Leveling measurement is mainly used to measure the height difference between various points, that is, to control the elevation of each control point. Leveling generally adds turning points between control points as needed. In the project, we can branch out from the control points to control the elevation of a certain part of the project.
Leveling uses a horizontal line of sight and a leveling rod to measure the height difference between two points on the ground, so that the height of an unknown point can be calculated from the height of a known point. Leveling measurement usually starts from the leveling origin or any known elevation point, and measures the elevation of each point 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.
1. Principle of leveling
Leveling is to directly measure the height difference between two points on the ground by using the horizontal line of sight provided by the level, with the help of a level ruler with divisions. Then based on the known point elevation and the measured height difference, the unknown point elevation is calculated.
As shown in the figure, erect a level ruler at two points A and B on the ground. Use the horizontal line of sight provided by the level instrument to intercept the readings a and b on the ruler. Then, the height difference between points A and B hAB is:
The height difference is equal to the backsight reading minus the frontsight reading.
2. Calculate the height of the unknown point
1. Height difference method
Height difference method—a method that directly uses the height difference to calculate the height of unknown point B.
After measuring the height difference hAB between points A and B, if the height HA of point A is known,
The height HB of point B is: HB=HA hAB
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2． Instrument height method
Instrument height method—a method of calculating the elevation of unknown point B using the height Hi of the instrument's line of sight.
As shown in the figure, the elevation of point B can also be calculated by the sight elevation Hi of the level,
that is:
Hi= HA a
HB= Hi- b
In construction surveying, sometimes an instrument is installed once and the elevation of multiple ground points needs to be measured. It is more convenient to use the instrument height method.
3. Intermediate method
When the point B to be measured is far away from the known point A, it is impossible to measure their height difference by placing the instrument once. At this time, choose a measurement route between A and B. Add some turning points, measure one station at every two adjacent points, and find their height differences. Then the height difference of AB is the sum of these height differences.
Transition point: Temporary ruler point, used as a transition point to transfer elevation. (Generally, a ruler pad is required at the turning point)
Measuring station: Each time an instrument is placed, it is called a measuring station.
For high-precision measurements, the instrument must be placed at roughly equidistant distances between the front and rear viewpoints, and the principle of equidistant effects of the curvature of the earth is used to automatically eliminate the effects of curvature on the front and rear sight points in the calculation of the height difference of the measuring station. Depending on the influence of the readings, this method is called the intermediate method and is a commonly used method in precision measurement.