[GeoJSON] AsJSON(geometry)

[John Herring]

David, Chris, 

	In ISO 19107: Spatial schema, which is also the geometry abstract
specification for OGC, it states that CRS specifications will push down any
composition hierarchy. Therefore if a feature (or for that matter a feature
collection or data set) has a SRS value (SRS is an older version of CRS),
then nothing under it needs it unless it wants to override the value for the
container (such an override is not considered a good idea, by the way).

	Hence, the "srs:" on the feature means that the geometries have the
same SRS unless they override it. Similarly, you do not carry an SRS for
each array in a list of coordinate lists or even point in a coordinate list
(even if that list or point represents yet another geometry objects). 

Regards,
John

You do what you can when you can because you can.

The opinions expressed in this email are 
purely my own and do not necessarily 
represent the opinions of any organization
or otherwise sane person or persons.

John R. Herring
Architect, Spatial Products
Oracle Corporation
One Oracle Drive
Nashua, New Hampshire 03062
ph: 1 603 897 3216
fx: 1 603 897 3334

Annue cœptis - Novus Ordo Seclorum
  


-----Original Message-----
From: geojson-bounces at lists.geojson.org
[mailto:geojson-bounces at lists.geojson.org] On Behalf Of Chris Holmes
Sent: Monday, March 19, 2007 4:26 PM
To: bitner at gyttja.org
Cc: geojson at lists.geojson.org
Subject: Re: [GeoJSON] AsJSON(geometry)



David William Bitner wrote:
> 
> 
>      >
>      >  { "id": "alesia",
>      >       "title": "Alesia",
>      >       "classname": "settlement",
>      >       "geometry": {
>      >        "type": "point",
>      >        "value": [47.535, 4.478, 0.0]
>      >       }
>      >       "srs": "EPSG:4326",
>      >     }
>      >
>      > instead of
>      >
>      >  { "id": "alesia",
>      >       "title": "Alesia",
>      >       "classname": "settlement",
>      >       "geometryType": "point",
>      >       "spatialCoordinates": [[47.535, 4.478, 0.0]],
>      >       "srs": "EPSG:4326",
>      >     }
>      >
>      > ?
>      >
>      > Chris
>      >
> 
>     I'm in favor of the above.
> 
>     Sean
> 
> Should the SRS go in with the geometry as well?
> 
> { "id": "alesia",
>        "title": "Alesia",
>        "classname": "settlement",
>        "geometry": {
>           "type": "point",
>           "value": [47.535, 4.478, 0.0]
>           "srs": "EPSG:4326",
>         }
> }

I would think so.  Though I do think it'd be nice if we didn't have to
include the srs with every geometry, since it's a lot of redundant
information, and there's a pretty good chance that with most collections of
features you're going to have the same srs.

I'm not sure how 'json' it is, but if possible I'd like to have something
like let the srs inherit from the collection, and then let the individual
geometry object over-ride if it chooses to.  Another twist could be to
define a default 4326 for everything, and if geometries (or collections if
we like that) want to over-ride they can.

Though as a server side implementor I am happy to include it each time, as
that's what you have to do for GML, so I do it already.

Chris

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--
Chris Holmes
The Open Planning Project
http://topp.openplans.org