[Geojson] coordinate order

[Christopher Schmidt]

On Mon, Mar 10, 2008 at 03:55:58PM -0400, Norman Vine wrote:
> Tim Schaub writes:
> > Sean Gillies wrote:
> > > Tim Schaub wrote:
> > >>
> > >>
> > >> Regarding the order of elements in the coordinates array for GeoJSON 
> > >> geometries, we say that the first two elements are in x, y order (lon, 
> > >> lat for dd).  This is what we have said from the start.
> > >>
> > >> The change to the spec is to accommodate CRS that define a different 
> > >> coordinate order.  Instead of requiring all clients to know about all 
> > >> CRS coordinate order conventions, we require that GeoJSON geometries 
> > >> referencing a CRS that defines non-xy coordinate order include a 
> > >> "coordinate_order" member in the CRS object.
> > >>
> > >> This means a point geometry that references EPSG:4326 
> > would look like this:
> > >>
> > >> {
> > >>     "type": "Point",
> > >>     "coordinates": [-180.0, 90.0],
> > >>     "crs": {
> > >>         "type": "EPSG",
> > >>         "properties": {"code": 4326},
> > >>         "coordinate_order": [1, 0]
> > >>     }
> > >> }
> > >>
> > > 
> 
> I thought we meant that if the coordinate order was different then X,Y 
> then even if crs was present that one must supply "coordinate_order"
> 
> So proper access to the coordinate array coords[] for 'simple clients' 
> could still be done eg.
> 
> for point in coords:
>   x = point[coordinate_order[1]]
>   y = point[coordinate_order[0]]

No. 

"For type "Point", each element in the coordinates array is a number
representing the point coordinate in one dimension. The order of
elements follows x, y order (or longitude, latitude for coordinates in
decimal degrees)."

  x = point['coordinates'][0]
  y = point['coordinates'][1]

Always. No questions. No alternatives. 

> Note that for clients that know the CRS this might simplify to
> 
> for point in coords:
>   lat = point[0]
>   lon = point[1]
> 
> As this is implied by the CRS

No. For clients that know the CRS, they can do:

point[crs['axis'][0]] = points[coordinate_order[0]]
point[crs['axis'][1]] = points[coordinate_order[1]]

By default, we assume 
  crs['axis'] = ['x', 'y']

And therefore 
  coordinate_order = [0, 1]

Leads to:

 point['x'] = points[0]
 point['y'] = points[1]


If, instead, 
  
  crs['axis'] = ["y","x"]

then you must explicitly state:

  coordinate_order = [1, 0]

So that:

  point[crs['axis'][0]] = points[coordinate_order[0]]
  point[crs['axis'][1]] = points[coordinate_order[1]]

remains true.

The benefit is that if:

  crs['axis'] = ["x", "y", "z"]

you can do:  
  coordinate_order = [0, 1, 2]

And then:
  
  point[crs['axis'][0]] = points[coordinate_order[0]]
  point[crs['axis'][1]] = points[coordinate_order[1]]
  point[crs['axis'][2]] = points[coordinate_order[2]]

Or, more generally:

for axis in range(len(axis)):  
  point[crs['axis'][axis]] = points[coordinate_order[axis]]

And you can know what your point['x'], point['y'], point['z'] are, based
on your understanding of the CRS.

EPSG has definitions like:

<axis>
    <CoordinateSystemAxis gml:id="epsg-axis-39"
       gml:uom="urn:x-ogc:def:uom:EPSG:9001">
        <axisAbbrev>Y</axisAbbrev>
        <axisDirection codeSpace="EPSG">east</axisDirection>
    </CoordinateSystemAxis>
</axis>
<axis>
    <CoordinateSystemAxis gml:id="epsg-axis-40"
          gml:uom="urn:x-ogc:def:uom:EPSG:9001">
      <axisAbbrev>X</axisAbbrev>
      <axisDirection codeSpace="EPSG">north</axisDirection>
    </CoordinateSystemAxis>
</axis>

In this case, you would have, in your knowledge of the CRS: 

crs['axis'] = ['Y', 'X']

and you would have, in your data:

  coordinate_order = [1, 0]

Because crs['axis'][coordinate_order[0]] = "X" is true, which matches to
what the spec tells you *must* be true -- that X is first, and Y is
second.

Regards,
-- 
Christopher Schmidt
MetaCarta