The angle, at a point A on a surface, between the tangent to the meridian at A and the tangent to the geodesic from A to the point B whose geodetic azimuth is wanted. The angle is measured from the direction of north clockwise. (Until 1985, the U. S. National Geodetic Survey (NGS) and some other organisations measured the angle from the direction of south clockwise. ) The angle is called the forward azimuth for the line AB. The azimuth at B between the tangent to the meridian and the tangent to the geodesic is called the back azimuth or reverse azimuth of the line AB. The geodesic is not, except in special cases, a plane curve. However, for short lines of the length commonly used in triangulation, the small difference between the tangent to the geodesic and the tangent to the actual line of sight between A and B can be neglected.