Specify crs when exporting to GeoJSON

[pmav99]

If the information is available, v.out.ogr should probably specify the "crs" attribute when exporting to GeoJSON. ogr2ogr seems to support this feature.

$ pacman -Qs gdal
local/gdal 2.0.1-6
    A translator library for raster geospatial data formats

A GeoJSON file with no crs:

$ cat sample.json
{
    "type": "Polygon",
    "coordinates": [
        [
            [ -84.32281494140625, 34.9895035675793 ],
            [ -84.29122924804688, 35.21981940793435 ],
            [ -84.24041748046875, 35.25459097465022 ],
            [ -84.32281494140625, 34.9895035675793 ]
        ]
    ]
}

Reprojecting:

$ ogr2ogr -f GeoJSON -t_srs "EPSG:3857" 3857.json sample.json

$ ogr2ogr -f GeoJSON -t_srs "EPSG:4326" 4326.json sample.json

The new files:

$ cat 3857.json
{
"type": "FeatureCollection",
"crs": { "type": "name", "properties": { "name": "urn:ogc:def:crs:EPSG::3857" } },
 
"features": [
{ "type": "Feature", "properties": { }, "geometry": { "type": "Polygon", "coordinates": [ [ [ -9386772.821532785892487, 4162454.812297557480633 ], [ -9383256.718231666833162, 4193793.993894479703158 ], [ -9377600.378138564527035, 4198533.089648159220815 ], [ -9386772.821532785892487, 4162454.812297557480633 ] ] ] } }
]
}

$ cat 4326.json
{
"type": "FeatureCollection",
"crs": { "type": "name", "properties": { "name": "urn:ogc:def:crs:OGC:1.3:CRS84" } },
                                                                                
"features": [
{ "type": "Feature", "properties": { }, "geometry": { "type": "Polygon", "coordinates": [ [ [ -84.32281494140625, 34.989503567579298 ], [ -84.291229248046875, 35.219819407934352 ], [ -84.24041748046875, 35.25459097465022 ], [ -84.32281494140625, 34.989503567579298 ] ] ] } }
]
}

By the way, when crs is not defined ogrinfo seems to default to EPSG:4326 which I guess is the most reasonable choice, although I think I would prefer something more explicit (e.g. a warning):

$ ogrinfo -so -al sample.json 
 
ERROR 4: GeoJSON Driver doesn't support update.
Had to open data source read-only.
INFO: Open of `sample.json'
      using driver `GeoJSON' successful.
Layer name: OGRGeoJSON
Geometry: Polygon
Feature Count: 1
Extent: (-84.322815, 34.989504) - (-84.240417, 35.254591)
Layer SRS WKT:
GEOGCS["WGS 84",
    DATUM["WGS_1984",
        SPHEROID["WGS 84",6378137,298.257223563,
            AUTHORITY["EPSG","7030"]],
        AUTHORITY["EPSG","6326"]],
    PRIMEM["Greenwich",0,
        AUTHORITY["EPSG","8901"]],
    UNIT["degree",0.0174532925199433,
        AUTHORITY["EPSG","9122"]],
    AUTHORITY["EPSG","4326"]]

[pmav99]

Hmm... it seems ogr2ogr's implicit choices can lead to bugs :

If we erase the CRS field from 3857.json:

$ cat 3857.json
{
"type": "FeatureCollection",
"features": [
{ "type": "Feature", "properties": { }, "geometry": { "type": "Polygon", "coordinates": [ [ [ -9386772.821532785892487, 4162454.812297557480633 ], [ -9383256.718231666833162, 4193793.993894479703158 ], [ -9377600.378138564527035, 4198533.089648159220815 ], [ -9386772.821532785892487, 4162454.812297557480633 ] ] ] } }
]
}

Then ogrinfo assumes that EPSG is 4326:

$ ogrinfo -so -al 3857.json
ERROR 4: GeoJSON Driver doesn't support update.
Had to open data source read-only.
INFO: Open of `3857.json'
      using driver `GeoJSON' successful.

Layer name: OGRGeoJSON
Geometry: Polygon
Feature Count: 1
Extent: (-9386772.821533, 4162454.812298) - (-9377600.378139, 4198533.089648)
Layer SRS WKT:
GEOGCS["WGS 84",
    DATUM["WGS_1984",
        SPHEROID["WGS 84",6378137,298.257223563,
            AUTHORITY["EPSG","7030"]],
        AUTHORITY["EPSG","6326"]],
    PRIMEM["Greenwich",0,
        AUTHORITY["EPSG","8901"]],
    UNIT["degree",0.0174532925199433,
        AUTHORITY["EPSG","9122"]],
    AUTHORITY["EPSG","4326"]]

reprojecting does not make any changes to the coords (which is reasonable I guess):

$ ogr2ogr -f GeoJSON -t_srs "EPSG:4326" 4326_implicit.json 3857.json

$ cat 4326_implicit.json
{
"type": "FeatureCollection",
"crs": { "type": "name", "properties": { "name": "urn:ogc:def:crs:OGC:1.3:CRS84" } },
"features": [
{ "type": "Feature", "properties": { }, "geometry": { "type": "Polygon", "coordinates": [ [ [ -9386772.821532784029841, 4162454.812297557014972 ], [ -9383256.718231666833162, 4193793.993894479703158 ], [ -9377600.378138564527035, 4198533.089648158289492 ], [ -9386772.821532784029841, 4162454.812297557014972 ] ] ] } }
]
}

but this particular (coords, crs) combination is obviously garbage:

$ ogr2ogr -f GeoJSON -t_srs "EPSG:3857" 3857_implicit.json 4326_implicit.json

Failed to reproject feature 0 (geometry probably out of source or destination SRS).
ERROR 1: Terminating translation prematurely after failed
translation of layer OGRGeoJSON (use -skipfailures to skip errors)

[rouault]

Quoting the geojson spec ( ​http://geojson.org/geojson-spec.html#coordinate-reference-system-objects ) : "The default CRS is a geographic coordinate reference system, using the WGS84 datum, and with longitude and latitude units of decimal degrees." . So OGR behaviour follows the spec.

And the new geojson spec ( ​https://www.ietf.org/id/draft-ietf-geojson-00.txt ) deprecates the use of other CRS : "Other coordinate reference systems, including ones described by CRS objects of the kind defined in [GJ2008] are NOT RECOMMENDED"

[Nikos Alexandris]

If I understand correctly, this ticket can be closed?

[neteler]

It seems that the specs are respected, so not much we can do here. Closing.