3.1 Map projections

For any point on the Earth’s surface True North is towards the Geographic North Pole (The Earth’s axis of revolution).

This fact is independent of any map system, datum or spheroid. However, when a map projection surface is introduced, it is impossible to maintain a parallel map North that still meets at a single point.

In the example below a vertically wrapped cylinder such as those used in Transverse Mercator map projections includes the North Pole but the straight blue line on the globe will become slightly curved on the surface of the cylinder. The black line in the diagram shows the direction of Map North (Grid North) and clearly they are not the same.

When the cylinder is unwrapped, the two lines look like below.

Because all True North lines converge to a single point, the angle from True to Grid North is referred to as the ‘Convergence’ Angle.

Convergence is the True Direction of Map North.

In the case of the Universal Transverse Mercator Projection, the convergence within one map zone can vary from -3 degrees to + 3 degrees.

When correcting a true North Azimuth to Grid, this convergence angle must be subtracted from the original azimuth. It is essential that a North Arrow is drawn in order to correctly visualize the relative references.

When magnetic north is included we have three different North References to contend with.

These can be in any order with several degrees of variation between them so a clear North Arrow is essential on all well plans and spider maps.

When correcting from one reference to the other it is common practice to set the company reference North straight upwards and plot the others around it. Grid North is the preferred company reference, so all quoted azimuths would be referenced to Grid and the North arrow is centered on Grid North.

In this example the well is heading 60 degree Grid with magnetic North at 6 degree west of True and Grid North 2 degree East of True. Depending on which reference we use, the azimuth can be expressed three different ways. It is easy to see how confusion can occur.