Convergence of the Meridian
P is a reference location in the map projection - based SRF
The central meridian appears as a straight line (the Y-axis)
Meridians may or may not curve (one exception is the Mercator map projection)
T is the tangent line to the meridian at P
? is the Convergence of the Meridian
The convergence of the meridian is the angle between SRF North and true North on the OBS (positive clockwise)
The angle ? is the rotation angle needed to rotate a vector in the map projection-based SRF to true North when siting a reference vector onto a canonical LTP
The formula for computing the convergence of the meridian ranges from very simple (0 for the Mercator map projection-based SRFs) to very complex for Transverse Mercator map projections.