Are you anti-podal? So are many geographic edges!

Actually, not many, but it feels like a common first test case people try when they start playing with the geography type. “How far is it between 0,0 and 180,0?”

There’s a big problem with anti-podal edges though: they don’t have a determinate path. That is, to get from (0,0) to (180,0) it doesn’t matter what direction you travel, just start moving. Any other pairing of points generates a single great circle describing the shortest path joining them. So anti-podal points make very bad components of geometry: they don’t define a path, and they can’t bound an area, because only the end points have a determinate location.

Which brings me to my problem. How do I handle geography objects in PostGIS that include anti-podal segments? On the one hand, since they are impossible to do calculations against, I should just disallow them in all cases, and throw an error. On the other hand, people think they have meaning and stick them into functions all the time. There are also a few functions (like ST_Length) where it’s actually possible to calculate a valid answer given an antipodal input (because we know that antipodal points are exactly one half-circumference apart, even if we don’t know what direction someone might travel between them).

What do you think? Is there a best answer? Comments most welcome!