Direction

Kinds of Direction

Azimuth: the horizontal direction of one object from another expressed as an angle in degrees of arc clockwise (i.e., to the east) from the north. It is expressed as a numerical value between 0º and 359º; thus, an azimuth of 90º represents an object that is due east of the observer. Commonly used in relation to celestial objects and marks the point where the vertical arc through the star and the observer intersects the horizon.

Bearing: the horizontal direction of one object from another expressed as an angle of arc between 0° to 90° either clockwise or counterclockwise from north or south. Values are usually expressed as a combination of two letters and a numerical value between 0° and 90° (examples include N 54° E, S 20°W) but may also be given using just letters (N, NW, ESE, etc.).

Heading: the course or direction in which an object (a ship, aircraft, vehicle, person, etc.) is moving, usually expressed as points of the compass, such as E, NW, ESE, or N 15º W, or clockwise from north in values of 0º to 359º (e.g., a heading of 256º).

[Page 111]Slope direction: In GIS, slope direction is a commonly derived raster data set, usually calculated from digital elevation models (DEM). Slope direction is normally stated as the angle from north (0º to 359º) and is often used to calculate the amount of solar radiation received on a surface.

Flow direction: Like slope direction, flow direction is normally calculated from DEMs but it is often calculated as one of only four or eight cardinal directions (N, NE, NW, S, SE, SW, E, W). Thus, in raster-based watershed analyses, each cell is assumed to drain into one of its four- or eight-nearest neighbor cells.

Relative Direction

In textual or verbal representations, directions may be given in an abstract form from a given or named place or in relation to the movement of the observer, for example, “to the right of point X,” “continue straight ahead,” “on the left bank of the river,” or along a path (e.g., “north on Highway 95,” “down the Amazon”) and assume a knowledge of the direction the observer is facing or moving at the time of the observation. In some cases, conventions may apply; for example, the left or right bank of a river always assumes the observer is facing downstream.