Is it normal for the geomipterrain to return any elevation on negative X and Y?
Yeah, I think it’d extrapolate. You shouldn’t use it outside of the correct range, though.
I`m trying to understand something …
If the heightfield image is 513x513, geomipterrain is “showing” us a terrain with visible geometry 512x512. Where are the missing pixels and are they the last ones from the heightfield image?
p.s. Maybe I need to review the GeoMipMapping algorithm … 
Hm …
I was wrong …
Geomipterrain shows geometry with size 513x513 pixels …