[DeletedUser]

@rover83 I assume it would therefore be important to differentiate between geodetic north and geographic north, as their 90° latitude will have been calculated on a different basis.

[mathteacher]

@terminologist 

I think that the term geographic north is derived from the term geographic north pole. That’s the spot where the earth’s axis intersects the earth’s surface. It’s a different point from where a compass needle points, which is magnetic north. The difference in compass direction that you have to travel in order to reach the geographic north pole is the compass variation.

In ellipsoidal models, the minor of axis of the ellipsoid is aligned to the best of our ability with the geographic north pole. Thus, geodetic north and geographic north should be the same thing.

But, in practice, geographic north is used casually to mean all manner of things. If you include it as a term, it might be good to define it as synonymous with geodetic north.

As to Grid North vs Geodetic North, this diagram may help. Remember that the grid is a two-dimensional being derived from an ellipsoid. Its purpose is to simplify calculations. This is a model of a Lambert Conformal projection, but the same principles apply to others. Grid North is parallel to the central axis while geodetic north is toward the minor axis. The angle between these two is the convergence angle.

 

[DeletedUser]

@mathteacher Thanks. You have been very helpful.

[mathteacher]

Tough job; best wishes for total success.