Validate + canonicalize an altitude-type value (case-insensitive), throwing
input/invalid-altitude for anything other than MSL/AGL. Returns the
upper-cased canonical form — the one geo field normalized for storage, since
it is a closed two-value enum rather than freeform/numeric.
Validate + canonicalize an altitude-type value (case-insensitive), throwing
input/invalid-altitudefor anything other thanMSL/AGL. Returns the upper-cased canonical form — the one geo field normalized for storage, since it is a closed two-value enum rather than freeform/numeric.