Vertical control survey

Small area elevation control survey can be divided into leveling and trigonometric leveling. Usually, the first-level control of survey area is established by third-and fourth-class leveling, and then root leveling and trigonometric leveling are developed. Root leveling is mostly used in flat areas to measure the elevation of plane control points such as triangle points, advanced terrain control points and root points, so that these control points can be used as stations to measure the elevation of broken points during surveying and mapping. In mountainous areas and control points located on high-rise buildings, the elevation is measured by trigonometric leveling. In addition, the third-and fourth-class leveling also directly provides elevation control for the design and construction of various construction projects.

First, third and fourth leveling

As the first-level control of survey area, the layout of third-and fourth-class leveling lines should generally be arranged as a closed loop, and then the attached leveling lines and node networks should be encrypted. Only under special circumstances, such as mountainous areas, are branch lines allowed to be laid. The accuracy of the level used shall not be less than DS3, the magnification of the level telescope shall not be less than 28 times, and the calibration value of the leveling tube shall not be greater than 2 ". The spirit level generally adopts a double-sided ruler, and the ruler should be equipped with a level. Before measurement, the liquid level must be checked and corrected. The level also needs to be checked, such as whether the ruler is bent, whether the zero point is worn, and so on.

Leveling routes are generally laid along railways, highways and other roads with small slopes and convenient measurement as far as possible; Avoid crossing lakes, swamps and rivers as much as possible. The datum point should be selected in the position with solid soil, low groundwater level and easy observation. Places that are easily submerged, damp, vibrating and sinking should not be selected as leveling points. After the leveling points are selected, leveling marks and leveling marks shall be buried, and points shall be drawn for future reference.

The observation procedures, records, calculation, inspection methods and technical requirements of third-and fourth-class leveling are as follows: Table 6-7.

Table 6-7 Technical Requirements for Third and Fourth Leveling

Note: When the round trip is poor, L in the table is the route length between leveling points (km); When calculating the closure difference of the attached or closed route, L is the length (km) of the attached or closed route.

1. Station observation plan

The third and fourth class leveling mainly adopts double-sided leveling method. The observation procedure of this station is as follows:

Leveling the instrument with a circular level;

1) Look back at the black ruler, read the upper and lower alignment, strictly level the bubbles in the pipe, and read the center line;

2) Look ahead at the black ruler, read the upper and lower aiming lines, strictly level the bubbles in the leveling tube, and read the middle line;

3) Look forward at the red ruler and read the middle thread (check whether the bubble is centered);

4) Look back at the red ruler, strictly level the bubbles in the leveling tube, and read the medium wire.

The above observation procedures are referred to as "before and after". See Table 6-8 for the order of observation and recording. Fourth-class leveling can also adopt the observation procedure of "back, back, front and front".

2. Calculation and verification of stations

(1) sight distance part

Backward distance (15) = ((1)-(2)) ×100.

Forward-looking distance (16) = ((4)-(5)) ×100.

Sight distance difference before and after (17) = (15)-(16)

Cumulative difference of sight distance before and after (18)= this station (17)+ the previous station (18)

See Table 6-7 for line-of-sight tolerance.

Table 6-8 Fourth-class Leveling Observation Manual

sequential

(2) Height difference part

Forward sight gauge (9)=(6)+K 1-(7) Reading difference between black and red surfaces.

The reading difference between the black and red surfaces of the rear sight gauge (10)=(3)+K2-(8)

Height difference measured on black surface (1 1)=(3)-(6)

Height difference measured on the red surface (12)=(8)-(7)

The difference of height difference between black and red surfaces (13)=( 10)-(9)

Because the zero difference K 1 and K2 of the red and black surfaces of the front sight ruler and the rear sight ruler are not equal (one is 4.787m and the other is 4.687m, with a difference of 0. 1 m), the verification calculation of the term (13) is as follows.

( 13)=( 1 1)-( 12) 0. 1

See Table 6-7 for height difference tolerance.

If the tolerance on the station exceeds the limit, the station needs to be re-measured. If the inspection is qualified, calculate the average height difference of the station:

( 14)=[( 1 1)+( 12) 0. 1]/2

Then, move the instrument to the next station for observation.

3. Check the calculation of each page

When the whole leveling route is completed, check the calculation page by page for errors. The method is as follows:

Sight distance part:

Last stop (18) = ∑ (15)-∑ (16)

Total sight distance =∑( 15)+∑( 16)

Height difference part:

∑( 1 1)=∑(3)-∑(6)

∑( 12)=∑(8)-∑(7)

When the total number of stations is even:

∑( 14)=[∑( 1 1)-∑( 12)]/2

When the total number of sites is odd:

∑( 14)=[∑( 1 1)-∑( 12) 0. 1]/2

Step 4 organize the results

If the elevation difference closure difference of the leveling line meets the tolerance requirements specified in Table 6-7, the elevation of each point of the leveling line can be calculated. When the leveling route is attached leveling route, closed leveling route or branched leveling route, the indoor calculation method is the same as that introduced in Chapter 2. When the leveling route is a node network with only one node, the elevation of the node is calculated by the weighted average method. See measurement calibration.

Now the related problems in the measurement will be further explained.

1) The third level must be observed back and forth, and the line-of-sight interval is calculated by reading the upper and lower wires. When using DS 1 and invar ruler, one-way observation of two turning points can be adopted, and the observation procedures of both methods are back-front-front-back, that is, black-black-red-red.

2) Fourth-class leveling, except branch leveling and one-way double-turning-point observation which must be observed back and forth, closed leveling and attached leveling route can be observed in one direction. The observation program of each station can also be back-back-front-front, that is, black-red-black-red. When using a single-sided ruler, the back-front-front-back reading procedure is adopted, but the instruments must be rearranged between two observations, and the height of the instruments must be changed, and the station inspection is carried out by using the double instrument height method. Fourth-class leveling can directly read the sight distance (accuracy to meters) without reading the upper and lower wires.

3) The number of stations in the forward survey and reverse survey of each section of the third-and fourth-class leveling should be even, otherwise the zero-point error should be corrected by adding ruler. When changing from forward measurement to backward measurement, the two scales must be exchanged and the instrument should be repositioned.

4) The third-class leveling at each station shall not aim at the light twice, and the fourth-class leveling shall aim at the light twice as little as possible.

5) When the work is intermittent, it is best to end the observation on the leveling point. Otherwise, two fixed points which are firm and reliable and convenient for placing the ruler should be selected as intermittent points and marked. After the intermission, the test is about to take place. If the test results meet the requirements, you can continue to observe, otherwise you must re-observe from the previous benchmark.

6) In a station, only when all inspections meet the tolerance requirements can the station be moved. If one of them exceeds the standard, it can be re-measured at the station immediately, but the height of the instrument must be changed. If the instrument is found to be out of specification after moving to the site, it should be retested from the previous reference point or intermittent point.

7) When the number of stations per kilometer is less than 15, the closure difference shall be calculated according to the tolerance formula of flat land, and when the number of stations exceeds 15, it shall be calculated according to the tolerance formula of mountain land.

8) When the image is clear and stable, the line-of-sight length of Grade III and IV can be enlarged by 20% according to the specified length.

9) In leveling network, the length of the attached leveling route between nodes or between nodes and advanced points shall be 0.7 times of the specified value.

10) When observing the third-class and fourth-class leveling with a single-sided ruler, the limit of the difference between the two height differences measured before and after changing the height of the instrument is the same as the limit of the difference between the height differences measured on the black and red surfaces.

Second, the root leveling

When the basic vertical spacing of mapping (see Chapter 8, Section 4) is 0.5m, the elevation of the root point is determined by the root elevation.

The horizontal route of the graph root can be arranged as an additional route, a closed loop or a node network passing through the graph root point. The length of connection path or closed loop between advanced points shall not exceed 8km, the length of path between nodes shall not exceed 6km, and the branch line shall not exceed 4km. See Section 3 of Chapter 2 for the observation method and record calculation of root leveling, and see Table 6-9 for the technical requirements.

Table 6-9 Main Technical Indexes of Leveling at Tugen

Third, trigonometric leveling

When the ground slope in mountainous area is large or the basic vertical spacing of mapping is more than 0.5m, the elevation of the root point and other plane control points can be determined by trigonometric leveling.

Figure 6- 18 trigonometric leveling principle

Triangle elevation principle, as shown in figure 6- 18, the elevation HA of point A is known, and to measure the elevation HB of point B, the theodolite can be placed in A, and the vertical distance I from the pile top to the center of the horizontal axis of the instrument is called the instrument height; Erect a beacon at point B and measure the beacon height VB; If the wire in the telescope is aimed at the vertex of the target and the vertical angle α α is measured, the height difference hαb can be calculated by trigonometric formula according to the distance d between AB points (the distance between the root points of the graph is known), that is,

hαb=Dtanαα+iα-vb(6-23)

When d is greater than 300m, the curvature of the earth (increasing reading) and atmospheric refraction should also be considered according to the formula.

Architectural engineering survey

Where: d-horizontal distance;

αa- vertical angle;

K—— atmospheric vertical refraction coefficient 0.14;

R-radius of curvature of the earth, R=6370km.

finally

HB=HA+hab (6-24)

Triangular elevation survey is generally divided into two levels: the first level triangular elevation line starts and ends at the elevation point of direct leveling, and the number of its sides does not exceed 7; The second level is attached to the first level route, and does not exceed 10 edge. The first level is used for elevation measurement of advanced terrain control points, and the second level is used for map root points. See table 6- 10 for the technical requirements of trigonometric leveling. During observation, I and V are accurately measured to 5mm with a steel ruler, and the opposite observation method is adopted until the last edge. When the closing difference does not exceed the allowable value, the sign of closing difference is corrected for the height difference of each side according to the principle of being proportional to the length of the side. Finally, starting from the starting point elevation, the elevation of each point is calculated in turn by using the corrected height difference of each side.

Table 6- 10 Main technical indexes of trigonometric leveling