First, the basic principles of measurement work
Surveying and mapping disciplines divide the complex and diverse objects and shapes on the earth's surface into two categories: features and landforms. Objects fixed on the ground or with obvious boundaries, such as rivers, houses, roads, lakes, etc. , known as features; The ups and downs of the ground, such as mountains, valleys and steep cliffs, are called landforms. Features and landforms are collectively called topography.
When surveying and mapping topographic maps or setting out the location of buildings, it is impossible to survey all the terrain in the survey area or set out the location of all the buildings at a certain point. As shown in point A in figure 1-9(a), at this time, only the nearby terrain can be surveyed or the location of nearby buildings (as shown in figure, building P) can be set out, and the terrain behind the mountain and in the distance cannot be observed. Therefore, it is necessary to measure at several points (stations) in different regions, and finally splice the topography of each region into a complete topographic map, as shown in figure 1-9(b). The same is true for construction lofting. However, any survey work will inevitably produce errors, so the survey of each point (station) should adopt certain procedures and methods, follow the basic principles of measurement, prevent the accumulation of errors and ensure the quality of surveying and mapping results.
Figure 1-9 Topography and Topographic Map Schematic Diagram
Therefore, the following basic principles should be observed in the actual measurement work:
1) In the survey layout, the principle of "from whole to part" should be followed; The principle of "from high to low" should be followed in measuring accuracy; In the measurement procedure, the principle of "control first, then break parts" should be followed.
2) In the process of measurement, the principle of "check at any time to prevent errors" should be followed.
Second, the concept of control measurement
Follow the survey principle of "control first, then break the part", and measure the plane position (coordinates) and elevation of some points with control significance in the survey area with high precision as the basis for mapping topographic map or construction lofting. This step is called control survey, which is divided into plane control survey and elevation control survey. The forms of plane control survey include traverse survey, triangulation survey and intersection survey. Its purpose is to determine the coordinates x and y of a series of control points in the survey area; The forms of elevation control survey include leveling, photoelectric ranging trigonometric leveling, etc. Its purpose is to measure the height difference between control points, so as to obtain the elevation of each control point, as shown in figure 1-9(a), where A, B, C, D, E and F are plane control points, and the geometric network formed by connecting these control points is called plane control network, as shown in figure 654. Through traverse measurement (including angle measurement, distance measurement, etc. ) and calculation, get the coordinate x and y values of control points such as a, b, c, d, e and f. At the same time, starting from the leveling points with known elevation in the survey area, a closed leveling route is formed through control points such as A, B, C, D, E and F, and leveling and calculation are carried out, so as to obtain the elevation h of these control points.
Third, the concept of detail investigation.
The work of determining the position of feature points of ground objects and landforms is called detail measurement. Based on control points, detail measurement determines the position of breakpoints by measuring the horizontal distance, height difference and angle relative to a known direction between control points and breakpoints (topographic feature points), and then draws these breakpoints on drawings or computer screens according to a certain proportion. After determining a certain number of breakpoints, draw a topographic map with reference to the field, as shown in figure 1-9(b). Roads, bridges and houses represented on the map are ground objects, which are drawn with designated schematic symbols. A set of closed curves in the middle of the figure represents two connected hills and their ups and downs in the field investigation area. These closed curves are called contour lines. Using contour lines to represent landforms is the most commonly used method. See Chapter 7 for its principle.
Figure 1- 10 theodolite mapping method
Conventional detail measurement usually uses flat plate mapping method or theodolite mapping method. Figure 1- 10 is a detailed survey by theodolite surveying and mapping method. Place a theodolite at control point A and orient it at another control point B. Then aim the straightedge erected at the corner of the house 1, 2, 3 in turn, and read out the corresponding angles β 1, β2, β3 and distances D 1, D2. According to the angle and distance, use protractor and ruler to draw the plane position of room angle 1, 2, 3 according to the proportion on the drawing board. At the same time, the elevation of these breakpoints is obtained according to the measured data, and marked on the right side of the breakpoint position.
Fourth, the concept of construction lofting
Construction lofting (lofting) is to mark the location of buildings (structures) on the site design drawings as the basis of construction. In order to make the building position relationship determined on the ground correct, the construction lofting also needs to follow the basic principle of "control first and then break".
As shown in Figure 1-9(b), Building P (shown by dotted line in the figure) is designed near control points A and F, and it is required to be calibrated on the spot. According to the control points A and F and the design coordinates of the building, the lofting data such as the water level angle β 1, β2 and the horizontal distance D 1, D2 are calculated. Then at the control point A, the directions indicated by the water level angles β 1 and β2 are measured by instruments, and the horizontal distances D 1 and D2 are measured along these directions, and 65438 is determined on the spot. The methods introduced above are polar coordinate method commonly used in construction lofting, in addition to rectangular coordinate method, direction (angle) intersection method and distance intersection method.
Because the construction control network in construction lofting is a whole, it has the corresponding precision and density. No matter how big the scope of the building (structure) is, the positions of the points of the building (structure) lofting from each control point will certainly form a whole.
Similarly, according to the known elevation of construction control network and the design elevation on building drawings, the site design elevation position of buildings (structures) can be measured by leveling.
Five, the basic work of measurement
To sum up, the essence of control survey, detail survey and construction lofting is to determine the position of points. Detailed drawing survey is to draw the points on the ground on the drawings or provide users with measurement data and results, while construction lofting is to measure the points of buildings (structures) on the design drawings on the ground as the basis of construction. It can be seen that all the points to be measured are inseparable from the three basic observations: distance, angle and height difference. Therefore, distance measurement, angle measurement and height difference measurement (leveling) are the three basic tasks of measurement. All kinds of engineering professionals should master these three basic skills.
Review exercises
1. What's the difference between measurement and measurement?
2. What is the geoid? What are its characteristics and functions?
3. What are absolute elevation, relative elevation and height difference?
4. Why must the influence of the curvature of the earth be considered in height difference measurement (leveling)?
5. What is the difference between the plane rectangular coordinate system in measurement and the plane rectangular coordinate system in mathematics?
6. How is the Gaussian plane rectangular coordinate system established?
7. It is known that a point is located in the 20th band of the 6-degree band of Gaussian projection. If the natural coordinate y of the point in the rectangular coordinate system of the projection belt Gaussian plane is -306588.25438+00m, find the common divisor y of the horizontal coordinate of the point and the longitude L0 of the central meridian of the belt.
8. On the first floor of a hotel, the absolute elevation of indoor ground plus or minus 0 is 28.6m, the design elevation of outdoor ground is-1.68m, and the design elevation of parapet is+68.80 m. What are the absolute elevations of outdoor ground and parapet respectively?