[wfs-dev] Re: [SFsql.rwg] RE: Axis Ordering in GML - an FYI

Carl Reed OGC creed at opengeospatial.org
Mon May 23 12:04:34 EST 2005

Roel -

Thanks for the clarification and additional information. The following is 
not to incite more dicussion but is merely provided as informational :-)

As this latest discussion on axis order was progressing, I browsed around 
the web looking for other standards and products use of geographics and how 
the axis were ordered. Kind of a best practices study. I found that the 
majority of other standards that express or encode WGS 84 based coordinates 
do so as latitude/longitude (see below).



Axis Order best practice:

OASIS CAP: A WGS-84 coordinate value is here represented as a 
comma-delimited latitude/longitude pair, measured in decimal degrees 
(un-projected). Latitudes range from -90 to 90 and longitudes range 
from -180 to 180

OMA MLP API: Example -
  <Point srsName="www.epsg.org#4326" gid="some_thing">


      <Y>30 27 45.3N</Y>

      <X>45 25 52.9E</X>


IETF DHCP Location Draft RFC: Latitude and Longitude are represented in 
fixed-point 2s-complement binary degrees, for the economy of a smaller 
option size compared to the string  encoding of digits in [4]. Datum is WGS 

IETF PIDF Location Draft RFC: example (is GML!)
  <gml:Point gml:id="point96" srsName="epsg:4326">
         <gml:coordinates>31:56:00S 115:50:00E</gml:coordinates>
netCDF: Appears to be Long/Lat :-)

Climate Data Markup Language (CDML): Lat/Long

Portland State University class on CRS: Geographics are expressed at 

POSC Epicenter Model - Lat/Long

GeoVRML: Either. Lat/Long is the default. GD - "<latitude> <longitude> 
<elevation>" or <longitude> "<latitude> <elevation>". The order of latitude 
and longitude is controlled by the geoSystem field. If "latitude_first" is 
specified, the order is latitude then longitude. If "longitude_first" is 
specified, the order is longitude then latitude. If neither is specified, 
"latitude_first" is the default.

SEDRIS - Lat/Long

West Virginia GIS Standard best Practices - Lat/Long

Carl Reed, PhD
CTO and Executive Director Specification Program

The OGC: Helping the World to Communicate Geographically


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----- Original Message ----- 
From: "Nicolai, Roel R SIEP-EPT-SCIG" <Roel.Nicolai at shell.com>
To: "Martin Daly" <Martin.Daly at cadcorp.com>; <wfs-dev at opengeospatial.org>
Cc: "PostGIS Development Discussion" 
<postgis-devel at postgis.refractions.net>; <sfsql.rwg at opengeospatial.org>
Sent: Monday, May 23, 2005 2:47 AM
Subject: RE: [SFsql.rwg] RE: Axis Ordering in GML

> Gents,
> There are two versions of WGS 84 floating around nowadays:
> - EPSG code 4326, which defines latitude before longitude 
> (urn:ogc:def:crs:EPSG:6.6:4326)
> - CRS84, with longitude before latitude (urn:ogc:def:crs:OGC:1.3:CRS84)
> See OGC document 04-077.
> Don't mix these two up or you map will look rather odd.
> On the other point I need correct Martin: The EPSG definitions of 
> geographic crs's now specify the units to "degree (supplier to define 
> representation)".
> Hence, it is quite legal for the specification to prescribe decimal 
> degrees in conjunction with the use of EPSG codes, or for the supplier 
> provides additional information stating that the coordinate values in 
> decimal degrees if the spec does not mention that.  For example GML 
> prescribes decimal degrees.  What you cannot do is use your own 
> representation of degrees or you own preferred axis order AND:
> 1. not tell anyone about it or agree it in a small group and assume that 
> is authoritiative for the rest of the world
> 2. overwrite what the spec prescribes.
> The sum of it is that if you feel you must use longitude before latitude, 
> use OGC:CRS84;  DON'T use EPSG:4326, because that means latitude before 
> longitude.  ANY software that reads "EPSG:4326" should expect that the 
> latitude value comes before the longitude value.  If it internally swaps 
> these values the software makes a joke of the OGC specifications and of 
> every attempt to standardize.
> The whole idea of standardisation is to agree to do things a certain way 
> and to avoid one's own preferred solution.  The "one" in the previous 
> sentence may mean an individual person, a data supplier, a working group, 
> a particular group of users etc.
> Best regards,
> Roel Nicolai
> -----Original Message-----
> From: sfsql.rwg-bounces+roel.nicolai=shell.com at opengeospatial.org
> [mailto:sfsql.rwg-bounces+roel.nicolai=shell.com at opengeospatial.org]On
> Behalf Of Martin Daly
> Sent: Friday, May 20, 2005 9:19 AM
> To: wfs-dev at opengeospatial.org
> Cc: PostGIS Development Discussion; sfsql.rwg at opengeospatial.org
> Subject: [SFsql.rwg] RE: Axis Ordering in GML
>> Question for the group. Is the GML below legal?
>> <gml:Point
>> srsName="EPSG:4326"><gml:coordinates>-172.335,18.53916667</gml
>> :coordinates></gml:Point>
>> I have built a GML point that references EPSG:4326 but has
>> the easting
>> before the northing.
> Oh no, not again.  Do you *any* idea what you have just started?
>> If I check your WFS 1.0 servers, will I find them returning GML in
>> easting/northing order or northing/easting order for
>> EPSG:4326? This is
>> both a theory and practice problem. In theory, what should
>> happen, and
>> in practice, what are people doing?
> Cadcorp's experience is that the theory is lat/lon, but the practice is
> lon/lat.  Another worm squirming out of the can is that the
> representation of ordinates in EPSG 4326 is not supposed to be in
> decimal degrees.  There are differences of opinion (!) as to whether or
> not this applies to data transfer mediums, or just to user-facing
> representation.
> Let battle commence,
> Martin
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