How to identify the co-ordinate system (CRS, SRS) of a GeoJSON file?

Question

Suppose you have an arbitrary GeoJSON file, and the publisher didn't give much supporting documentation - or perhaps it's been provided by a third party who don't know much about it.

Is there a standard way to identify what CRS or SRS the file is in? For example, is there a readily available tool (like a web tool) that can read such a file and identify this?

I'm writing some documentation for a process that is to be used for preparing spatial boundary data files for a process, for converting files from arbitrary publishers into accepted formats, and I'd like a step that users (who usually won't have much specific GIS experience) can follow to see if they need to convert their file's CRS/SRS and if so, what from.

I've seen Identifying Coordinate System of Shapefile when Unknown? but that seems to be exclusive to Shape files.

Answer 1

Assuming your data provider sticks to the format specification, the CRS is in geographic coordinates with WGS84 datum, unless specified otherwise. See also section 3 of the GeoJSON specification.

If your data comes with an unspecified CRS, I know of no other method than having to guess. If the people working with this data know where they should be located, a first educated guess would be a CRS used by the government or military of that area, an information that can be obtained easily for pretty much every region.

Also, you should definitely have a word with your data provider then.

Answer 2

Officially, "the coordinate reference system for all GeoJSON coordinates is a geographic coordinate reference system, using the World Geodetic System 1984 (WGS 84) [WGS84] datum, with longitude and latitude units of decimal degrees." That is also known as EPSG:4326.

Originally in the 2008 specification, an alternative CRS (Coordinate Reference System) could be given.

"crs": {
    "type": "name",
    "properties": {
        "name": "urn:ogc:def:crs:OGC:1.3:CRS84"
    }
}

That support was removed in the 2016 specification. Though that document goes on to basically say that continued use of assigning a CRS does not conflict with the specification, so some software may continue to support the behavior. Again, since it is not part of RFC 7946, it should not be expected to be available in software that complies with RFC 7946.

Main source: https://geojson.org/

Answer 3

This site (https://mygeodata.cloud/converter/geojson-to-latlong) provides a converter for coordinate systems. You can convert three per day for free and then you have to subscribe to convert more.

If you downloaded an archive file from a provider, you should upload that as it may have a metadata file with the coordinate system listed.

If you don't have any indication of what the coordinate system is click on the left link for 'coordinate system' and search for the country that your map covers. This usually works for me. Then on the right 'coordinate system' link select: World, which will give you the normal 180 to -180 output. Click 'Show in a Map' to preview the new coordinate layout. If it's good then click 'Convert' to download a new package.