[Geojson] circles & units

[Tim Schaub]

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.