Measuring distance is one of the most basic tasks in geodesy. There are different ways to measure distance, as well as a large number of instruments designed to carry out these works. So, let's consider this issue in more detail.
Direct method for measuring distances
If it is required to determine the distance to an object in a straight line and the terrain is available for research, such a simple device for measuring distance as a steel tape measure is used.
Its length is from ten to twenty meters. A cord or wire can also be used, with white markings after two and red after ten meters. If it is necessary to measure curvilinear objects, an old and well-known two-meter wooden compasses (sazhens) or, as it is also called, “Kovylok”, is used. Sometimes it becomes necessary to make preliminary measurements of approximate accuracy. They do this by measuring the distance in steps (based on two steps equal to the growth of the person measuring minus 10 or 20 cm).
Measuring distances on the ground remotely
If the measurement object is located in the line of sight, but in the presence of an insurmountable obstacle that makes it impossible to directly access the object (for example, lakes, rivers, swamps, gorges, etc.), distance measurement is applied by a remote visual method, or rather methods, since there are several varieties of them:
- High precision measurements.
- Low precision or approximate measurements.
The first includes measurements using special instruments, such as optical rangefinders, electromagnetic or radio rangefinders, light or laser rangefinders, ultrasonic rangefinders. The second type of measurement includes such a method as geometric eye measurement. Here is the determination of the distance by the angular magnitude of objects, and the construction of equal right-angled triangles, and the method of direct resection in many other geometric ways. Consider some of the methods of high-precision and approximate measurements.
Optical distance meter
Such measurements of distances to the nearest millimeter are rarely needed in normal practice. After all, neither tourists nor military intelligence officers will carry large and heavy objects with them. They are mainly used in professional surveying and construction work. Often used in this case is a device for measuring distance, such as an optical rangefinder. It can be either with a constant or with a variable parallax angle and be a nozzle for a conventional theodolite.
Measurements are made according tovertical and horizontal measuring rails with a special mounting level. The measurement accuracy of such a rangefinder is quite high, and the error can reach 1:2000. The measurement range is small and is only from 20 to 200-300 meters.
Electromagnetic and laser rangefinders
An electromagnetic distance meter refers to the so-called pulse type devices, the accuracy of their measurement is considered average and can have an error of 1, 2 and up to 2 meters. But on the other hand, these devices have a great advantage over their optical counterparts, as they are optimally suited for determining the distance between moving objects. Their distance units can be calculated in both meters and kilometers, so they are often used when conducting aerial photography.
As for the laser range finder, it is designed to measure not very large distances, has high accuracy and is very compact. This is especially true for modern portable laser tape measure. 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 over the entire length.
Ultrasonic range finder
This is one of the simplest and most convenient devices. It is light and easy to operate and refers to devices that can measure the area and angular coordinates of a separately given point on the ground. Nevertheless, in addition to the obvious advantages, it also has disadvantages. Firstly, due to the short measuring rangedistance units for this device can only be calculated in centimeters and meters - from 0, 3 and up to 20 meters. Also, the measurement accuracy may change slightly, since the speed of sound propagation directly depends on the density of the medium, and, as you know, it cannot be constant. However, this device is great for quick small measurements that do not require high accuracy.
Geometric eye methods for measuring distances
Above we talked about professional ways of measuring distances. And what to do when there is no special distance meter at hand? This is where geometry comes in. For example, if you need to measure the width of a water barrier, then you can build two equilateral right triangles on its shore, as shown in the diagram.
In this case, the width of the river AF will be equal to DE-BF Angles can be adjusted using a compass, a square piece of paper, and even using the same crossed twigs. There shouldn't be any problems here.
You can also measure the distance to the target through an obstacle, using also the geometric method of direct resection, by constructing a right triangle with the apex on the target and dividing it into two scalene ones. There is a way to determine the width of the barrier with a simple blade of grass or thread, or a way with the thumb out…
It is worth considering this method in more detail, as it is the simplest. On the opposite side of the barrier, a noticeable object is selected (you must knowits approximate height), one eye is closed and the raised thumb of the outstretched hand is pointed at the selected object. Then, without removing the finger, close the open eye and open the closed one. The finger turns out to be shifted to the side in relation to the selected object. Based on the estimated height of the object, approximately how many meters the finger visually moved. This distance is multiplied by ten and the result is the approximate width of the barrier. In this case, the person himself acts as a stereophotogrammetric distance meter.
There are many geometric ways to measure distance. To talk about each in detail, it will take a lot of time. But they are all approximate and are only suitable for conditions where accurate measurement with instruments is impossible.
