Click [Big Buffalo Surveying and Mapping] above to follow the concept of our measurement control points. Measurement control points refer to a series of points laid out within the area to be measured before the measurement operation is carried out to complete the measurement oper

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The concept of measurement control points

Measurement control points refer to a series of points laid out within the area to be measured before the measurement operation is carried out to complete the measurement operation of the entire area. In short, control points have accurate coordinate positions, and instruments can be used to deduce the coordinates or elevations of other features to draw maps or carry out engineering construction.

In engineering surveying, control points are divided into plane control points and elevation control points. Plane control points are control points with known plane coordinates, and elevation control points are control points with known elevation coordinates. In actual measurement, we establish the coordinate system in a space, and the position of the point in the coordinate system is represented by (X, Y, Z). (X, Y) represents the position of the point in the plane of this space (vertical and vertical), and Z represents the position of the point in the elevation (vertical) of this space. Therefore, plane control points are represented by (X, Y), and elevation control points are represented by (Z). Plane control points and elevation control points can overlap, referred to as control points, represented by (X, Y, Z) (X, Y, Z represent data), and can be selected according to different measurement requirements.

Measurement control points Arrangement principles of control points

, plane When selecting control points

, first investigate and collect the existing topographic maps of the survey area and the results of control points. Generally, the control network is designed on the topographic map of scale (1:10000-1:1000000). Based on the existing national control points in the survey area or the available control points established by other engineering departments near the survey area, determine the plan for joint testing with them and the location of the control network. After the network layout plan is initially determined, the accuracy of the control network can be estimated, and the initial control points can be adjusted if necessary. Then go to the field to explore, check, modify and implement the points. If the starting edge is to be determined, the position of the starting edge should be given priority. If there is no previous topographic data in the survey area, the site needs to be surveyed in detail. Based on the distribution of known control points, terrain conditions, surveying and construction needs, etc., the location of the conductor points should be reasonably planned and signs established.

highway plane control network should meet the following requirements.

(1) Adjacent wire points must be visible. For steel ruler measuring wires, the terrain between adjacent points must be flat to facilitate measurement of side lengths.

(2) The conductor point should be selected in a place with hard and stable soil to facilitate marking of the point and placement of instruments.

(3) The conductor point should be selected in a place with higher terrain and a wide view to facilitate encryption, expansion, search, fragment measurement and construction stakeout.

. Elevation control points

Elevation control points are usually established by the leveling method and become level points. The selection of leveling point should meet the following requirements:

(1) The leveling point should be selected in a solid and stable place that can be stored for a long time, convenient for testing.

(2) horizontal route should be laid out along the road with a small slope as much as possible, and try to avoid crossing rivers, lakes, swamps and other obstacles.

(3) When selecting the leveling point, further encryption of the elevation control network should be taken into consideration.

(4) should be considered to facilitate joint testing at national benchmarks.

(5) The horizontal network should be laid out as a supporting route, node network or ring network.

(6) For special leveling points for highway projects, they should be selected within the range of 50-300m from the center line on both sides of the highway route. The distance between leveling points is generally 1-15km. Mountainous and hilly areas can be appropriately dense; leveling points should also be added on both sides of bridges, both ends of tunnels, passes and other large structures.

In actual pipeline measurement work, using RTK for measurement requires establishing a GPS control network. In order to enable the GPS control network to obtain more accurate conversion parameters during constrained adjustment, the number of control points should be selected based on the actual situation. To measure horizontal coordinates (X, Y), elevation coordinates (Z) are not required and at least two control points must be used. If both horizontal coordinates (X, Y) and elevation coordinates (Z) are required, at least three control points must be used. The reason is that when performing four-parameter calculations, at least two sets of coordinate system coordinates of two control points need to be involved in the calculation to meet the minimum control requirements; when performing elevation fitting, three points need to be used to participate in the elevation calculation, and the control point coordinate library is used for weighted averaging.

The control points should be evenly distributed within the scope of the project measurement area. It is best to distribute them on the edge of the entire operation area to completely surround the measurement area. If it cannot be completely surrounded, the solution can be solved by appropriately adjusting the position of the control points and adding new control points, and avoid the short side controlling the long side. The scope of the measurement area is approximately square, as shown in Figure 2-1. The control points are distributed at nearly 120° with the geometric center of gravity of the measurement area as the center. The scope of the measurement area is approximately strip-shaped, as shown in Figure 2-2. The two control points should be distributed on both sides of the long side, and the third point is located in the center. The network type designed in this way is conducive to more accurately calculating the coordinate conversion parameters of the measurement area, and is also conducive to the expansion and connection of the control network in the measurement area in the future. The control network should be based on edge connections and supplemented by point connections, which can reduce the workload.

Figure 2-1 Square survey area control point layout

Figure 2-2 Strip survey area control point distribution map Calculation and use of control points

, Calculation of control points

The work of determining the plane position of the control points is called plane control survey ; the work of determining the elevation of the control points is called elevation control survey .

(1) Plane control measurement

The principle of plane control measurement is to establish a plane control network and then calculate the coordinates of the control points. Commonly used methods for plane control measurement include triangulation, traverse measurement, and intersection legal point measurement. The more commonly used method is traverse measurement. In addition, with the promotion of GPS global positioning system technology, the use of GPS technology for plane control measurement has been widely used. The following mainly describes the method of wire control measurement:

The polyline formed by connecting adjacent control points into a straight line is called a wire, and the corresponding control point is called a wire point. Conductor measurement is to sequentially measure the horizontal distance between the conductor sides and the horizontal angle between two adjacent conductor sides. Based on the starting data, calculate the azimuth angle of each side and obtain the plane coordinates of the conductor points.

According to different situations and requirements, a single wire can be laid out as attached wire , closed wire and branch wire, as shown in Figure 3-1. It is a common method for establishing a plane control network in small areas. It is often used in built-up areas with complex distribution of ground objects, hidden areas and strip areas with many visual obstructions.

Figure 3-1 Distribution form of wires ① Closed wire: Starting from a known control point BM, and finally returning to this point, forming a closed polygon. There should be at least one directional edge connected to the known control point of the closed wire. This wire form has 3 checking conditions, including 1 polygon interior angle sum condition and 2 coordinate increment conditions.

② Attached wire: The wire starts from a known control point BM1 and ends at another known control point BM2. This wire form has 3 checking conditions, including 1 coordinate azimuth angle condition and 2 coordinate increment conditions. ③Branch line: Starting from a known control point BM, it is neither attached to another known control point nor closed to the original starting control point. Since the branch wire lacks inspection conditions, it is generally limited to use in topographic survey root wires.

(2) Elevation control measurement

The principle of elevation control measurement is to calculate the elevation of control points by establishing an elevation control network. The methods of elevation measurement include leveling method, electromagnetic wave ranging triangulation elevation measurement method, etc. Commonly used leveling methods. Elevation control uses the high-level leveling points laid out near the survey area as the starting point for calculating elevation, and lays out the root leveling network in the survey area, and tries to jointly measure all high-level level points near the survey area.

The following describes the leveling method:

Leveling is a method of measuring the height difference between two points on the ground using the level and level ruler . 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 results, necessary corrections must be made according to the normal height system adopted to obtain the correct elevation.

Figure 3-2 Leveling measurement

In addition, if there are no known control points within the survey area, we can select appropriate points, use RTK to conduct static measurements, and send the static data files to relevant units for control point coordinate calculation to obtain accurate control point coordinate information.

. Control point application example

is used in combination with RTK as an example. The steps are briefly described as follows: First, establish a GPS control network according to the project scope. The control points are arranged at reasonable locations, and the number of control points can be adjusted according to the network type; secondly, based on the existing control points, through parameter calculation, point correction is performed in RTK to convert to the corresponding coordinates of the points in the required coordinate system; finally, through the point measurement function, the coordinates of other points in the survey area are measured to obtain basic surveying and mapping data.

Summary

Control points play a huge role in surveying and mapping work. They are both the foundation and the guarantee. Therefore, surveying workers should select appropriate control points according to the actual requirements of the project and arrange them reasonably and scientifically. Based on this, the surveying and mapping work can be carried out more smoothly and efficiently.