Mapnik Users ->
I've been using Mapnik of late for desktop/print map development, and
really enjoying not having to fire up a graphical program as much to
do basic desktop cartography. However I've still thought it would be
useful to georeference some of mapnik's png output to view in
graphical programs like QGIS and uDig against existing data. This
helps to get a quick sense of the resolution of your output to the
input data.
Using an ESRI world file would allow for single mapnik pngs (or jpegs)
to be opened in a variety of gis viewers and since GDAL also reads
projection information gdal2tiles.py could even convert mapnik
graphics to tiled kml overlays in a single step.
I think i've got a simple solution hacked up, but I'm curious if
anyone has suggestions to improve upon it...
If I take the hello world tutorial download (http://trac.mapnik.org/attachment/wiki/XMLGettingStarted/hello_world.zip
) and modify it a bit this is what writing a world file might look
like using the mapnik python bindings:
cheers,
Dane
----------------
#!/usr/bin/env python
from mapnik import *
def render_to_wld(map, path, x_rotation=0.0, y_rotation=0.0):
"""
Outputs an ESRI world file that can be used to load the resulting
image as a georeferenced raster in a variety of gis viewers.
A world file file is a plain ASCII text file consisting of six
values separated
by newlines. The format is:
pixel X size
rotation about the Y axis (usually 0.0)
rotation about the X axis (usually 0.0)
negative pixel Y size
X coordinate of upper left pixel center
Y coordinate of upper left pixel center
Info from: http://gdal.osgeo.org/frmt_various.html#WLD
"""
scale = map.scale()
extent= map.envelope()
upper_left_x_center = extent.minx+(scale/2)
upper_left_y_center = extent.maxy+(scale/2)
wld_string = '%f\n%s\n%s\n-%f\n%f\n%f\n' %
(scale
,y_rotation,x_rotation,scale,upper_left_x_center,upper_left_y_center)
wld_file = open(path, 'w')
wld_file.write(wld_string)
print 'world file output written to %s:' % path
print wld_string
m = Map(10000,5000,'+proj=latlong +datum=WGS84')
m.background = Color('steelblue')
s = Style()
r=Rule()
r.symbols.append(PolygonSymbolizer(Color('#f2eff9')))
r.symbols.append(LineSymbolizer(Color('rgb(50%,50%,50%)'),0.1))
s.rules.append(r)
m.append_style('My Style',s)
lyr = Layer('world')
lyr.datasource = Shapefile(file='../data/world_borders')
lyr.styles.append('My Style')
m.layers.append(lyr)
m.zoom_to_box(lyr.envelope())
filepath = 'map/hello_world_in_pure_python'
raster = '%s.png' % filepath
world_file = '%s.wld' % filepath
render_to_file(m, raster)
render_to_wld(m, world_file)
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