[GeoJSON] spec clarification of CRS

Jukka Rahkonen jukka.rahkonen at latuviitta.fi
Wed Dec 11 21:48:00 PST 2013


Raj Singh wrote:

> On Dec 11, at 3:42 PM, Martin Daly <Martin.Daly at cadcorp.com> wrote:
>
>>> I'd like to suggest that in section 3.1 Named CRS [1], a sentence be
>>> added
>>> that says, "EPSG:4326 identifiers such as EPSG:4326 and
>>> urn:ogc:def:crs:EPSG:6.6:4326 are prohibited, as they define coordinate
>>> reference systems with Y-X axis order, which is illegal according to
>>> section
>>> 2.1.1."
>>> (a good reference on the issue is [2]).
>>
>> -1
>>
>>> As we move into 2014, confusion still reigns on this issue. And the
>>> current
>>> wording of the GeoJSON spec implies that EPSG:4326 is OK, which is an
>>> internally contradictory statement. Don't make the same mistake OGC did
>>> a
>>> decade ago!
>>
>> I, for one, completely disagree. GeoJSON is explicit and consistent
>> about axis order of the coordinates. All that it says is " use any CRS
>> you like, but *always transfer the coordinates in this pre-defined
>> order*".
>>
>> That way, clients can be 100% sure that they can take the coordinates on
>> face value, with or without a priori knowledge of the CRS, and with or
>> without the ability to dereference any CRS definition.
>>
>> No mistakes, no confusion. At least on the part of GeoJSON.
>>
>> Martin
>>
>
> Correct me if I'm wrong, but I'm still feeling contradiction within the
> spec:
>
> 1) Section 2.1.1 says to me that any "positions" in a GeoJSON file will
> "follow x, y, z order". If that is true, then...
> 2) Section 3.1 says "urn:ogc:def:crs:OGC:1.3:CRS84 shall be preferred over
> legacy identifiers such as EPSG:4326". That says to me that EPSG:4326 is
> not preferred, but allowed. However, EPSG:4326 doesn't follow x, y, z
> order. So doesn't that make it illegal to use in GeoJSON?
>
> I'm not trying to open or re-open any discussion on CRS support. I just
> want to know if GeoJSON only supports CRSes that follow x, y, z order. If
> so, then section 3.1 is ambiguous. If not, then section 2.1.1 is
> misleading.



Hi,

I am sure that you have read the whole sentence

" The order of elements must follow x, y, z order (easting, northing,
altitude for coordinates in a projected coordinate reference system, or
longitude, latitude, altitude for coordinates in a geographic coordinate
reference system) "

I understand that people have principles but I do not see anything really
misleading in giving a place as
{ "type": "Point", "coordinates": [25.0, 70.0] }
and saying that they are EPSG:4326 coordinates.


If some coordinate system has elements for giving easting, northing,
altitude, or longitude, latitude, altitude then GeoJSON supports it.
EPSG:4326 belongs without a doubt to that group.

What section 3.1 says is that " A CRS object may indicate a coordinate
reference system by name. " Take it as for GeoJSON EPSG:4326 is just a
name.



-Jukka Rahkonen-

> ---
> Raj
> _______________________________________________
> GeoJSON mailing list
> GeoJSON at lists.geojson.org
> http://lists.geojson.org/listinfo.cgi/geojson-geojson.org
>





More information about the GeoJSON mailing list