Method
Measurement Ruler Distance
Measurement ruler directly measures the distance between two points, which is divided into steel ruler measurement distance and Invar baseline measurement distance. The steel ruler is made of thin steel strip and is 20 meters, 30 meters or 50 meters long. When the measured distance is greater than the length of the ruler, it is necessary to calibrate the straight line first and then measure in sections. The accuracy of steel ruler measuring distance is generally higher than 1/1000. Invar baseline ruler is a linear ruler or strip ruler made of Invar alloy steel with a small temperature expansion coefficient. The commonly used linear ruler has a length of 24 meters and a steel wire diameter of 1.65 mm. The two ends of the ruler are connected to a reticle with a millimeter scale, and the graduation of the reticle is 80 mm. When measuring the distance, use a 10 kg weight to pull the ruler through the pulley to make the ruler into a catenary shape. Working length. The Invar baseline ruler is minimally affected by temperature changes, and the measuring distance accuracy is as high as 1/1000000. It is mainly used to measure the baseline of the triangulation network and other high-precision side lengths.
Line-of-sight measurement
A method of measuring the distance between two points according to the principles of optics and trigonometry using a measuring instrument with a line-of-sight device. Commonly used theodolite, plate meter, level and scaled ruler for measurement. Observe its position on the vertical ruler through the two line-of-sight wires of the telescope. The interval between the line-of-sight wires on the ruler is called the ruler interval or the reading of the line of sight. The distance between the instrument and the ruler is a function of the ruler interval. For most instruments, the ratio of distance to ruler interval is 100 when designing. The accuracy of line-of-sight measurement can reach 1/300～1/400.
Ranging by parallax method
Use theodolite to measure the horizontal angle of the fixed-length baseline horizontal ruler, and use the triangle formula to calculate the horizontal distance between the instrument and the baseline. This horizontal angle is called the parallax angle. The distance between the fixed marks at both ends of the baseline horizontal ruler is generally 2 meters. The ruler is equipped with a level and a sight so that the horizontal ruler is placed horizontally and the ruler surface is perpendicular to the survey line. The accuracy of the parallax method is low.
Electromagnetic wave distance measurement
In the 1940s, electromagnetic wave distance finder appeared. Using it to measure distance, the measurement range is long, the distance measurement accuracy is high, and the work efficiency is high. Therefore, electromagnetic wave measurement Distance has become an ideal method of measuring distance.
Measuring device
Dual-image line-of-sight device
Dual-image line-of-sight device is a high-precision line-of-sight measurement Device. Can be used for low-level wire measurement. The ruler is optically used to form a double image in the telescope's field of view. The distance between the two images is the distance between the rulers. Since the two ends of the ruler interval are scored and close together, the ruler interval can be measured more accurately with the addition of a micrometer device. In order to reduce the influence of atmospheric refraction, the ruler is mostly changed from vertical to horizontal. The accuracy of measuring distance with dual image tachymeter is higher, up to 1/2000. There are dedicated dual-image tachymeters, but they are more used as additional tachymeter devices for other measuring instruments. The optical wedge dual-image line-of-sight device is an optical wedge (Figure 2), which can be installed in front of the objective lens of the telescope of other measuring instruments to cover a part of the objective lens. Place a horizontal or vertical ruler at the other end of the survey line. In the telescope field of view, you can see the conformation of the ruler through the objective lens through the wedge and the conformation of the ruler directly through the objective lens without passing through the wedge. The optical wedge deflects the light by an angle 嗘, thereby measuring the length l on the ruler. The value of 嗘 is determined according to the requirement that the line-of-sight multiplying constant is equal to an integer such as 100 or 200 during design. Some dual-image tachymeters have automatic reduction performance, which can directly measure the horizontal distance. Some dual-image tachymeters use a fixed-length ruler, and the angle of light deflection 嗘 is a variable value. Use an optical method to measure the 嗘 angle that changes with the distance, and then press The trigonometric formula finds the distance.
Tachymeter without a ruler
A tachometer that can measure the distance without placing a ruler on the point to be fixed. The angle value he and the baseline length l necessary for measuring the distance are all obtained on the instrument. In some non-scale tachymeters, the 嗘 angle is a fixed value, and the baseline length varies with the distance to be measured (Figure 3). This scaleless tachymeter is mainly composed of a baseline ruler, a fixed pentagonal prism, an optical wedge, a movable pentagonal prism with an index line, and a telescope. When measuring the distance, select a target on the to-be-fixed point, and enter the objective lens for imaging after being refracted by the optical wedge. At the same time, it enters the objective lens for imaging without being refracted by the optical wedge. Moving the movable pentagonal prism can make the two images of the target coincide in the field of view of the telescope. At this time, the length of the index line is intercepted on the baseline ruler l, multiplied by the line-of-sight multiplied by a constant to calculate the distance. In other instruments, the baseline length l is fixed, and the 嗘 angle changes with distance. The accuracy of measuring distance with a rulerless tachymeter is poor, but it is very convenient to measure the distance from the station to the top of the mountain and cliffs and other difficult-to-climb places, and it can be used for topographic mapping in areas with large ups and downs.