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Ex c s s
3.1 Map projections
Fo ny po n on h E h’s su f c u o h s ow ds 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 this example 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.
Figure 21: Map projection surface exercise showing it is impossible to maintain a parallel map North.
When the cylinder is unwrapped, the two lines look like figure 22a.
Because all True North lines converge to a single point, the angle fom u oGd o hs f do sh ‘on gnc’Angl.
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.
Figure 22a: The ‘Convergence’ Angle.
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