Filament rangefinders: purpose, device, rules of use

There are many types of measuring (rangefinder) equipment. A filament rangefinder is present in almost any theodolite model. Thanks to him, such an additional option as determining the distance is realized.

The need to measure the distance with a theodolite arises when a tacheometric or horizontal survey is performed. A filament rangefinder is a pair of rangefinder filaments. The procedure is roughly as follows:

• first, the height of the device (theodolite) is set in relation to the stationary point;
• then the leveling rod is mounted at the place to which you want to measure the distance;
• direct the pipe to a reading close to the height of the apparatus itself;
• take readings on two ranging lines (top and bottom);
• determine the value of the rangefinder reading according to a special formula that takes into account the coefficient, the difference in the readings on the staff;
• enter the obtained result into the log of the results of the tacheometric survey.

The next step is to set the horizontal position. For this, in the process of office processing of the results, a different formula is used, which takes into account the angle of inclination of the sighting beams. To simplify the work, using a theodolite with an inverse display, the rangefinder filament located on top is oriented to a close value (in decimeters).

A filament rangefinder, which allows measuring line lengths, is present in the overwhelming majority of models of geodetic equipment. The network includes a pair of main ranging lines. Their projection through the telescope forms a parallax angle. In this case, the distance separating the ranging lines and the focus of the lens are of great practical importance. To measure the distance, use strips with a centimeter scale.

First, a count is taken showing the number of centimeters visible through the telescope separating the projections of the filaments. The rangefinder coefficient is taken equal to 100. Judging by the available information, the accuracy of optical filament rangefinders is about 1: 400 (0.25%) of the measured distance. For a more accurate measurement of long lines, it is advised to divide them into segments of 50-100 m. With this approach, the error is reduced by 1.5-2.5 times.

Most often, the parallax angle is constant. In this case, in order to determine the distance between two points using the rangefinder, you need to add:

• the gap from the edge of the focus to the staff;
• focal length;
• the distance between the lens and the theodolite's torsion axis.

The so-called constant term of the rangefinder is rigidly and unambiguously specified in any design. Its size is several centimeters; the exact figure is given in the data sheet of the rangefinder. When measuring large distances or low accuracy requirements, the constant term can be ignored. A consequence of the theory of the filament rangefinder is that during the measurement the staff should be normal to the line of the sight. When measuring the slope distance, the visible section of the staff is replaced by another section.

When, due to obstacles (reservoirs, pits, buildings), the distance cannot be measured with a tape, it is determined by an indirect method.Be sure to carry out a control measurement, building an additional triangle on the basis, and then, if there are no excessively large discrepancies, you need to calculate the arithmetic mean. Nityanaya, like any other rangefinder, works by “solving” a special long isosceles triangle AMN.

Measurement of the distance in devices with a constant base and a changing angle is carried out taking into account the radian, painted in arc seconds. But more often they use rangefinders with a stable angle and a changing base. If internal focusing is provided, the focal length is changed by moving the focusing component. In this case, the formula for determining the distance is used, including the coefficient, the result of the rangefinder reading on the staff and the correction. The correction level is selected empirically, using a horizontal basis up to 150 m long.

This distance is divided into segments of 10 m. To compensate at least partially for the effect of vertical refraction, horizontal slats are used. Then you will have to place the ranging filaments horizontally (in relation to the pipe grid). The correction for bringing the line to the horizon is determined taking into account the slope of the horizon line. The filament rangefinder allows you to measure lines with a maximum length of 300 m, while the error can reach 0.3%.

It may seem that this value is too high. But in fact, for topographic and geodetic surveys, such an error is quite acceptable. You can use a filament rangefinder to solve a number of other problems arising in engineering geodesy. Important: sometimes the generally accepted coefficient of 100 for such a device turns out to be incorrect and does not give good results. In this case, the real exact factor is calculated by dividing the focal length by the interval from one to the other ranging filament.

Some filament rangefinders include checker bars with centimeter divisions. When the light rays, leaving the rangefinder filaments, pass through the lens to the front focus, they hit the staff at two points. A factor of 100 is convenient when the parallax angle is 34.38 degrees.

In the next video, you will measure the horizontal angle with a theodolite.