Essential Guide to Surveying Tribrachs: Functions, Types, and Best Practices

Land surveying is a job that brings results only if you have the proper equipment at hand. If you are working with a dody tripod, you might as well throw away all the measurements you get as precision is key no matter the type of land surveying you are doing.

One of the more underestimated pieces of equipment are surveying tribrachs. Those who are amateur surveyors or have just heard about surveying may think that a tripod is the one essential piece of equipment that you have to have at hand. A surveying tribrach is as essential, if not even more, compared to tripods.

The tripod may be the base, but a survey tribrach provides stability to all the equipment you will be using during a survey and allows you to get the most accurate results as possible.

What is a Tribrach?

A surveying tribrach is a type of plate that helps to attach any kind of surveying equipment to a tripod, including GNSS accessories, total stations, theodolites, etc. A tribrach for surveying will allow you to place any kind of equipment in the same position over a certain marker point. You set it by loosening or tightening a lock which allows to to adjust the instrument’s base along a horizontal plane. It comes with two triangular metal plates, attached to the corners by leveling thumbscrews.

The central part of the bottom plate has a 5 / 8-11 UNC thread hole which is used to attach it to a tripod. The top plate has three holes which are spread at fixed distances. Surveyors have to level the screws and lock the mechanism in place and make sure that it is secured. A tribrach with an optical plummet will have a crosshair sight which comes in handy when positioning the instruments correctly over a certain survey marker.

Older types of tribrachs require a plumb line so they are positioned vertically. They also needed much longer to set. However, some modern types of total stations come with a built-in laser plummet that speeds things up greatly. 

The Job of a Tripod and Tribrach?

A tripod is an essential part of your surveying arsenal built both for accuracy and endurance as well. A proper tripod can withstand a lot, from being carried over various types of rough terrains to simply being thrown in the back of a van. It is constantly being exposed to various elements, whether it is hot or cold, dry or humid, or whatever.

It has to be durable enough to support heavy-duty island rotating instruments and has to absorb vibrations usually caused by passing traffic or heavy machinery, which is where fiberglass tripods come in handy.

On the other hand, a tribrach used in surveying has to endure the same abuse. For that reason, having a surveying tribrach that can withstand all of the elements and situations in the field is crucial. If distorted, it may cause you to get false readings, which will require you to replace it and set a proper one.

Why Have a Tribrach With an Optical Plummet?

A tribrach used in surveying is there to help level you over the backsight point. However, while old types of surveying equipment used plumb bobs to do the same, newer ones come with tribrachs with an adjustable focus, optical plummets, or openings for a laser plummet.

You can use adapters to convert the thread on your existing tripod, or you can simply get a tribrach adapter with an optical plummet. These may be a bit more expensive than tribrachs without an optical plummet, but they help you save a lot of time when you are in the field. 

You can never be sure about the elements or the conditions you are going to be facing in the field. So having a bit of help to level things up goes a long way, and you wouldn’t mind spending a few extra bucks to speed up the setup process and get more accurate results.

How to Use a Tribrach With a Laser Plummet?

Setting up a tribrach in surveying with a laser plummet depends on the horizontal position and the plate level. These two parts are interactive, and to make the necessary tribrach adjustment you will need to do the following:

1. Pick two of the tripod legs and position the crosshairs over the marker, all while looking through the optical plummet. 
2. Secure all three legs on the surface you are working on.
3. Use the thumbscrews to put the crosshairs back over the marker, also while looking through the optical plummet.
4. Adjust the tribrach level. You can do that by adjusting the tripod legs.
5. Re-center the crosshairs with the help of a thumbscrew. Fine-tune by leveling the tripod legs.
6. Make the final adjustments with your thumb to center the tribrachs over the marker.

If the target is at the same elevation as the feet of the tripod, it should be fine. But if you notice a great difference between the target’s elevation and the marker, it will seem that it is moving laterally in the optical plummet. You should also understand that tribrachs are precision instruments, so they need to be checked from time to time. Constant tribrach calibration is needed, even after setting everything once.

Surveying Tribrach FAQs

Here are a few additional answers to the essentials of tribrach use in surveying.

What is a tribrach used for?

A tribrach used in surveying helps to attach various survey instruments including theodolites, total stations, GNSS antennae, or targets.

How do I adjust a tribrach?

You can use the optical plumb sight as a vertical reference to place a plumb over a survey reference point. Constant readjustments during surveys are important to get accurate results.

How do I plumb a tribrach?

Tribrachs usually come with two screws on the optical plumb arm, adjustable according to your needs. One of them adjusts the crosshair holder from left to right, and the other moves it up and down. Turning them simultaneously in opposite directions will cause them to move diagonally. Use the tiny screws to move the crosshair holder until the hairs are centered with the triangle.

What is a laser plummet?

A laser plummet is a device that emits a consistent beam in a vertical direction, helping surveyors get an accurate plumb line.