d3.geo.{centroid,area}

[mbostock]

This would be useful for computing the lat-lon centroid:

d3.geo.centroid = d3.geo.path().projection(d3_identity).centroid;

Jason points out that we should probably expose d3.geo.area as well, both using spherical coordinates.

[jasondavies]

We should use a spherical polygon area weighting though, otherwise this will be wrong. This should also comply with our interpretation of polygon inside/outside.

[jasondavies]

Do we want to return area in (°)² for consistency with our use of degrees elsewhere? Aside from coordinates being in degrees, we also have d3.geo.{circle,greatArc,graticule}.precision, d3.geo.graticule.extent and d3.geo.circle.angle.

If so, we might also make a backwards-incompatible change to d3.geo.greatArc.distance to return degrees, too.

Alternatively, we could simply return steradians and say that this is consistent with d3.geo.greatArc.distance, and simpler to use when converting to real-world units by multiplying by Earth's radius² or radius.

[mbostock]

I would go with steradians.

[mbostock]

Added in 3.0.