[Geojson] circles & units

[Matt Priour]

I disagree.
In the absence of a stated CRS, GeoJSON spec states:
"If no crs member can be so acquired, the default CRS shall apply to the 
GeoJSON object.

The default CRS is a geographic coordinate reference system, using the WGS84 
datum, and with longitude and latitude units of decimal degrees."

Thus, in absence of any additional information, AND IF the circles spec 
stated that radius MUST be stated in CRS coordinate units (as has been 
suggested), then the following information could be determined:
All units are in decimal degrees of latitude & longitude in the WGS 84 
datum.
circle:
   center: 0.0 deg longitude, 0.0 deg latitude
   radius: 1.0 degrees

point:
    x: 0.5 deg longitude
    y: 0.0 deg latitude

Most geometric operators would probably convert to a reasonable geometric 
(rather than geographic) coordinate system before doing Point in Polygon or 
Area calculations, but that is beyond the concern of GeoJSON.

As long as the requirement is "radius MUST be stated in CRS coordinate 
units" then all the required information is known.
However, if the requirement is "radius MUST be assumed to be meters unless a 
specific measurement unit is indicated", then you are right. All kinds of 
information is unknown and requires outside information to know even the 
basic positional information.

Matt Priour

--------------------------------------------------------------------------------------------

From: Tim Schaub
Sent: Friday, November 04, 2011 4:25 PM
To: geojson at lists.geojson.org
Subject: [Geojson] circles & units

I'll try not to combine or confuse concerns here.  This thread is
strictly about my concern with the circles proposal with regard to
units.

https://github.com/GeoJSONWG/geojson-spec/wiki/Proposal---Circles-and-Ellipses-Geoms

An otherwise lightweight client cannot do a whole lot with circles.
Take, for example, these two objects:

    [{
        "type": "Circle",
        "coordinates": [0, 0],
        "radius": 1
    }, {
        "type": "Point",
        "coordinates": [0.5, 0]
    }]

Without knowing the number of meters per unit for the geometries' CRS,
a client cannot answer the very simple question: is the point in the
circle?  Lacking this same information, a client cannot even render
the two objects.

This makes circles unique among GeoJSON objects.  Without the EPSG
database, a service that returns CRS details*, or some other
dependency, circles cannot be used together with other
features/geometries any interesting way.

Tim

* No handy service that I know of returns the number of meters per
linear unit for a given CRS.  Though this is critical for so many
things, specs like WMS & WMTS assume everybody knows this, and it
drives me nuts.

-- 
Tim Schaub
OpenGeo http://opengeo.org/
Expert service straight from the developers.
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