Adding excerpt from: http://docs.opengeospatial.org/is/12-063r5/12-063r5.html#35
```
Axis direction indicates the positive increment along an axis. The handedness
of a 2- or 3-dimensional coordinate system may be derived from the directions.
Requirement: the following constraints shall apply:
i. For geodetic CRSs having an ellipsoidal 2-D coordinate system, the
two-dimensional ellipsoidal coordinate system axes are geodetic latitude,
positive northwards, and geodetic longitude, positive eastwards. Axis direction
shall be ‘north’ and ‘east’ respectively.
ii. For geodetic CRSs having an ellipsoidal 3-D coordinate system, the
three-dimensional ellipsoidal coordinate system axes are geodetic latitude,
positive northwards, geodetic longitude, positive eastwards, and ellipsoidal
height, positive upwards. Axis direction shall be ‘north’, ‘east’ and ‘up’
respectively.
iii. For geodetic CRSs having a geocentric Cartesian coordinate system,
in WKT strings the axis directions shall be ‘geocentricX’, ‘geocentricY’ and
‘geocentricZ’ respectively. The first axis of the earth-centred 3D Cartesian
coordinate system lies in the equatorial plane such that a vector pointing in
the positive direction passes through the intersection of the equator and the
prime meridian. The second axis is in the equatorial plane such that a vector
pointing in the positive direction passes through the intersection of the
equator and the meridian of 90°E. The third axis is perpendicular to the first
two such that it completes a right-handed coordinate system; it is
approximately parallel to the earth’s rotation axis, positive towards the north
pole.
iv. For projected CRSs, except for coordinate systems centred on a pole,
the axis direction shall be ‘north’ or ‘south’ and ‘east’ or ‘west’.
For coordinate systems centred on a pole the direction for both axes will
be ‘south’ (for the north pole case) or ‘north’ (for the south pole case); the
axes direction shall be supplemented with a <meridian> description. This is the
value of the meridian that the axis follows from the pole. The prime meridian
from which the meridian value is reckoned is given through the <prime meridian>
object; if no <prime meridian> object is in the WKT string then the IRM or
Greenwich meridian shall be assumed.
v. For vertical CRSs, the axis direction shall be ‘up’ or ‘down’.
vi. For temporal CRSs, the axis direction shall be ‘future’ or ‘past’.
vii. In engineering CRSs the horizontal directions are only approximate,
the set of directions indicating whether the coordinate system is left-handed
or right-handed. (In the 2D case, the handedness is when viewed from above the
plane of the system). For engineering CRSs with polar coordinate systems the
direction of the rotational axis shall be ‘clockwise’ or ‘counterClockwise’.
The specified direction from which the rotation is measured shall be given
through the supplementary object <bearing>; the bearing value shall be given in
the unit defined through <axis unit>.
See 6.5 for comment on case sensitivity.
NOTE The requirements in 7.5.4, iv) for projected CRSs with a coordinate
system centred on a pole and in 7.5.4, vi) for engineering CRSs with a polar
coordinate system require the enumeration for <axis direction> in 7.5.1 to be
an extended version of the codelist given in ISO 19111:2007, 9.4.
```
I think this adds clarity for some of the concerns raised by @JonathanGregory.
Thoughts?
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