LEVELING

Definition of leveling

Leveling is the process use to determine a difference in elevation between two point.

A process of determining the height of one level relative to another. It is used in surveying to establish the elevation of a point relative to a datum, or to establish a point at a given elevation relative to a datum. 

Principle of leveling

To obtain horizontal line of sight with respect to which vertical distances of the points above or below this line of sight are found. 

Figure 1 : Principle of leveling

Purpose of leveling (general usage)

To find the elevations of given points with respect to a given datum. To establish points at given elevations or different elevations with respect to the given or assumed datum.

Leveling term

a. Bench Mark (BM)

point with known height above mean sea level (or other reference datum). These are permanent points (e.g. unchanged by weather conditions) and are provided by the Department of Lands and Surveys.

 

 Figure 2 : Bench Mark

b. Temporary Bench Mark (TBM)

a point of known height above a pre-defined level. This level is not absolute and is defined locally by the surveyor for the purpose of the survey. Based on the TBM the survey may then later be reduced to absolute levels if the level of TBM is known.   

  Figure 3 : Temporary Bench Mark 

c. Back Sight (BS)

first staff reading taken immediately after setting up the instrument. 

 Figure 4 : Backsight (BS)

d. Foresight (FS)
Last staff reading taken before moving the instrument to another location.

 

Figure 5 : Foresight (FS) 

e. Intermediate sight (IS)

All readings taken between a BS and a FS. 

  Figure 6 : Intermediate sight (IS)

 

f. Change point (CP)

A point that has back sight and fore sight reading.

Figure 7 : Change point (CP)

g. Reduced level (RL)

Reduced level is the vertical difference between the level point and datum line or the mean sea level. Reduced Level in surveying refers to equating elevations of survey points with reference to a common assumed datum. 

h. Datum

Reduced Level in surveying refers to equating elevations of survey points with reference to a common assumed datum.

 
i. Mean sea level (MSL)

Mean sea level is established by taking the average height of tides in the sea over a very long period (generally 18years). Mean sea level or MSL is used as datum level for all important surveys. 

Figure 8 : Mean sea level (MSL) 

Type of  level instrument

Figure 9 : Type of  level instrument

 a. Automatic level  

It uses a compensating system of prisms suspended in the telescope to ensure that the line of collimation is horizontal. Simply center the pill bubble as above. 

b. Digital level 

That reads and records the staff for you (at much greater cost).

c. Tilting level  

The telescope is hinged. An accurate bubble tube (usually with a split bubble viewer) is used for accurate leveling of each shot. The Tilting level replaced the Dumpy level which required 3 foot screws to be adjusted for each shot.

Instruments use in leveling

Figure 10 :  Instruments use in leveling

Temporary adjustment for level 

This adjustment are performed at every setup of instrument.  

• Setting up of leveling : This operation includes fixing the instrument on the tripod and also approximate leveling by leg adjustment.
• Leveling up : Accurate leveling is done with the help of foot screws and by using plate levels. The object of leveling up the instrument is to make its vertical axis truly vertical.
• Elimination of parallax ( Focusing of the eye peace
  & Focusing of object glass) : If the image formed by the objective does not lie in the plane of the cross hairs, there will be a shift in the image due to shift of the eye. Such displacement of image is termed as parallax. Parallax is removed in two stages.

         

Two peg test

A two peg test is a test for error within a surveying level. This test gives a fairly accurate check for the amount of collimation error. Collimation error is the difference between what the level hairs in the scope say is level, and what is actually level. 

     

 

Figure 11 :  Two peg test

• Setup and mark on the ground (with wooden pegs driven into the earth, or roofing nails in tar) two point some 30m apart. Set up the the level exactly mid-way between them. 
• Take measurements of back sight and foresight for this first setup. The height difference dh1 will be free of the effects of collimation error.
• Next, move the level to a position just beyond the fore staff position (about 10m).
• Then repeat the readings.  

Example:

 

A = (a2 - 3e)

B = (b2 - e)

The actual correction = (a2 - 3e)    -  (b2 - e)

                                   = (7.86 - 3e) -  (6.36 - e) 

                                   = 1.5 - 2e    ...............equation (i)

The actual difference =    a1 -  b1

                                   = 3.75 - 4.25 

                                   = - 0.5         ................equation (ii)

∴ equation (ii) = equation (i)

                - 0.5 = 1.5 - 2e

                   2e  = 2

                     e  = 1m/jarak

actual reading at A (a2) = 7.86 - 3e

                                       = 7.86 - 3(1)

                                       = 4.86

actual reading at B (b2) = 6.36 - e

                                       = 6.36 - (1)

                                       = 5.36

                                  

Inverse staff reading

Figure 12 :  Inverse staff reading

Series level procedure, booking and calculation

Figure 13 :  Series level data and procedure

• The instrument (L1) is set up approximately ∓5m away from the BM and a staff reading to the nearest 0.001m is observed and booked. 
• The staff is move to IS point A and B. If any levels are required to objects in the immediate area of the level, then intermediate levels are taken and booked in the intermediate sight column.
• The staff is moved to the FS position point C and again the observation is observed to 0.001m and booked. (example of the booking sheet)
• Care should be taken that the BS distance is equal or nearly equal to the FS distance.
• This point C is referred to as a change point and must be a firm physical object.

• The level is then moved to (L2) approximately ∓5m from staff, as shown in the diagram above. BS reading is then observed to the staff at point C to the nearest 0.001m. 
• The staff is move to IS point D.
• The staff then moved to the next change point E and a FS reading taken.
• The level is then moved to (L3) approximately ∓5m from staff, as shown in the diagram above.
• These steps are repeated until the end of the levelling run is reached - the last point (FS) should always be a point of known RL. 

 

a. Rise and fall method

Adjustment          = 48.71 - 48.701

                             = 0.009

Total BS is 3 ∴    =  0.009 / 3

                             = 0.003 per station 

Arithmetic check

Σ (BS) - Σ (FS) = 6.962 – 8.131 = - 1.169m 

Σ (RISES) – Σ (FALLS) = 1.515 – 2.684 = - 1.169m  

LAST (RL) – FIRST (RL) = 48.701 – 49.87 = -1.169m

  

b. Height of collimation method (HOC)

Adjustment          = 48.71 - 48.701

                             = 0.009

Total BS is 3 ∴    =  0.009 / 3

                             = 0.003 per station 

Arithmetic check

Σ (BS) - Σ (FS) = 6.962 – 8.131 = - 1.169m 

LAST (RL) – FIRST (RL) = 48.701 – 49.87 = -1.169m




Longitudinal Section and Cross Section Procedure 

Figure 14 : Longitudinal Section and Cross Section Design. (longitude distance is option)

 Figure 15 : Longitudinal Section and Cross Section procedure

• Set-up a level at point of station 1 and perform temporary adjustment .(Refer Figure 15)
• Set-up staff on BM/TBM as back sight (BS) and at point a1 until point c2 as intermediate sight (IS). Take and record the reading.
• After that, move a level to station 2 and set-up it. Make sure the staff is still at point c3 as a back sight (BS) and the other staff move to point d1 until point e3 as  a intermediate sight (IS). Take and record the reading.  
• Close at BM/TBM (started point) as a fore sight. 
• During the process, make sure the bubble of staff and a level always center. 
• Then, determine the adjustment reduced level.  
• Calculate and plotting the result. 

Contouring using grid method procedure

Figure 16 : Contour using grid method design. (contour distance is option)

 Figure 17 : Contouring using grid method procedure

• Set-up a level at point of station 1 and perform temporary adjustment .(Refer Figure 16)
• Set-up staff on BM/TBM as back sight (BS) and at point a1 until point c2 as intermediate sight (IS). Take and record the reading.
• After that, move a level to station 2 and set-up it. Make sure the staff is still at point c3 as a back sight (BS) and the other staff move to point d1 until point e3 as  a intermediate sight (IS). Take and record the reading.  
• Close at BM/TBM (started point) as a fore sight. 
• During the process, make sure the bubble of staff and a level always center. 
• Then, determine the adjustment reduced level.  
• Calculate and plotting the result.

 Figure 18 : Interpolation method calculation