The primary conceptual difference (though there are also many functional
differences) between the 2d and 2dsphere indexes, is the type of coordinate
system that they consider. Planar coordinate systems are useful for certain
applications, and can serve as a simplifying approximation of spherical
coordinates. As you consider larger geometries, or consider geometries near
the meridians and poles however, the requirement to use proper spherical
coordinates becomes important.
I don’t know anything about geo, so I’ll leave this up for experts to comment on.
✚ There’s actually something I like about this announcement: the fact that MongoDB decided to use an existing standard instead of coming up with its own custom solution.
Original title and link: New Geo Features in MongoDB 2.4 ( ©myNoSQL)