principle of leveling

the instrument is set up and correctly levelled in order to make the line of sight through the telescope horizontal . if the telescope is turned through 360 ُ . a horizontal plane of sight is swept out . vertical measurement from this plane .

using a graduated levelling staff . enable the relative elevation of ground points to be ascertained . consider , with the instrument set up approximately midway between ground point A and B . if the reducced level (RL) of point A known and equal to 100.000 m above OD 

(AOD) then the reading of 3.000 m on a vertically held staff at A gives the reduced level of horizontal line of sight as 103.000 m AOD . this sight onto A is termed a backsight (BS) and the reduced level of the line of sight is called the height of the plane of collimation ( HPC) thus ;

RLa + BS = HPC

the reading of 1.000 m onto the staff at B is called a foresight (FS) and shows the ground point B to be 1.000 m below HPC ; therefore its RL = (103.00 - 1.000) = 102.000 m AOD .

An alternative approach is to subtract the FS from the BS . if the result is positive then the difference is a rise from A to B and if negative a fall . i.e

(3.000 - 1.000 ) = + 2.000 m rise from A to B 

therefore , RLb  =  100.000 + 2.000 = 102.000 m AOD

this then is the basic concept of leveling which is further developed in .

the field data are entered into a field book that is pre-drawn into rows and columns . an example of levelling observations from a practical project, observation are booked using either the rise and fall or the HPC method .

it should be clearly noted that , in practice , the staff readings are taken to places of decimals , that is , to the nearest millimetre . however , in the following desripion only one place decimals is used and the numbers kept very simple to prevent arithmetic interfering with an understanding of concepts outlined ,

the field procedure for obtaining elevations at a series of ground point is as follows .

the instrument is set up at A, from which point a horizontal line of sight is possible to the TBM at 1A the first sight to be taken is to the staff held vertically on the TBM and this is called a 

basic principle of leveling

backsight (BS) the value of which (1.5 m ) would be entered in the appropriate column a levelling book . sight to point 2 A and 3A where further levels relative to the TBM are required sight arenintermediate (IS) and are again in the appropriate column of the levelling book . the final sight from this instrument is set up at 4A and is called the foresight (FS) it can be seen from figure that this is as far as one can go with this sight if . 

for instance the staff had been placed at X it would not have been visible and would have had to be moved down the slope , towards the instrument at A until it was visible As foresight 4A is as far one can see from A . it is also called the change point (CP signifying a change of instrument position to B . to achieve continuity in the levelling the staff must remain at exactly the same point 4A although it must be turned to face the instrument at B . it now becomes the BS for the new instrument set-up and whole procedure as before ,

thus one must remember that all levelling commences on a BS and finishes on FS with as many IS in between as are required ; and that CPs are always FS/BS . Also , it must be closed back into a known BM to ascertain misclosure error .