How could I generate a CRS field inside the GeoJSON object created by Leaflet layer.toGeoJSON?

Question

I'm using Leaflet to transform some drawn polygons to GeoJSON and send them to a backend. But the GeoJSON objects generated by layer.toGeoJSON are like this:

{
 "type":"Feature",
 "properties":{},
 "geometry":{
    "type":"Polygon",
    "coordinates":[[[18.69873046875,51.481382896100975],[18.69873046875,51.890053935216926],[20.456542968749996,51.890053935216926],[20.456542968749996,51.481382896100975],[18.69873046875,51.481382896100975]]]
  }
 }

When I receive that GeoJSON object in the backend of my application (Django based) and create a GEOS object from it, epsg:4326 is assumed. Even when epsg:3857 is used for the map.

I know GeoJSON spec supports CRS definition, but I don't know how to include it in the GeoJSON object generated by toGeoJSON.

Any suggestion?

Answer 1

You can just set the crs field:

object.crs =  {
  "type": "name",
  "properties": {
    "name": "epsg:3857"
    }
  }

Note however that the coordinates you give in your example really are epsg:4326 coordinates, which is also what leaflet would output by default. I don't think you need a change here.

Answer 2

This is a link to the comment made by @johanvdw saying the CRS for all GeoJSON is epsg:4326 (WGS84) https://www.rfc-editor.org/rfc/rfc7946#section-4

Answer 3

I am not an expert but I would like to share what I did to tackle it. Since GeoJSON is basically a string, I added a string specifying the CRS (3857 in this example) manually. Might be useful to someone.

json = geo_df.to_json()
attr = grid_out.split(',', 1)
json_with_crs = attr[0] + ', "crs": { "type": "name", "properties": { "name": "urn:ogc:def:crs:EPSG::3857" } },' + attr[1]