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Tribrach Basics: Components and General Characteristics

Leica-Tribrach

Surveying tribrachs are important accessories for various applications in surveying. A tribrach is an attachment plate used to afix a surveying instrument – like a total station or GNSS antenna, for example – to a tripod so that the instrument can be moved from place to place without affecting the submillimeter precision. How does it work, and what do you need to know before using one?

Tribrach Components

Typically, a tribrach consists of two triangular metal plates connected at the corners by leveling footcrews, a bubble level, a locking mechanism, and an optical plummet. The base plate and upper plate are usually connected with three studs. Below is a cross section of a modern tribrach.

cross section of a modern tribrach

By turning the footscrews, the upper tribrach plate can be moved in relation to the base plate. By turning the footscrews differently, the upper tribrach plate can achieve an angle tilt compared to the base plate of about 10 degrees.

Leveling Your Instrument

To accurately level your instrument, you need a horizontal plane projected through the supporting pins on the upper tribach plate. Ideally, this will be parallel to the reference plane for your bubble, or at least within the tolerance of the bubble specification of the circular level. The red lines in the above figure show the upper and lower planes. The image below shows supporting pins on the upper tribach plate.

supporting pins on the upper tribach plate

Circular levels are generally specified to the level plane. For example, if a tribrach is specified with 8’/2 mm, this means that a tilt of the level plane of 8 feet moves the bubble 2 millimeters. The bubble is adjustable with three Allen screws so that you can ensure it remains well calibrated (indicated by the centrally printed circle on the bubble glass below).

a circular bubble level on a tribrach

Before you begin:

  • Make sure the instrument is calibrated.
  • Use the digital bubble to level your instrument. Turn the instrument 180 degrees to ensure that the digital (or analog) bubble is centered.
  • Adjust the circular level of the tribrach using the adjusting pin, which is typically provided in the instrument container itself or with the tribrach.

If you need help in choosing or using your tribrach, please contact us. Our knowledgeable experts would be glad to assist you.