Horizontal direction
A horizontal direction is the leg of a horizontal angle . In geodesy , horizontal directions are measured very precisely with a theodolite or tachymeter .
The measurement is carried out on a horizontally mounted pitch circle that contains an angular division (e.g. 400 gon or 360 °). Regardless of the zero point of this division, an angular value is read off the pitch circle for each telescope aiming. The prerequisite for correct results is that the pitch circle is recorded during the entire measurement, i.e. This means that the zero point does not change.
The further use of the horizontal directions depends on the task at hand:
- An angle can be calculated from the difference between two horizontal directions (e.g. for the reverse cut )
- By comparing the horizontal direction with direction angles in a coordinate system , the orientation unknown can be calculated, that is, the direction of the zero point on the pitch circle related to north ( block orientation ).
The horizontal directions that have been determined in the second telescope position with a theodolite or tachymeter to one and the same target point in the first telescope position differ by exactly 200 gon.
If there are device errors ( sighting axis errors , tilting axis errors ), the difference between the measured values determined in the two telescope positions deviates from 200 gon. The device errors falsify the measured horizontal directions with opposite signs in the telescope positions, so that the mean value of the measured values in both telescope positions is error-free, taking into account the difference of 200 gon.