# Direction measurement

The direction of measurement is a fundamental task of geodesy , geometry , navigation and other areas of technology . It consists of an oriented (mostly geographically north related) angle measurement to determine horizontal angles , oriented measuring beams or courses in a defined reference system .

Directions and distances are the most important measured values when solving geometric tasks, above all

In nautical science and hiking, some quick, less precise methods of direction measurement are mentioned as bearing . They are mainly done with a magnetic compass .

## Reference directions

Directional measurements require a reference direction that gives orientation to the mere angular measurement . Reference directions can be absolute or relative .

Absolute reference directions:

• geographic north = astronomical north: the horizontal direction that leads to the geographic North Pole . The result of a direction measurement related to geographic / astronomical north is called azimuth , a true course in navigation and a beam oriented in image measurement.
• magnetic north: Horizontal projection of the field lines of the magnetic field of the earth (or a celestial body ). The result is called magnetic direction compass -Peilung, miss-setting o course. Ä.
• Roundabout north: using a north-looking centrifugal instrument or a gyro-stabilized platform. The results of such direction measurements are called gyroscopic bearings, gyroscopic course, or the like.
• Grid north: North direction in a coordinate system or a map . With this reference, so-called directional angles result , which are mainly used in geodesy.
• Plumb line : the local vertical in the earth's gravity field . Angles related to the perpendicular are called the zenith angle or zenith distance, the extension to 90 ° is called the elevation angle .

Relative reference directions:

In a surveying network ( triangulation ), directions oriented “north” mostly relate to a reference ellipsoid . Such calculated azimuths are called ellipsoidal - in contrast to the measured astronomical azimuth (see above).

## Measuring instruments

Directional measurements are made depending on the accuracy and subject area with:

automatically looking north
relative direction

The accuracies are 1–10 ° (sports, navigation), hundredths of a degree up to 1 "(surveying) and 0.1" to 0.01 "(satellites, vertical deviation , astronomy ).

Side bearing to a light barrel

In nautical science , direction measurements with respect to the ship's axis are also common, they are called side bearings . If you add the course , you get the direction true (no. 1) or negative (no. 2).

Such “measurements” are also made intuitively as a pedestrian or cyclist. In particular, a stationary bearing as a location indicates the risk of a collision; if this should not be counted better for human directional hearing, for which the localization is important. The spatial position is judged relative to the vertical direction with the sense of balance for degree accuracy.