from Wiktionary, Creative Commons Attribution/Share-Alike License
- adj. having axes at right angles
- adj. being corrected for the curvature of the Earth
from the GNU version of the Collaborative International Dictionary of English
- adj. Having the axes at right angles to one another; -- said of crystals or crystalline forms.
from The Century Dictionary and Cyclopedia
- In crystallography, pertaining to the three systems in which the axes are at right angles with each other. See crystallography.
ortho- + -metric (Wiktionary)
Where the geoid comes into play is determining the orthometric elevation of a point, because Ortho Height is measured from the geoid.