optical plummet

optical plummet

A device on some transits and theodolites; used to center the instrument over a point, in place of a plumb bob, which moves in a strong wind.
McGraw-Hill Dictionary of Architecture and Construction. Copyright © 2003 by McGraw-Hill Companies, Inc.
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The NETH503 5" Theodolite has a 30x telescope with illumination, 360 degree full transit, digital reading of both horizontal and vertical angles, dual-side LCD display, photoelectric incremental encoder, optical plummet, alkaline or rechargeable battery pack selectable.
Instruments are centered over the vertex by suspending a plumb bob beneath the instrument or by using an optical plummet. For accurate surveys, the plumb bob suspended beneath the instrument or the optical plummet must be aligned with a surveyors nail in a stake or a surveyors nail driven into an asphalt or concrete surface.
Procedures for Centering an Instrument The procedures for centering an instrument over a point will vary depending on whether the instrument uses a plumb bob or an optical plummet. A common feature of instruments that are designed to measure horizontal angles is the ability to make a slight lateral movement after the instrument is mounted on the tripod.
TIP Although the optical plummet will be used to center the instrument, a plumb bob can be used to center the tripod.
The last step is to secure the instrument, level it with the spirit level mounted on the instrument and check the optical plummet. If the instrument is not centered, it is loosened and centered again.
For high-accuracy work, the RFP should always specify that the optical plummets and bubble levels of the tribrachs, which hold the antennas on the tripods, be checked before and after each day of observations to ensure proper calibration.

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