[Feature Request] automatically reproject to WGS84 if saving a file as GeoJSON

geopandas: '0.5.0'

I had this projection issue as well before: https://stackoverflow.com/questions/56806907/gis-shapefile-converted-to-geojson-has-unexpected-coordinate-format

As GeoJSON only supports WGS84. does it make more sense to automatically reproject the GeoDataFrame being saved as a GeoJSON file? and also of course, give users a warning

https://macwright.org/2015/03/23/geojson-second-bite.html

[austinorr]

I disagree, I think most people want to export their geodataframe to geojson without having to know trivia like epsg:4326 is the only valid geojson projection. Users don’t get to use their own crs for geojson (according to the spec), so if their frame is not projected, we should assign 4326, and if it is projected we should be converting their data to the correct projection so that the output is valid with a warning that the tool ‘had to reproject from <orig proj> to epsg:4326’.

[sgillies]

@snowman2 I think it’s worth pointing out that OGC urn:ogc:def:crs:OGC::CRS84 is the official system for GeoJSON, not EPSG 4326. Using the latter as shorthand for the former is going to be problematic in the future where systems based on GDAL 2 and GDAL 3 need to coexist, yes?

The sentence in the GeoJSON spec that you referenced means to acknowledge that projected data which happens to be structured like GeoJSON is often useful in some contexts. Rasterio’s rio-shapes command lets a user output GeoJSON-like data for features extracted from a raster dataset in the projection of the raster, for the purpose of piping the data into rio-rasterize (for example) and not suffering any degradation of coordinates. It’s not even necessary to put the CRS in the data for these applications as the CRS is communicated out of band. That’s what “prior arrangement” means.

The sentence does not say that it’s correct to write projected GeoJSON-like data, add a “crs” object to it, and call it “GeoJSON” with the expectation that all GeoJSON readers can understand it.

Summary: GeoPandas ought to let users export projected GeoJSON-like data for their own needs. A warning when doing so might be appropriate. Putting a CRS object on the GeoJSON like GDAL has done is no longer correct and reduces interoperability.