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Tacheometry is a surveying method that measures distance and elevation without advanced technology. The stadia tacheometry method is commonly used with a theodolite and leveled rod. The equipment is light and requires two operators, but accuracy decreases with distance.
Tacheometry is a method of measuring both the horizontal distance and vertical elevation of a point in the distance, without the use of sophisticated technology, such as electronic distance measurement (EDM) or satellite transmissions. Traditional sensing techniques involving logging, pacing, or odometers aren’t used either. It is considered less accurate than more modern surveying methods, but is still of practical value in topographic mapping for regions without access to high technology.
There are several types of systems, including stages, subtension rods, and optical wedge systems. The stadia tacheometry method is the most commonly used, however, and incorporates an operator-controlled theodolite and a leveled rod with precise, measured marks on it held by another surveyor at a distance. The theodolite is essentially a customized telescope with horizontal and vertical reticles. It is pointed at the staff, and vertical and horizontal angles are displayed in relation to the staff marks, which determine distance and elevation. The two horizontal marks on the theodolite are known as stadia hairs, which are equal distances above and below a horizontal line and cross a vertical center line in a cross shape.
The theodolites used in this process have varying levels of sophistication. The first types made in the early 19th century had fixed stadia hair and the ability to flip over and face in the opposite direction, so that a reference point could be established to reduce measurement errors. Some newer theodolites have movable horizontal stadia hairs and their position can be measured with a micrometer for more accurate horizontal and vertical sighting. The former were referred to as transit instruments and are still used for basic topographic mapping and quick measurement uses, such as in archeology and geology, where precise distance and height measurements are not required.
One of the advantages of tacheometry is that it is a fast surveying method, and if you are using a basic theodolite, the equipment is quite light and easy to carry in the field. It requires only two operators, one to hold the grader rod with stadium hair marks and one to measure it with the remote theodolite. The accuracy of measured distances decreases as the distance between the leveling staff and the theodolite increases. At a distance of one-quarter mile (402 meters), the process is considered fairly accurate, and at a distance of 1 mile (1.609 meters), the error in horizontal distance is approximately 32 feet (9.75 meters) 4 inches ( 10.16cm) vertically.
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