[Geojson] [OSGeo-Standards] EPSG + Coordinate Ordering (Was Re:GeoJSON '1.0'?)
creed at opengeospatial.org
Fri Mar 14 08:21:22 PDT 2008
I understand your concerns. However, the axis order issues transcends the
requirements for rapid development or any one application. Standardized
approaches to using geospatial content (including payloads) and services is
of paramount importance in many industries and enterprises. For example, the
location services industry as represented by the Open Mobile Alliance, the
e-business industry as represented by OASIS and the internet industry as
represented by the IETF all rely honesty and consistency of how a CRS is
expressed even if the default is WFS 84-2d. The key words are honesty and
consistency. In other words, if one chooses to ignore the axis order as
expressed in the CRS definition and express the geometry coordinate order
differently (the crux of the lat-lon/lon-lat issue), then the
application/payload must say so. The problem we have had is that
applications state the 4326 CRS and then code the coordinates as lon-lat but
say that this is the case. The focus of the axis order manifesto is
providing guidance so that this does not happen and that honesty and
consistency are achieved.
As to the drawing and the short pieces on datums, that was written by Roger
Lott, chairman of the OGP and formerly Head of Survey BP Exploration. I
should have provided the reference! I will update ogcnetwork today.
Finally, as Metacarta is an OGC member, feel free to participate in the
process and provide what I know to be valuable implementation experience.
----- Original Message -----
From: "Christopher Schmidt" <crschmidt at metacarta.com>
To: "Carl Reed" <creed at opengeospatial.org>
Cc: "Frank Warmerdam" <warmerdam at pobox.com>; <geojson at lists.geojson.org>;
<standards at lists.osgeo.org>; "Paul Ramsey" <pramsey at cleverelephant.ca>
Sent: Friday, March 14, 2008 5:37 AM
Subject: Re: [Geojson] [OSGeo-Standards] EPSG + Coordinate Ordering (Was
> On Thu, Mar 13, 2008 at 05:50:27PM -0600, Carl Reed wrote:
>> Not to be a bother, but Please wait until I send you the axis order
>> guidance document the OGC has been working hard on. You may find the
>> document useful and we would encourage your feedback.
> After waiting more than 4 months for the current discussion paper on WMS
> Tiling to be made available on the OGC website, I'm not particularly
> convinced that waiting on the OGC to provide guidance is in the
> best-interest of a rapid-development specification design process.
>> Axis order is a non-trivial element of any geo application and the
>> implications of having different approaches to dealing with CRS and axis
>> order in different communities will be deadly in terms of consistency,
>> sharing, and interoperability in general.
> That's fine if you expect to be able to use GeoJSON without translation to
> some data format that is designed to encode that information. GeoJSON is
> not designed for that. The primary use case of GeoJSON is for the
> *simple* cases -- and no matter what the simple case looks like, it will
> be trivial (so far as I can tell) for a user to map that into data that
> makes sense in their client if they have a knowledge of CRS, because the
> definition of coordinate reference systems includes the information
> needed to reverse it.
>> And just to make things interesting, remember that a CRS does not have to
>> be related to the earth :-)
> How is that relevant? We don't describe things as relating to the earth?
> Only to X,Y,Z in a projected coordinate system. How they map to the
> earth is the job of the CRS, not of GeoJSON.
>> Also, read http://www.ogcnetwork.net/node/338 which provides excellent
>> grounding as to what happens to the same "point" on the surface of the
>> earth when different datums are used. One more reason using full CRS
>> metadata such as defined in the EPSG database is so important.
> Absolutely. Though I'm not convinced that that drawing is realistic: I'd
> be interested in seeing the source data (vector + imagery) that that
> applies to. But we're not talking about ignoring the metadata defined in
> the EPSG database: we're talking about how to *map it to GeoJSON* --
> specifically, how you take a CRS, read GeoJSON, and get out the data in
> the coordinate ordering you care about.
>> Oh yeah, I forgot - (x,y,z) in South Africa is typically defined using
>> reverse Lo-Gauss, which means that they use an upside down coordinate
>> system (at least from us Northerners perspective). In other words, y "is
>> directed south and not north.
> Sure. And Y can also be Westing, as in
> but that doesn't mean that there *isn't* a y. I don't care if z means
> "into the earth": Just that it is labeled Z. Application understanding
> of a particular esoteric CRS is required in order to do fancy things
> with it, but if you only care about working in projected space, then
> none of this matters.
> Christopher Schmidt
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