gml:axisDirection is the direction of this coordinate system axis (or in the
case of Cartesian projected coordinates, the direction of this coordinate system axis at the
origin). Within any set of coordinate system axes, only one of each pair of terms may be
used. For earth-fixed CRSs, this direction is often approximate and intended to provide a
human interpretable meaning to the axis. When a geodetic datum is used, the precise
directions of the axes may therefore vary slightly from this approximate direction. The
codeSpace attribute shall reference a source of information specifying the values and
meanings of all the allowed string values for this property.