Azimuth is the horizontal angle, measured clockwise between a reference direction and the line to an observer or designated point. There are three base (reference) directions or azimuths: true, grid, and magnetic azimuth. True azimuth is an azimuth referenced to north as defined by the axis of rotation of the earth. Grid azimuth is an azimuth referenced to grid north. It differs from true azimuth by the amount of the grid convergence. Magnetic azimuth is an azimuth referenced to the local direction of the earth's magnetic field.