[GeoJSON] Removing CRS from GeoJSON

[Eric Lemoine]

On Wed, May 15, 2013 at 6:41 AM, Tim Schaub <tschaub at opengeo.org> wrote:
> So as to avoid hijacking Sean's thread, I'll start a new one here.
>
> I'm in favor of restricting the allowed coordinate reference systems for
> GeoJSON objects to 1: latitude, longitude coordinates relative to an
> ellispoidal CRS based on the WGS84 datum.
>
> I think the second best alternative would be to restrict to 2 CRS: CRS84 or
> EPGS:3857.
>
> I don't like the "allow any CRS and let axis order follow the CRS" because I
> think it either reduces interoperability or imposes an unreasonable burden
> on web clients (I don't know of a good web service - or really want to
> depend on one - that provides axis order information for arbitrary CRS URN,
> and the table is too big to ask every client to carry around).
>
> I apologize for having missed earlier "discussion" [1]. I haven't dug down
> to that epoch in my inbox yet.
>
> And I'm in favor of the proposed RFC to IETF [2].
>
> https://github.com/GeoJSONWG/draft-geojson/pull/2
>
> [1]
> http://lists.geojson.org/pipermail/geojson-geojson.org/2013-April/000712.html
> [2]
> http://lists.geojson.org/pipermail/geojson-geojson.org/2013-April/000713.html


So, as discussed with Tim at the FOSS4G-NA conference, we, at
Camptocamp, use GeoJSON to transport projected data. Very common use
case of ours: EPSG:21781 (swiss projection) data stored in PostGIS,
exchanged between client and server through custom GeoJSON-based web
services. GeoJSON is currently the perfect format for us.

If, in the future, the GeoJSON spec restricts the allowed coordinate
reference systems to either WGS84 only, or to WGS84 and EPSG:3857,
then it will mean that, by using GeoJSON for EPSG:21781 data, we
violate the GeoJSON standard. This is a problem, as, for example,
GeoJSON parsers may assume that the data is WGS84, and transform the
parsed data to some other projection (the display projection).

My first reaction to this was: "Couldn't GeoJSON just transport
coordinate numbers and assume absolutely nothing about the data's
coordinate reference system?". But Tim said to me that some people
have catalogs of GeoJSON documents, and they need to somehow tell
their users what coordinate systems they use. I think that makes
sense.

As discussed with Tim, WSG84 could be assumed when no "crs" value is
provided only, and, if the "crs" property is present then parsers
should assume nothing about the coordinate system. What do you think?

Very sorry for waking up a bit late on this. And thanks Tim for the
discussion at FOSS4G-NA.

Cheers,

--
Eric Lemoine

Camptocamp France SAS
Savoie Technolac, BP 352
73377 Le Bourget du Lac, Cedex

Tel : 00 33 4 79 44 44 94
Mail : eric.lemoine at camptocamp.com
http://www.camptocamp.com