[Geojson] Valid values for numbers in points etc?

[Sean Gillies]

On Thu, Mar 7, 2013 at 1:14 PM, Francis Galiegue <fgaliegue at gmail.com> wrote:
> On Wed, Mar 6, 2013 at 6:03 PM, Michael Geary <mg at mg.to> wrote:
>> The meaning of the coordinate values depends on the coordinate reference
>> system (CRS) the GeoJSON file uses.
>>
>> The default CRS is WGS84 (EPSG:4326), where the values do represent
>> longitude, latitude, and optional altitude.
>>
>> If you specify a different CRS in the GeoJSON file, then the coordinate
>> values are easting, northing, and again optional altitude. The meaning of
>> the easting and northing depends on the CRS. I believe altitude is in meters
>> in all cases; someone correct me if I'm wrong.
>>
>> As an example, my PolyGonzo library accepts GeoJSON in either the default
>> WGS84, or alternatively in Spherical Mercator (EPSG:3857) coordinates, also
>> known as Google Mercator. These are coordinates in "meters" on the projected
>> Spherical Mercator plane. If you use this in the GeoJSON, it allows slightly
>> faster display of the polygons because it only takes a single multiplication
>> to convert each coordinate to a pixel location instead of a sequence of
>> trigonometric operations.
>>
>
> Hmm, so the schemas I have written should be really split in two. I
> guess there is no defined way to tell from the get go if one GeoJSON
> representation is written using one coordinate reference system or
> another? How do APIs know whether one or the other coordinate system
> is used?

http://geojson.org/geojson-spec.html#coordinate-reference-system-objects

-- 
Sean Gillies