Levelling

1. Horizontal Line Pull of Gravity Height Reduced Level Level line Mean Sea Level Levelling

2. DEFINITIONS • Level Surface: A level surface is a surface, all points of which are normal to the pull of gravity. I.e. a line at right angles to the string line on a freely suspended plumb bob. The open sea provides a good example of a level surface. • Horizontal Surface: A horizontal surface is one which passes through a point at right angles to the pull of gravity at that point. • Datum surface: The reference plane used in surveying is called a DATUM. In this country the main datum surface is the mean sea level at Newllyn in Cornwall and heights above this plane are referred to as being ABOVE ORDNANCE DATUM (AOD). • Height: The height of a point can be regarded as the distance measured vertically to that point from a level reference plane or surface. • Reduced level: The reduced level of a point is its height above a reference plane

3. Flying Levelling Practice • Flying levelling is used when the reduced level of a point is to be found from another point of known reduced level. • Used to determine a TBM from a OBM or MBM • Consists of a series of BS and FS only • Should always “close” the survey back onto the starting point or another OBM • BS is BACK SIGHT and is the initial staff reading with the staff on a KNOWN LEVEL (e.g. OBM or change point) • FS is FORE SIGHT and is the staff reading with the staff on a point whose levels you wish to determine.

4. Levelling Staff (2.195) Line of Sight View through eye piece 2.195 A 21.000m B 2 2 2 1 First staff position NOTE: Position of staff is important. Position of instrument is only for convenience This first staff reading onto a known level is termed the BACKSIGHT (BS) reading

5. Calculations • The line of sight is termed the HORIZONTAL PLANE OF COLLIMATION (HPC). • Hence HPC = Known reduced level + BS • The plane of reference has now been established (HPC) • i.e. HPC = 21.000 + 2.195 • HPC = 23.195 • The next reading of the staff can be referred to this • The staff is next placed at the point whose reduced level is desired (B) and the telescope of the level is rotated to target the new staff position, care being taken not to alter the adjustment of the instrument (i.e. don’t knock it).

6. Levelling Staff (23.195) View through eye piece Line of Sight (3.195) 3.195 3 3 2 1 Next staff position This staff reading onto the point whose desired level is required is termed the FORESIGHT reading (FS).

7. Calculations • The new staff reading is taken, 3.195m (say). • This indicates that point B is 3.195m below the level of the line of sight or HPC, which was calculated to be 23.195m. • The reduced level of point B is therefore: - 23.195m – 3.195m = 20.000 Hence Reduced Level = HPC - FS NOTE: Intermediate sights are treated the same way as FS except for being booked in the IS column

8. Booking the Readings

9. Always start with comments on staff position

10. Add staff reading in BS column and in row referring to point A

11. Calculate the HPC

12. Move staff to B and add comments

13. Enter reading at B in FS column and row corresponding to B

14. Calculate the RL at B

15. Continuing with the Flying levels • Having ascertained the level at B we can proceed to move the instrument forward using the recently discovered level at B as our known point. i.e. B now becomes the BS point • So at B we take both a FS and a BS reading. • Point B is a “change point” (Use change plate or at least a solid point to rest staff on) • Note that this means when the instrument is moved the staff stays stationary • And when the staff is move the instrument stays stationary. • MOVE ONE or the OTHER – NOT BOTH TOGETHER

16. Levelling Staff Line of Sight 3.155 B 20.000m Always use the correct row

17. And Calculate HPC The HPC now reflects the line of sight in the latest instrument position Levelling now proceed by repeating previous actions i.e. Move staff, take FS, calculate RL, Move instrument, Take BS, Calculate HPC etc

18. Staff Staff Target finder (In line with staff) Target finder (Look at this and beyond towards staff) Eye-piece Eye-piece MOVE TELESCOPE UNTIL TRIANGLE APPEARS OVER STAFF Telescope Telescope Finding the Target • Most telescopes have a targeting system along the top of the barrel (similar to a rifle target finder). The telescope should be aimed at the staff using this targeting system prior to trying to find the staff through the eyepiece.

19. LEVELLING SOFFITS Inverted staff readings An inverted staff reading can be used to determine the reduced level of a point above the line of sight of the instrument such as a ceiling, underside of a bridge, balcony etc. As the name suggests, the staff is simply turned upside –down, the bottom placed against the point that the level is required, and then read. An important difference between inverted staff readings and other types is that they are treated as negative quantities, both in the booking of the readings and the reduction of the levels.

20. backsight foresight 0.174 2.111 1.603 1.440 1.505 2.192 0.738 3.890 D A = 33.550 B E C Inverted staff example

21. INTERMEDIATE STAFF READINGS • Intermediate staff readings are those staff readings taken after a backsight is read and before a foresight. • Each reading in turn is subtracted from the collimation level (HPC) to deduce the reduced level of the point • The problem with intermediate sights is that the arithmetic check will only relate to the back sight and the fore-sight • Hence any arithmetic error could go unnoticed • Systematic errors due to mal-adjustment of the instrument cannot be eliminated as in flying levelling where the equal fore-sight distance and back sight distance compensated for the mal-adjustment errors • Instrumental errors of this nature can be eliminated by the use of a REVERSIBLE LEVEL.

22. backsight Intermediate sights 1.440 0.738 0.174 2.111 1.603 1.843 F D A = 20.450 E C B INTERMEDIATE STAFF READINGS -2