How to draw a marker with Mapnik?

Question

After setting up a tile server accordingly this instruction https://switch2osm.org/serving-tiles/manually-building-a-tile-server-20-04-lts/ my offline tile server working nicely.

And after helping of ThomasG77 in this question How to create a map to PNG file with specific box coordinates by Mapnik? now I can generate PNG file with specific region (box) by this code on Python:

import mapnik

mapnik_xml = "openstreetmap-carto/mapnik.xml"
map_output = "region_map_mapnikXml.png"

# Create a map object
m = mapnik.Map(600,300)
mapnik.load_map(m, mapnik_xml)
bbox = mapnik.Box2d(5034980.57, 5062621.68, 5146300.06, 5347045.97)
m.zoom_to_box(bbox)
mapnik.render_to_file(m, map_output)
print(f"Rendered image to {map_output}")

But now I am facing the following problem: I need to generate PNG file of specific region with marker (something like small image e.g. green pin) in center of this region. How I can do that?

Answer 1

You can find below a sample to do it.

import json
import mapnik

mapnik_xml = "openstreetmap-carto/mapnik.xml"
map_output = "region_map_mapnikXml.png"
epsg_3857 = "+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 +nadgrids=@null +no_defs"
bbox_array = [5034980.57, 5062621.68, 5146300.06, 5347045.97]

# Create a map object
m = mapnik.Map(600,300)
# Uncomment if you want a standalone sample
# m.srs = epsg_3857
# Comment if you want a standalone sample without loading
# the OpenStreetMap style
mapnik.load_map(m, mapnik_xml)

# Create point style
s = mapnik.Style() # style object to hold rules
r = mapnik.Rule() # rule object to hold symbolizers

# Create symbolizer
point_sym = mapnik.MarkersSymbolizer()
point_sym.fill = mapnik.Color('green')
point_sym.allow_overlap = True
point_sym.width = mapnik.Expression("20")
point_sym.height = mapnik.Expression("20")

r.symbols.append(point_sym) # add the symbolizer to the rule object
s.rules.append(r) 
m.append_style('center', s)

# Create datasource
ds = mapnik.MemoryDatasource()
f = mapnik.Feature(mapnik.Context(), 1)

f.geometry = f.geometry.from_wkt("POINT({} {})".format((bbox_array[0] + bbox_array[2]) / 2, (bbox_array[1] + bbox_array[3]) / 2))
ds.add_feature(f)

center_layer = mapnik.Layer('center_layer')
center_layer.srs = epsg_3857
center_layer.datasource = ds
center_layer.styles.append('center')
m.layers.append(center_layer)

bbox = mapnik.Box2d(*bbox_array)
m.zoom_to_box(bbox)
mapnik.render_to_file(m, map_output)
print(f"Rendered image to {map_output}")