What is a Tribrach

Blog what is a tribach

A tribrach is a special plate that can be used to attach any type of surveying equipment (including theodolites, total stations, GNSS antennae) to a tripod. It will allow you to place the instrument in the same position over a specific marker point with a much greater level of precision by loosening and tightening a lock that adjusts the instrument base along a horizontal plane. It has two triangular metal plates that are connected at the corners by leveling thumbscrews.

The center of the bottom plate has a 5 / 8-11 UNC thread hole that’s used to attach it to the tripod. The top plate has three small holes or slits that are spaced a fixed distance apart. The leveling screws and locking mechanism must also be carefully secured in place. The optical plummet will have a bullseye or crosshair sight that can be used to position the instrument over a specific survey marker or ground control point. Older tribrachs require a plumb line to be positioned vertically, so they took longer to set up. Some modern total stations also have laser plummets built in.

How to Use a Tribrach with a Laser Plummet

The process of setting up a tribrach with an optical laser plummet depends on horizontal position and plate level, because the two are interactive. To set up a laser tribrach, you should take the following steps:

While looking through the optical plummet, pick up two of the tripod legs to position the crosshairs over the marker.
Secure all three legs into the grass, bitumen or concrete.
Look through the optical plummet, and use the thumbscrews to put the crosshairs back over the marker.
Adjust the tribrach’s level by loosening the tripod screws and sliding the legs.
Re-center the crosshairs with the thumb screws, while fine-tuning the level by adjusting the tripod legs.
Once the tribrachis almost centered over the marker, make any final adjustments with the thumb screws.
Loosen the tribrachfrom the tripod head and slide it about 4-5 mm, so it’s centered over the marker and is level.

If the target is at about the same elevation as the tripod’s feet, this procedure will work fine. But if there’s a big difference between the target’s elevation and the marker or the instrument’s bottom plate (such as when the target is in a deep hole or on a Bilby tower), it will appear to be moving laterally in the optical plummet as the tribrach is being adjusted. Because they’re precision instruments, tribrachs also need to be checked on a regular basis. This will ensure the most precise results.

If you’re looking for a laser tribrach or some other model, you can find what you need at Engineer Supply. We have a broad selection from some of the best manufacturers in the industry, and we can offer them to you at a competitive price. Feel free to look at what we have in stock, so you can find a product that will meet your specific needs.

Frequently Asked Questions

What is a tribrach used for?

Blog what is a tribach

A tribrach is an attachment plate that can be used with a variety of survey instruments. Some of them include but may not be limited to:

Theodolites
Total Stations
GNSS antennae
Targets

By using a tribrach with a laser plummet, you can be sure the instrument you use will be placed in the same position over a specific marker point and with a much greater level of precision.

How do I adjust a tribrach?

If you want to get the most accurate measurements possible, your tribach will need to be adjusted so it’s perfectly level or plumb over a survey reference point. It uses an optical plumb sight that can be used as a vertical reference, but rough field conditions can move it out of alignment. This can lead to inaccurate measurements from your survey instruments. That’s why periodic readjustment is necessary to get the best results, which isn’t hard to do.

How do I plumb a tribrach?

Most tribrachs have two small screws on the optical plumb arm under the instrument, which can be adjusted according to your needs. One of them adjusts the crosshair holder from left to right, while the other one moves it up and down. If you turn them simultaneously in opposite directions, you can move it diagonally. Use the tiny screwdrivers to move the crosshair holder until the hairs are centered on the mark at the center of the triangle. You have the right adjustment if all three sights hit the same plumb spot.

What is a laser plummet?

A laser plummet is an optical device that projects a consistent beam of light or infrared radiation in a vertical direction, which can be helpful if you need to get an accurate plumb line. Feel free to look at the broad selection of tools we have at Engineer Supply.

How do I use a tribrach with a laser plummet?

The optical plummet on a laser tribrach has a bullseye or crosshair sight that can be used to position the instrument over a survey marker or ground control point. This process depends on the relationship between the horizontal position and top plate level, because the two of them are interactive. You can use the optical plummet to adjust the tribrach by adjusting the thumbscrews to put the crosshairs over the marker while adjusting the level of the tribrach and the position of the tripod legs.

If you’re looking for a broad selection of tribrachs and other tools used for surveying and construction, be sure to look at what we have at Engineer Supply.