Hi, Aerilius--
I found out why I was having altitude problems. I don't program in ruby, but it looks close enough to other languages iI do program that I was able to muddle through it and realize that Altitude is not an attribute in GeoReference, so get_attribute("GeoReference","Altitude") returns 0. By changing Altitude to ZValueCentered, I was able to get a negative value in inches that is seemingly correct for my elevation. I cannot find a definitive reference on this, and there is also ModelTranslationZ which has the identical value for me, so I'm unclear which is the appropriate way forward. However, both solve my problem. So in conclusion, I've replaced line 253 by @ground = -@model.get_attribute("GeoReference","ZValueCentered").to_f.to_m and that solved my problem.
Now I'm struggling with an odd error. For some reason when exporting a group to multigeometry the KML is missing the opening <MultiGeometry> tag, but it has the closing one. This causes an import error in KML until I manually add the XML tag.
EDIT: terrain_alt([0,0,0]).to_m doesn't seem to return any sensible values. I'm interested if you have an idea about this, too.