[Geojson] [OSGeo-Standards] EPSG + Coordinate Ordering (Was Re: GeoJSON '1.0'?)

Carl Reed creed at opengeospatial.org
Thu Mar 13 07:20:56 PDT 2008


The OGC Architecture Board has been crafting an Axis Order "Manifesto". One 
of the topics the manifesto provides guidance on is the axis order case that 
you are describing. I would strongly encourage this group to wait two weeks 
until after the OGC TC meetings. The manifesto will be discussed and 
hopefully approved at those meetings. There will then be guidance and rules 
concerning axis order that the entire community can use that will allow us 
to achieve a higher level of consistency and interoperability.

We would also encourage comments and thoughts on the manifesto prior to the 
OGC meetings. I need to make a few edits to the document and will then 
provide the list a copy.

Thanks and regards

Carl Reed

----- Original Message ----- 
From: "Christopher Schmidt" <crschmidt at metacarta.com>
To: <geojson at lists.geojson.org>
Cc: <standards at lists.osgeo.org>
Sent: Thursday, March 13, 2008 7:54 AM
Subject: [OSGeo-Standards] EPSG + Coordinate Ordering (Was Re: 
[Geojson]GeoJSON '1.0'?)

> Members of the OSGeo standards list:
> In discussion of the creation of the GeoJSON specification, we're
> running into a question of whether you can refer to something as
> EPSG:4326 and 'override' the coordinate order, insisting that it is
> x,y[,z] regardless of what EPSG says. Apparently this came up before in
> WMS 1.3.0 discussions: if anyone has more context, please respond to
> this message on the osgeo-standards list  (I will summarize the contents
> back to the GeoJSON list).
> More context is available at
> http://lists.geojson.org/pipermail/geojson-geojson.org/2008-March/000354.html
> On Thu, Mar 13, 2008 at 09:36:55AM -0400, Daniel Morissette wrote:
>> Christopher Schmidt wrote:
>> > On Thu, Mar 13, 2008 at 07:43:59AM -0400, Panagiotis (Peter) A. 
>> > Vretanos wrote:
>> >> So if you use EPSG:4326 and are encoding the coordinates in x,y (or
>> >> long,lat) ... well, Raj said it best "You can't do that."!
>> >
>> > *Why?* The answer is obviously not the same for GeoJSON as it is for
>> > WMS: In WMS, there was no explicit statement that said "Ignore the CRS:
>> > It's always in x, y." If the reason that there is a problem is because
>> > the spec is poorly written, then that problem is *solved* with GeoJSON.
>> >
>> > Assuming that you override the ordering of the CRS in your spec: why
>> > can't you use EPSG:4326?
>> >
>> I don't know if you're allowed by the EPSG license to refer to EPSG and
>> explicitly override the coordinate order... others such as Frank
>> Warmerdam seem to know that license fairly well and could comment on
>> that (see for instance
>> http://www.mail-archive.com/geotools-devel@lists.sourceforge.net/msg03279.html)
> I know that most people participating in this particular spec writing
> process are of the opinion that using the letters "EPSG" does not
> somehow make us beholden to EPSG in any way. (Obviously, distributing
> their data would change this.)
>> Anyway, back to the point, I
>> believe (but could be wrong since I was not there at the time) that an
>> attempt had been made by the WMS 1.3.0 RWG before I joined to simply
>> state that all CRS use x,y ordering as you suggest here, but later on,
>> as part of the ISO review process, it was pointed out that OGC could not
>> do that. What I remember of it is essentially that the RWG was told by
>> ISO that "if you refer to the EPSG database then you cannot pick and use
>> only portions of it, you must adhere to all of it, including coordinate
>> order. Period."
> It seems like a weird requirement, and I hope that someone more
> intimately familiar with that part of the process can comment on why
> that might be. Certainly, to a layman, there's no material on the web
> that indicates something like this, and as I pointed out above, EPSG is
> just a set of letters, so trying to claim that it forces something
> special on users of those letters seems... a bit odd, at best.
> Hm, maybe someone from the OSGeo standards list can point me where to go
> with this issue. I've added that list as a CC.
>> I don't know if coordinate order will be an issue if you stay away from
>> ISO. Personally I still consider that this coordinate order issue ruined
>> the simplicity of WMS... if Geojson can avoid it that would be for the 
>> best.
> There is no doubt in my mind that GeoJSON will be x,y[,z], always. The
> only question is "How do we get there in a way that isn't shooting
> ourselves in the foot?" Since it seems like part of the problem in this
> case was ISO, maybe the answer is just "avoid ISO": since we're working
> with a spec that's designed for content on the web, that doesn't seem
> like a terribly painful decision to make to me, but maybe others have
> problems with it. (IETF is a more natural home for GeoJSON than ISO, in
> other words.)
> Regards,
> -- 
> Christopher Schmidt
> MetaCarta
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