09,0.218,-1.05,1.37 +units=m +no_defs">
Correct?
4. What generate_image.py is actually doing, is
- Tell Mapnik program to take further instructions from environment
variable MAPNIK_MAP_FILE which can be set in Windows with command SET,
and if that does not exist file named osm.xml.
- Pass the size of the resulting image to Mapnik program with imgx and
imgy variables.
- Tell Mapnik to zoom to bounding box that is defined, in the output
projection, with vabiable "bbox"
- And finally render the zoomed box out to image file.
- The projection definition in generate_image.py, as well as prj.forward
stuff in needed because in this script the user is asked to give the
extents of the resulting map in lon-lat bounding box, and this has to be
converted first to "bbox" that is using the units as in the output
projection.
Am I still tuned?
5. Finally, if I have defined the srs of the output map to Finnish KKJ
in the osm.xml file as before in number 3, I thought I possibly could
give the extents of the resulting map directly by writing the coordinate
array in KKJ projection inside the bbox = Envelope(). Naturally
commenting out some previous lines where the envelope gets calculated
from lon-lat coordinates.
Is my theory correct?
6. I tried blindly to feed in KKJ coordinates both into Envelope() and
also directly to m.zoom_to_box(bbox) but with no success. All I get
back are error messages telling me that I am feeding in numbers whith
are of format integer, or real or something else, depending on how I
wrote the coordinates, but not as DOUBLE which is the type that some C++
code is only willing to eat.
Is there some simple way to modify generate_image.py in such a way that
I could get the coordinates I give to passed on as DOUBLE for the Mapnik
for Windows?
With best regards,
-Jukka Rahkonen-
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