Lesson Note On Leveling Survey (Longitudinal and Cross-section)

Leveling Survey (Longitudinal and Cross-section)

Learning Outcome

By the end of this practical work students should be able to:

1.           Setup level equipment

2.           Take back sight, intermediate sight and fore sight reading

3.           Make correct booking

4.           Calculate reduced level of all the point taken

5.           Perform correction to the reduced level

6.           Plot a longitudinal profile and cross section with a suitable scale

Theory

Leveling is the art of determining relative height or elevations of different points on the earth surface. The elevation of a point has been defined as its vertical distance above and below a given reference level surface and usually a mean sea level. The leveling work can be carried out by using direct method or indirect method. The main equipment needed to carry out leveling works is level and staffs. The elevation of the point is calculated using Height of Collimation Method or by using Rise and Fall Method. The final works of the leveling is to transform the numerical data into graphic form either in map or drawing.

Equipment

1. level
Automatic level is used to compare points on the surface of the earth and series of heights observed must be relative to a plane called datum. Automatic level is a device that gives a truly horizontal line. The telescope of this instrument need only be approximately level and the compensating device, usually a pendulum system inside the telescope corrects for the residual mislevelment. This instrument has no bubble tube; therefore preliminary leveling is carried out using the conventional three-screw leveling head and a small target bubble mounted on the tribach which brings the collimation to within 10’ of the horizontal. A prismatic compensator fitted between the eyepiece and the objective lens make correction of slight tilt automatically.

2. Leveling staff

The staff used for ordinary leveling work is sectional and assemble either telescopically or by slotting one another vertically. Most modern designs are manufactured in an aluminum alloy BS4484:Part1:1969 requires length of 3 m, 4m or 5m extension. The graduations are in the form of an E or F shape and the graduations in the 100mm interval.

3. Staff bubble

An instrument to ensure the staff is erected vertically. It is place at the side of the staff.

4. Tapes

Tape is made of synthetic material, glass fiber or linen. The length of the tape is 10m, 20, 30m and 50m are generally available. The tape is graduated at every 5mm and figures every 100mm. the first and the last meter lengths are graduated in millimeters. Whole meter figures are shown in red at every meter.

Procedures

·                     Longitudinal leveling (60m length)

1.           The suitable position for the level to be set up is selected

2.           The level is set up. The temporary adjustment is made

3.           The staff is placed at the bench mark and the reading is taken

4.           The reading is noted in the form provided

5.           Another staff is placed at the distance of 7.5m from the first staff

6.           The reading of second staff is taken and is noted as the intermediate sight

7.           A distance of 7.5m is measured and the third staff position is placed. The reading is noted in the field sheet as an intermediate sight

8.           Step 5 to 7 is repeated until the staff cannot be read. The last reading of the staff before the level is moved is noted in the column foresight. The staff at the foresight position must not move until the back sight reading is taken

9.           The level is moved to new suitable position and the temporary adjustment is made

10.        The back sight reading is taken

11.        A cross section reading is taken at every 15m

12.        Steps 5 to 10 are repeated until the work is completed

13.        A fly level is performed back to the benchmark

14.        The HOC and the reduced level of all the staff positions is calculate

15.        The longitudinal and cross section profile of the road is plotted

·                     Cross section leveling (0m, 40m, 80m, 120m, 160m and 200m)

1.           Five staff positions is selected perpendicular to the longitudinal line at position A, B, C, D and E

2.           The staff is placed at point A and the reading is taken

3.           The reading is entered as intermediate sight if the level is still not being moved from the previous position

4.           Taking reading for points B, C, D and E is continued

5.           After completed cross section leveling, taking the longitudinal leveling is continued until completed

Conclusion

From this practical, we were able to setup the compass survey stations and their equipment correctly. We have taken the reading of three points of static things that have at our location from station A. There are 3.25m, 1.80m and 3.35m from station A

We have calculated the misclosure as mentioned in the local attraction method table. The corrected bearing and the final bearing is determined by followed the correct steps

Discussion and Recommendations

1.           Avoid taking measurement near magnetic sources such as hand phone, watch, electric cable and etc

2.           Make sure the bubbles are properly level

3.           Triple check with the work and reading

4.           Make sure the plum bob properly centered over the peg