Leveling

Leveling is an important method of measurement used to determine the height difference, or elevation, between one point on the ground and another. Leveling methods can be divided into two types: direct leveling and indirect leveling.    Direct leveling is measured through a level and a leveling rod. During the measurement process, attention should be paid to maintaining the accuracy of the level and eliminating errors, such as refraction errors, line of sight errors, etc. Indirect leveling is a method of calculating elevation by measuring the length and angle of height differences. Commonly used indirect leveling methods include triangular elevation measurement, elevation difference calculation, gravity leveling, etc. 

Introduction

Leveling, also known as " geometric leveling " , is a method of measuring the height difference between two points on the ground using a level instrument and a leveling rod. Place a level between two points on the ground, observe the level rulers erected at the two points, and calculate the height difference between the two points based on the readings on the rulers. Usually starting from the leveling origin or any known elevation point, the elevation of each point is measured station by station along the selected leveling route. Since the levels of different elevations are not parallel, the height difference between two points measured along different routes will be different. Therefore, when collating the national leveling survey results, necessary corrections must be made according to the normal height system adopted to obtain the correct the elevation.

Measuring instruments

The instrument used for level measurement is a level gauge, and the tools are a level rod and a ruler pad.

Level

According to the principle of level measurement, the main function of the level is to provide a horizontal line of sight and to take readings by pointing it at the level rod.

The level mainly consists of three parts: telescope, level and base.

Level rod

A level rod is a ruler used for leveling measurements. Its quality directly affects the accuracy of level measurement.

Leveling rulers are generally divided into tower rulers , board rulers and indium steel rulers . The tower ruler is retractable. Generally, the length of the tower ruler is 3 meters ( three sections ) , 5 meters ( five sections ), etc .; the board ruler is generally 2 meters or 3 meters .Indium steel rulers are generally 2 meters or 3 meters.

Operation steps

The use of a level includes the steps of instrument placement, rough leveling, aiming the level, fine leveling and reading.

Placement level

Open the tripod and make it at a moderate height. Visually make sure the head of the tripod is roughly level. Check whether the tripod legs are firmly placed and whether the tripod telescopic screw is tightened. Then open the instrument box and take out the level. Place it on the tripod head and secure the instrument firmly with the connecting screw. Attached to tripod head.

Rough leveling

Rough leveling is to use the bubble of the circular level to center the instrument so that the vertical axis of the instrument is roughly vertical, so that the collimation axis is roughly horizontal. During the leveling process, the bubble moves in the same direction as the left thumb.

Aiming at the level

First, perform eyepiece alignment, that is, point the telescope against a bright background, and turn the eyepiece alignment spiral to make the crosshairs clear. Then loosen the brake screw, rotate the telescope, aim the leveling rod with the rear sight and front sight on the telescope tube, and tighten the brake screw. Then observe from the telescope; turn the objective lens to align the light spiral to make the target clear, then turn the micro-motion spiral to align the vertical wire with the level rod.

When the eye moves slightly up and down at the eyepiece end, if there is relative movement between the crosshairs and the target image, this phenomenon is called parallax. The reason for the parallax is that the target imaging plane and the crosshair plane do not coincide. Since the existence of parallax will affect the accuracy of readings, it must be eliminated. The way to eliminate it is to re-align the objective lens carefully until the eye moves up and down and the reading remains unchanged. At this time, the images of the crosshair and the target are very clear from the eyepiece end.

Precision leveling and reading

Look at the bubble of the level tube with your eyes through the matching bubble observation window on the left side of the eyepiece. Turn the slightly tilted screw with your right hand to make the images at both ends of the bubble match, which means that the sighting axis of the level is accurately level. At this time, you can use the middle wire of the cross hair to read on the ruler. Levels mostly use inverted telescopes, so when reading, you should read from small to large, that is, from top to bottom. First estimate the number of millimeters, and then report the entire reading. Although leveling and reading are two different operating steps, during the implementation of level measurement, the two operations are regarded as a whole; that is, reading is taken after leveling, and after reading, it is also necessary to check whether the level bubbles of the tube are completely consistent. Only in this way can accurate readings be obtained