[Geojson] GeoJSON '1.0'?

Thu Mar 13 04:43:59 PDT 2008

Chris,

EPSG means "European Petroleum Survey Group".  This group maintains (or 
maintained ... I think someone else may manage the database now) a large 
database of coordinate system definitions usually referenced with a code 
like EPSG:4326.

I think what Raj may have meant is that that all coordinate system 
definitions (especially the ones from the EPSG database) include 
(whether implicitly or explicitly) a statement about the coordinate 
order which should be used when encoding the coordinates.

In the case of EPSG:4326 that order is lat, long because it is declared 
to be a geographic coordinate system (as opposed to a projected 
coordinate system) in the EPSG database.

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."!

The obvious thing to do is simply state: Do what the CRS says.

BUT ... there is legacy.

In the distant past, the WMS specification used EPSG:4326 in its 
examples.  HOWEVER, it used it with the coordinate order of long,lat 
which according to the EPSG database is not correct.  So, we ended up 
having a lot a client's built expecting EPSG:4326 to be in long,lat 
order (or x,y).

So now we are in a situation where you have some clients who incorrectly 
think that EPSG:4326 is long,lat and other who correctly think that 
EPSG:4326 is lat,long.  What to do?

One proposal is to explicitly have servers state the encoded coordinate 
order using some tag.  OGC, for example, is pondering using a tag called 
geometricCoordinateOrder in the output so that servers can say:  My 
coordinates are in the EPSG:4326 CRS but are being transmitted in 
long,lat order.

Not sure if this information helps but there you have it.  I hope I got 
the details right because I am not a CRS expert and have generally tried 
to avoid getting involved in the whole CRS-coordinate order discussion 
... especially in OGC where is seems to always spark heated debate.

Ciao.

Christopher Schmidt wrote:
> On Thu, Mar 13, 2008 at 01:25:59AM -0400, Christopher Schmidt wrote:
>> On Wed, Mar 12, 2008 at 11:20:04PM -0600, Tim Schaub wrote:
>>> For clarification:
>>>
>>> I'm responding the the current draft (which uses EPSG:4326) and saying 
>>> that I don't think coordinate_order is needed.
>>>
>>> The simple question is whether this should be a valid representation of 
>>> a point in the northern hemisphere:
>>>  ...
>>> I think it should.
>> I think it is not a valid representation of a point in the northern
>> hemisphere.
> 
> To clarify:
> 
> It is my understanding that using the "EPSG:4326" forces the assumption
> that the ordering is y, x. This understanding is based on feedback from
> conversations at FOSS4G2007, the most memorable of which was with Raj
> (from OGC), who essentially told me (as far as I remember) "You can't do
> that."
> 
> I have no idea what "You can't do that" means. The letters EPSG mean
> nothing special to me, and to claim that they do seems weird to me. 
> 
> Given that, I have no particular opinion of my own basd on my own
> knowledge. However, it appears to be a not-uncommon opinion that using
> the EPSG code in this way would be 'bad form'. 
> 
> Or maybe I really just misunderstood, and this should have all been
> resolved 6 months ago.
> 
> I don't know enough to have any personal prference: my understanding is
> based entirely on what I have been told, and there is no significant
> support for what I have been told that I am able to point to.
> 
> Regards,

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