The geoid is the surface of reference for astronomic observations and for geodetic leveling. A network of lines representing parallels of latitude and meridians of longitude forming a map projection. 1-6.12.11 Isogonic Line. A line drawn on a map or chart joining points of equal magnetic declination for a given time. The line connecting points of zero declination is the agonic line.
Lines connecting points of equal annual change are isopors.
A line on the surface of the Earth cutting all meridians at the same angle, a rhumb line. 1-6.12.13 Map Projection. An orderly system of lines on a plane representing a corresponding system of imaginary lines on an adopted terrestrial datum surface. A map projection may be derived by geometrical construction or by mathematical analysis. A mathematical figure closely approaching the geoid in form and size.