How much of Earth's land area has antipodal land area? Using the site antipodesmap one can quickly explore what the opposite point of the earth to any location would be. That is, the surface point connected to this point by a straight line through the centre of the earth.

It is easy to see that most land points have antipodes which lie somewhere in water, which made me wonder:

How much land area (as a fraction of total land area) has antipodal land area? 

And how would I best find this out? Can somebody produce a map of those areas, e.g. highlight those areas on a world map?
A rough idea, using the Lambert Conformal image at the Cartopy projection list and Gimp:



The gold area should be the overlap.  So basically parts of South America with Indonesia and SE Asia... and Antarctica with the far northern islands of the northern hemisphere, plus a few smaller areas from islands like New Zealand and Hawaii and even smaller.

That assumes I did properly invert it.  To do so, I just vertically flipped the image, and offset it by 180 degrees.  Thinking that should be all that is needed.

So in the end it looks like it's not a lot of area at all.  Further exact statistics would take some GIS work (probably quicker than GIMP in the end!)  A central starting guess would be 8.5% (the 29% land cover squared), but particulars will make it a bit higher or lower.