Distance meter on the ground. Methods of measuring distance

Measurement of distance is one of the most basic tasks in geodesy. There are different ways of measuring the distance, as well as a large number of instruments designed to carry out these works. So, consider this issue in more detail.

Direct method of measuring distances

If you want to determine the distance to the object along a straight line and the terrain is accessible for research, such a simple distance measuring device as a steel measuring tape is used. Its length is from ten to twenty meters. Another can be a cord or wire, with white designations in two and red in ten meters. If it is necessary to measure curvilinear objects, the old and well-known two-meter long wooden compass (sazhen) or, as it is called, the "Kovylek", is used. Sometimes it becomes necessary to make preliminary measurements of approximate accuracy. Do this by measuring the distance in steps (two steps are equal to the growth of the measuring minus 10 or 20 cm).

Measuring distances on site remotely

In the case of finding the object of measurement in the line of sight, but in the presence of an insurmountable barrier that makes direct access to the object impossible (for example, lakes, streams, swamps, ravines, etc.), remote sensing is used remotely by a visual method, or rather methods, as there is Their several varieties:

1. High-precision measurements.
2. Low-current or approximate measurements.

The first include measurements using special instruments, such as optical range finders, electromagnetic or radio range meters, light or laser range finders, ultrasonic range finders. The second type of measurement is a method such as a geometric eye measurement. Here and the definition of the distance in the angular size of objects, and the construction of equal rectangular triangles, and the method of direct intersection in many other geometric ways. Let's consider some of the methods of high-precision and approximate measurements.

Optical distance meter

Such measurements of distances to within a millimeter in ordinary practice are infrequent. After all, neither tourists, nor military scouts will carry with them overall and heavy objects. They are mainly used for professional geodetic and construction work. Often, such a device is used to measure distance, as an optical range finder. It can be either a constant or a variable parallactic angle and represent a nozzle to a conventional theodolite.

Measurements are made on vertical and horizontal measuring rails having a special setting level. The accuracy of the measurement of such a range finder is quite high, and the error can reach a value of 1: 2000. The range of measurement is small and is only 20 to 200-300 meters.

Electromagnetic and laser rangefinders

The electromagnetic distance meter refers to so-called pulse type instruments, the accuracy of their measurement is considered average and can have an error of 1.2 and up to 2 meters. But these devices have a great advantage over their optical counterparts, since they are optimally suited for determining the distance between moving objects. Units of distance can be calculated in both meters and kilometers, so they are often used in aerial photography.

As for the laser range finder, it is designed to measure not very long distances, has high accuracy and is very compact. This is especially true for modern portable laser roulettes. These devices measure the distance to objects at a distance of 20-30 meters and up to 200 meters, with an error of no more than 2-2.5 mm on the entire length.

Ultrasonic Rangefinder

This is one of the simplest and most convenient devices. It is easy and simple to operate and refers to devices that can measure the area and angular coordinates of a separately specified point on the terrain. Nevertheless, in addition to the obvious advantages, he also has disadvantages. Firstly, because of the short range of measurement, the distance unit of this device can be calculated only in centimeters and meters - from 0.3 to 20 meters. Also, the accuracy of the measurement may vary slightly, since the speed of sound transmission directly depends on the density of the medium, and it, as is known, can not be constant. Nevertheless, this device is excellent for fast small measurements that do not require high accuracy.

Geometric visual methods of measuring distances

Above we talked about professional ways of measuring distances. And what to do when there is no special distance meter at hand? Here, geometry comes to the rescue. For example, if it is necessary to measure the width of a water obstacle, then it is possible to construct on its shore two equilateral rectangular triangles, as shown in the diagram.

In this case, the width of the river AF will be equal to DE-BF. The angles can be verified with a compass, a square piece of paper and even with the same crossed twigs. There should not be problems here.

You can also measure the distance to the target through the obstacle, using also the geometric method of direct resection, constructing a rectangular triangle with a vertex on the target and dividing it into two versatile ones. There is a way to determine the width of a hedge by using a simple blade of grass or a thread, or a method using an exposed thumb ...

It is worth considering this method in more detail, since it is the simplest. On the opposite side of the barrier, an observable object is selected (it is necessary to know its approximate height), one eye is closed and the raised thumb of the outstretched hand is induced on the selected object. Then, without removing the finger, close the open eye and open the closed one. The finger is obtained relative to the selected object is shifted to the side. Based on the expected height of the object, it is approximately represented by how many meters the finger has visually moved. This distance is multiplied by ten and the result is an approximate width of the barrier. In this case, the person himself acts as a stereophotogrammetric distance meter.

Geometric methods of measuring distance are many. It would take a lot of time to describe everything in detail. But they are all approximate and are only suitable for conditions where accurate measurement with instruments is impossible.