If suitable, the tower bolt can also take on the function of a fixed height point (HFP). It consists of wrought iron, brass or steel and is installed horizontally in the masonry of a church tower or other stable building.
The classic securing of high points has become largely dispensable through the use of satellite-based methods (GNSS) in surveying. The measurements at high points that were previously often required for indirect derivation of the centering elements (laying down) are complex and depend on the local conditions. With the laying down, the inaccessible high station points on the building (such as button , helmet pole , etc.) are to be geometrically related to the safety points on the ground. The laying-down figure, in particular the base line of the viewing triangles, must be chosen so that the high point - which is often subject to changes - can be checked in its position by a local measurement from the ground and, if necessary, re-coordinated.
The tower bolts have a diameter of 2-3 cm and are about 20 cm long. They have ring-shaped thickenings on the walled-in shaft in order to remain permanently and unchanged in the masonry or mortar joint .
On the front, which protrudes a few centimeters from the wall, the bolt is thickened like a knob and has a round, cylindrical contact surface with a highest point for the measuring stick of a leveling device . In the middle of the bolt head, in the sense of the original function of a tower bolt, a 1–2 mm hole is usually made a few millimeters deep. During the measurements, this hole accommodates a vertically positioned needle as a target for angle measurements with a theodolite.
Since the end of the 19th century, public buildings and churches have been preferred since the end of the 19th century to ensure the long-term securing of the measurement results for position and height control points , which at the same time have a long lifespan and no longer have any subsidence in the subsoil.
In addition to the position coordinates in the UTM coordinate system or the meanwhile historical Gauß-Krüger coordinate system, the surveying office usually also has height information above normal height zero .
- Decree on the trigonometric fixed point field (TP decree), JOINT MINISTERIAL SHEET SAARLAND, January 15, 1969
- Drawing regulations for surveying cracks in North Rhine-Westphalia (drawing regulations Riss NRW) RdErl. D. Ministry of the Interior - III C 4 - 7120 BC 06/06/1997
- Overview of the control point data kept in AFIS in Baden-Württemberg (PDF)
- News from the Lower Saxony Surveying and Cadastre Administration, 1985, Issue 2 (PDF)