What Is Tacheometry?
Tacheometry is a method of measuring both 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 surveying techniques that involve taping, pacing, or odometers are also not used. It is considered less accurate than the most modern methods of surveying, but it is still of practical value in topographic mapping for regions that don't have access to high technology.
There are several different types of system, including the stadia, subtense bar, and optical wedge systems. The stadia tacheometry method is the most commonly used, however, and it incorporates a theodolite controlled by one operator and a level staff with precise, measured markings on it held by another surveyor at a distance. The theodolite is essentially a custom telescope with horizontal and vertical cross hairs. It is pointed at the staff, and vertical and horizontal angles are displayed in relation to markings on the staff, which determines distance and elevation. The two horizontal markings on the theodolite are known as stadia hairs, which are an equal distance above and below a horizontal line, and they cross a central vertical cross hair line.
Theodolites used in this process have varying levels of sophistication. The first types made in the early 19th century had fixed stadia hairs and an ability to flip over and sight in the reverse direction, so that a point of reference 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. Early ones were referred to as transit instruments and are still used for basic topographic mapping and quick measure uses, such as in archeology and geology, where precise measurements of distance and height are not required.
One of the advantages of tacheometry is that it is a rapid surveying method, and, if a basic theodolite is used, the equipment is fairly lightweight and easy to take into the field. It requires only two operators, one to hold the leveling rod with the stadia hair markings and one to measure it with the theodolite from a distance. The accuracy of the measured distances decreases as the distance between the leveling staff and theodolite increases. At a range of a quarter-mile (402 meters), the process is considered quite accurate, and, at a distance of 1 mile (1,609 meters), the error in horizontal distance is around 32 feet (9.75 meters) and 4 inches (10.16 centimeters) vertically.
It is alternative tool for surveying and reduces the cumbersome procedure of field measurement.
I think tacheometry method is less accurate because of the observation of angels.
No matter which instrument we use, the tacheometric method of surveying is the most interesting part of surveying which integrates all types of measurements. The end product - the topographic plan of an area -- is the most important product to the surveyor and customer.
I hope no one actually relies on tacheometry when measuring from a mile away! Thirty-two feet is quite a large distance to be off by, and I can't see how measuring from this far away would do any good to anyone.
My dad used to do roadside surveys back in the seventies and early eighties, and he used to use tacheometry for his surveys. It was all very basic and much more involved than today's methods.
Let's just say he had to be really good at math. There was a lot of trigonometry involved.
Nowadays, the GPS makes it so much simpler. Of course, it is probably a lot more on point, so that is another advantage.
@OeKc05 – Those guys are using equipment that features a GPS, so it is definitely more accurate than tacheometry. I think that tacheometry is just used when no other equipment is available.
It may have been great years ago, before we had the technology we have today, but I really haven't seen anyone out surveying land with a theodolite lately. This might be the main technique used in poorer countries, but I haven't heard of it being used around here.
Is tacheometric surveying less accurate than what I see those guys on the side of the highway doing? They have some sort of instrument on a stand, and one of them is standing kind of far away from the other. Both are adjusting some sort of knob and looking through something.
I know that they are doing some sort of survey, but I don't know what it is called. It does look a bit more high-tech than a theodolite, though.
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