Hello, I'll try to describe my use case:
We have some shapefiles wich we want to, as the final result, show on Google Maps. These shapefiles are made of thousands of polygons, each being of 250 x 250 meters. These polygons have an integer feature associated, lets say from 0 to 200. If the feature is below "50", the polygon must be rendered in red. If betweeen "50 and 150", in yellow, and if above "150" in green.
Our maximum visualization zoom level in Google Maps is 14, which gives a resolution of ~ 7.4 m/px at the location of interest. (each polygon is then ~ 34 px). The shapefile width is 93000 m and 250000 m height. With these data, we believe we need to render the shapefile to a file (PNG) of 12567 x 33784 px in size to keep the accuracy of the original shapefile.
The code to render the png is:
from mapnik import *
m = Map(12567,33784,"+proj=latlong +datum=WGS84")
s = Style()
a_rule=Rule()
a_rule.filter = Filter('[Value] > -87 and [Value] <= -78')
a_rule.symbols.append(PolygonSymbolizer(Color('#FF0000')))
b_rule=Rule()
b_rule.filter = Filter('[Value] > -78 and [Value] <= -72')
b_rule.symbols.append(PolygonSymbolizer(Color('#FFFF00')))
c_rule=Rule()
c_rule.filter = Filter('[Value] > -72')
c_rule.symbols.append(PolygonSymbolizer(Color('#00FF00')))
s.rules.append(a_rule)
s.rules.append(b_rule)
s.rules.append(c_rule)
m.append_style('My Style',s)
lyr = Layer('E')
lyr.datasource = Shapefile(file='C:/shapefiles/E')
lyr.styles.append('My Style')
m.layers.append(lyr)
m.zoom_to_box(lyr.envelope())
render_to_file(m,'C:/shapefiles/E.png', 'png')
With that file size (12567,33784), we get a memory error. Is it possible
to increase the memory heap used by python in order to generate this file?
If we reduce the file size (e.g. 8378, 22523), the file is rendered but due to some antialiasing effect the borders of the polygons are blurred. We attach a picture in which you can see this effect and another showing how we would like to see it, as shown in Mapinfo loading directly the shape file and with a thematic map.
Is it possible to disable antialiasing using mapnik?Once we have the file generated, we want to generate tiles for Google Maps from it either using gdal2tiles or tilecache + mapnik. In the first case, we need the png file to be georreferenced. Is it possible from mapnik to generate a world file for the png according to the extents and bound of the source shapefile?
We haven't explored yet the tilecache + mapnik approach. gdal2tiles provides an option to resample the source file in order to generate missing pixels for the desired zoom level, so we believe we can provide a tinier source file which we can generate with mapnik from the shapefile, and then resample to the desired zoom level without loosing accuracy. Is it possible to make something similar with mapnik + tilecache?
Thanks in advance. -- Félix
<<inline: mapinfo.png>>
<<inline: mapnik.png>>
_______________________________________________ Mapnik-users mailing list [email protected] https://lists.berlios.de/mailman/listinfo/mapnik-users
