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I would like to project all the features in a geojson from WGS84 (EPSG:4326) to Web Mercator (EPSG:3857) using python.

This geojson contains only polygons.

I found out the following shapely code combined with pyproj will do it. However after the projection has been applied then I need to manually build the output geojson with the new Web Mercator projection.

import pyproj
import json

from functools import partial

# on windows: conda install -c conda-forge shapely=1.5.17
from shapely.geometry import Polygon, Point, shape, mapping
from shapely.ops import transform

# the shapely projection function
datum_wgs84 = pyproj.Proj(init='EPSG:4326') 
projection_web_mercator = pyproj.Proj(init='EPSG:3857')
projection_wm_func = partial(pyproj.transform, datum_wgs84, projection_web_mercator)

# load GeoJSON file containing sectors
with open('my_input_file.geojson') as f:
  js = json.load(f)

# project GPS lat/lon coordinates to web mercator for each polygon in the geojson
for feature in js['features']:
  polygon = shape(feature['geometry'])
  print('{} bounding box (in WGS84): {}'.format(feature['properties']['my_polygon_name'], polygon.bounds))
  print('-> A sample pair in the original WGS84 projection: {}'.format(mapping(polygon)['coordinates'][0][0]))
  projected_polygon = transform(projection_wm_func, polygon)
  print('-> A sample pair in the projected "Web Mercator": {}'.format(mapping(projected_polygon)['coordinates'][0][0]))

# TODO put all the projected polygons together in a new output geojson

Is there a more compact way of doing this manual part to avoid most of the boiler plate code? Ideally some sort of function with inputs:

  • input geojson filename
  • output geojson filename
  • existing projection
  • final projection

Or at least a combination of functions to avoid the for loop.

  • What's wrong with 'for'? – nickves Jul 19 '17 at 14:55
2

This can be done without shapely. See below, adapted from this example.

from pyproj import Proj
import json
from pprint import pprint

in_file = '/path/to/fc.geojson'

p_web = Proj(init='EPSG:3857')

with open(in_file) as src:
    fc_in = json.load(src)

# Define dictionary representation of output feature collection
fc_out = {'features': [],
          'type': 'FeatureCollection'}

# Iterate through each feature of the feature collection
for feature in fc_in['features']:
    feature_out = feature.copy()
    new_coords = []
    # Project/transform coordinate pairs of each ring
    # (iteration required in case geometry type is MultiPolygon, or there are holes)
    for ring in feature['geometry']['coordinates']:
        x2, y2 = p_web(*zip(*ring))
        new_coords.append(zip(x2, y2))
    # Append transformed coordinates to output feature
    feature_out['geometry']['coordinates'] = new_coords
    # Append feature to output featureCollection
    fc_out['features'].append(feature_out)

pprint(fc_out)
0

You could try using ogr2ogr utility:

ogr2ogr -f "GeoJSON" output.geojson input.geojson -s_srs EPSG:4326 -t_srs EPSG:3857 
0

This will not be the final answer, but i will try to help you:

1) If you want a compact way just put the code in a python function. foor loops always are needed.

2) In order to put all the projected polygons together in a new output geojson, only must to build a dictionary and write wherever you want. in the next code i use ogr library:

    print ("Begin")

features =[]

# loop through the features in the layer
feature = layer.GetNextFeature()
counter=0
while feature:
  counter +=1
  if counter > 10 : break

  features.append(feature.ExportToJson())

  # destroy the feature and get a new one
  feature.Destroy()
  feature = layer.GetNextFeature()

geo_json_dict = {"type": "FeatureCollection",
                      "features": features
                      }

# open the output text file for writing
file = open('features.json', 'w')
file.write(json.dumps(geo_json_dict))

# close the data source and text file
datasource.Destroy()
file.close()
print ("End")

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