In a 2D polar coordinate system points can be described with coordinates. Another way of defining a point in a plane is by using polar coordinates. This is the distance d from the origin to the point concerned and the angle α between a fixed (or zero) direction and the direction to the point. The angle α is called azimuth or bearing and is measured in a clockwise direction. It is given in angular units while the distance d is expressed in length units.
Bearings are always related to a fixed direction (initial bearing) or a datum line. In principle, this reference line can be chosen freely. Three different, widely used fixed directions are: True North, Grid North and Magnetic North. The corresponding bearings are true (or geodetic) bearings, grid bearings and magnetic (or compass) bearings, respectively.
In raster images, direction is determined by the orientation of the neighboring pixels.