[Geojson] coordinate order

[Sean Gillies]

Tim Schaub wrote:
> Hey-
> 
> See the updated GeoJSON spec v5
> http://wiki.geojson.org/GeoJSON_draft_version_5
> 
> We had a discussion on IRC today about geometry coordinates.  We all 
> agree that we want to support more than two dimensions.  We also want 
> simple clients to be able to know which dimensions at least the first 
> two elements in a coordinates array refer to.
> 
> Regarding more than two dimensions, the spec talks about two dimensions 
> for point geometry and goes on to say "any number of additional 
> dimensions are allowed, and interpretation and meaning of these 
> coordinates is beyond the scope of this specification."
> 
> Does this sit well?  We say you've got to have two elements in the 
> coordinates array but more are allowed.
> 
> Regarding the order of elements in the coordinates array for GeoJSON 
> geometries, we say that the first two elements are in x, y order (lon, 
> lat for dd).  This is what we have said from the start.
> 
> The change to the spec is to accommodate CRS that define a different 
> coordinate order.  Instead of requiring all clients to know about all 
> CRS coordinate order conventions, we require that GeoJSON geometries 
> referencing a CRS that defines non-xy coordinate order include a 
> "coordinate_order" member in the CRS object.
> 
> This means a point geometry that references EPSG:4326 would look like this:
> 
> {
>     "type": "Point",
>     "coordinates": [-180.0, 90.0],
>     "crs": {
>         "type": "EPSG",
>         "properties": {"code": 4326},
>         "coordinate_order": [1, 0]
>     }
> }
> 
> The benefits of this are that it allows all clients simple access to the 
> proper dimensions in the geometry coordinates array and allows smarter 
> clients who are strict about CRS coordinate order to map to the correct 
> dimension in our coordinates arrays.
> 
> The drawbacks of this are that people like to argue that the EPSG has 
> the right to assign meaning to any sequence of numbers in a data 
> structure that references EPSG.  I say that is fine, they get access to 
> our coordinate_order array.  If the EPSG says that the first value in a 
> sequence refers to the northing of a point, then they look at the first 
> value in our coordinate_order array and know which element to use from 
> our coordinates array.
> 
> Makes things a bit more complex for clients who know about CRS and 
> simpler for the rest.

Tim, if we discourage/prohibit people from defining coordinate ordering
that conflicts with the specified CRS, it would be no harder for a
client that knows all CRS: they can just ignore the coordinate ordering
hint.

> 
> I think this thing is ready to let out into the wild.
> Tim

+1. I like the idea of writing an IETF RFC after we see how the CRS
story plays out. Would anyone want to help?

Sean