Heart screw
In surveying, the heart screw or stem screw is the powerful central screw with which a theodolite or a leveling instrument is attached to the surveying tripod from below.
In order to be able to use an optical plummet , the heart screw is usually hollow. A wire hanger for centered suspension of a plumb line is also available.
The centering of the measuring device over the measuring point is done by moving the instrument slightly on the tripod plate after the heart screw has been loosened a little. Then it is tightened firmly.
The standard thread in surveying is an inch 5/8 Whitworth thread (11 turns per inch, which corresponds to a pitch of 2.3 mm, flank angle 55 °). At 15.6 mm, the diameter is close to the metric thread M16 × 2 (15.8 mm).
Some military surveying instruments actually have M16 × 2, which is why such devices or tripods can only be used to a limited extent (the screw jams after one turn).
In contrast to this, Russian surveying instruments use the M16 × 1.5 thread.