Surveying And Levelling Pdf Notes

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2You have learned what the height of a ground point is. Now, however, you will need to know a more accurate definition of this term. When the height of a point is its vertical distance above or below the surface of a reference plane* you have selected, it is called the elevation* of that point. When the height of a point is its vertical distance above or below mean sea level (as the reference plane), it is called the altitude* of the point. Example Elevation of a point above a selected ground mark A

Altitude of the same point above mean sea level (amsl) 1.83 m

345 m

3. The vertical distance between two points is called the difference in elevation , which is similar to what you have learned as the difference in height (see Section 5.0). The process of measuring differences in elevation is called levelling , and is a basic operation in topographical surveys.

4. You can level by using different methods, such as: direct levelling, where you measure differences in elevation directly. This is the most commonly used method; indirect levelling, where you calculate differences in elevation from measured slopes and horizontal distances. Direct levelling

You have already learned about indirect levelling in Section 5.0, when you learned to calculate differences in elevation from slopes or from vertical angles. Now you will learn about direct levelling.

5. By direct levelling, you can measure both the elevation of points and the differences in elevation between points, using a level and a levelling staff (see Chapter 5). There are two kinds of direct levelling: differential levelling; and profile levelling.6. In differential levelling , you find the difference in elevation of points which are some distance apart (see Section 8.1). In the simplest kind of direct levelling, you would survey only two points A and B from one central station LS. But you may need to find the difference in elevation between: either several points A, B, ... E, surveyed from a single levelling station LS; or several points A ... F, surveyed from a series of levelling stations LS1 ... LS6, for example:7. In profile levelling , you find the elevations of points placed at short measured intervals along a known line, such as the centre-line of a water supply canal or the lengthwise axis of a valley. You find elevations for cross-sections with a similar kind of survey (see Section 8.2).

8. You can also use direct levelling to determine elevations for contour surveying (see Section 8.3), and for setting graded lines of slope(see Section 6.9), where you need to combine both differential levelling and profile levelling.

9. There are several simple ways to determine the elevations of ground points and the differences in elevation between ground points. You will use a level and a levelling staff with these methods. In the following sections, each method is fully described to help you choose between them. Table 10 will also help you to compare the various methods and to select the one best suited to your needs in each type of situation you may encounter.

2. A backsight (BS) is a sight taken with the level to a point X of known elevation E(X), so that the height of the instrument HI can be found. A backsight in direct levelling is usually taken in a backward direction, but not always. Backsights are also called plus sights (+ S), because you must always add them to a known elevation to find HI.

3. A foresight FS is also a sight taken with the level, but it can be on any point Y of the sight line where you have to determine the elevation E(Y). You will usually take it in a forward direction, but not always. Foresights are also called minus sights (-S) , because they are always subtracted from HI to obtain the elevation E of the point. Remember:

8. You can make the calculations more easily if you record the field measurements in a table , as shown in the example. You will not make any intermediate calculations. All BS's and all FS's must be added separately. The sum FS is subtracted from the sum BS to find the difference in elevation from point A to point B. A positive difference means that B is at a higher elevation than A. A negative difference means that B is at a lower elevation than A. Knowing the elevation of A, you can now easily calculate the elevation of B. In this case, E(B) = 100 m + 2.26 m = 102.26 m; this is the same as the result in step 7, which required more complicated calculations. This kind of calculation is called an arithmetic check.

9. Often you will need to use more than one turning point between a point of known elevation and another point of unknown elevation. To do this, you can use the procedure you have just learned, but you will need to record the field measurements in a table to make calculating the results easier.

The arithmetic sum of these differences should be equal to the calculated difference in elevation D(E) = +2.82 m. These columns will also help you to calculate the elevation of each turning point , and to check on the elevation of point B more carefully.

When you survey a future fish-farm site, you will use a very similar method. You can then prepare a topographic map of the site (see Chapter 9), which will become a useful guide for designing the fish-farm.

14. This is a survey method using straight open traverses , that is, several intermediate stations along one straight line. You know for example the elevation of starting point A, E(A) = 63.55 m. You want to know the distance of point B from point A, and its elevation. Because of the type of terrain on which you are surveying, you cannot see point B from point A, and you need two turning points , TP1 and TP2 , for levelling. Measure horizontal distances as you move forward with the level, from point A toward point B; try to progress along a straight line. If you cannot, you will need to use the broken open traverse survey method, which involves measuring the azimuths of the traverse sections as you move forward and change direction (see step 17).

15. Set out a table like the one in step 12, and add two columns to it for horizontal distances. Enter all your distance and height measurements in the main part of the table. Then, in the first additional column, record each partial distance you measure from one point to the next one. In the second column, note the cumulated distance , which is the distance calculated from the starting point A to the point where you are measuring. The last number in the second column will be total distance AB.

16. Conclusions . Point B is 1.55 m higher than A and its elevation is 65.10 m. It is 156.5 m distant from point A. The arithmetic check from the (BS- FS) differences agrees with the calculated difference in elevation.

18. You need for example to survey open traverse ABCDE from known point A. You require four turning points, TP1, TP2, TP3 and TP4. You want to know: the elevations of points B, C, D and E; the horizontal distances between these points; the position of each point in relation to the others, which will help you in mapping them.

Proceed with the differential levelling as described earlier, measuring foresights and backsights from each levelling station. Measure azimuths and horizontal distances as you progress from the known point A toward the end point E. All the azimuths of the turning points of a single line should be the same. This will help you check your work. 19. Make a table similar to the one shown in step 15, and add three extra columns to it for recording and checking the azimuth values. Enter all your measurements in this table. At the bottom of the table, make all the checks on the elevation calculations, as you have learned to do them in the preceding steps. Example

From point A of a known elevation, survey by traversing through five turning points, TP1 ... TP5, and find the elevation of point B. To check on the levelling error, survey by traversing BA through four other turning points, TP6 ... TP9; then calculate the elevation of A. If the known elevation of starting point A is 153 m, and the calculated elevation of A at the end of the survey is 153.2 m, the closing error is 153.2 m - 153 m = 0.2 m.

21. The closing error must be less than the permissible error, which is the limit of error you can have in a survey for it to be considered accurate. The size of the permissible error depends on the type of survey (reconnaissance, preliminary, detailed, etc.) and on the total distance travelled during the survey. To help you find out how accurate your survey has been, calculate the maximum permissible error (MPE) expressed in centimetres , as follows:

23. If you do not know the exact elevation of starting point A, you can assume its elevation, for example E (A) = 100 m. Start the survey at point A , and proceed clockwise along the perimeter of the area. Take levelling staff readings at TP1, TP2, B, TP3, etc., until you reach starting point A again and close the traverse. At the same time, make any necessary horizontal distance and azimuth measurements. Record your measurements either in two separate tables , one for plan surveying and one for levelling, or in one table which includes distance measurements. From the (BS-FS) columns, you can easily find the elevation of each point on the basis of the known (or assumed) elevation at point A. Make all the checks on the calculations as shown in steps 15 and 16. Find the closing levelling error at point A (see step 20). This error should not be greater than the maximum permissible error (see step 21). Example Topographical survey of a closed traverse by differential levelling

24. The square-grid method is particularly useful for surveying small land areas with little vegetation. In large areas with high vegetation or forests, the method is not as easy or practical. To use the method, you will lay out squares in the area you are surveying, and determine the elevation of each square corner.

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