RECIPROCAL LEVELING

RECIPROCAL LEVELING

Reciprocal leveling is employed to determine the difference in elevation between two points when it is difficult or impossible to keep backsights and foresights short and equal

In this method, two sets of rod readings are observed, (1) one set is taken with the instrument set up close to one point, and (2) another with the instrument set up close to the other point.

Considering the Figure 1 below, the equations to obtain the difference in elevation are the following

DE_A= (a – b) and DE_B = (a’ – b’)

TDE = (DE_A+ DE_B)/2 = [(a - b) + (a’ – b’)]/2

Elev. BM2 = Elev. BM1 ±TDE

where:

   DE_A = the mean difference in elevation between A and B with level at left

   DE_B = the mean difference in elevation between A and B with level at right

   TDE = the true mean difference in elevation between A and B

 

Procedure:

1. Set two hubs either on the 2^nd or 3^rd floors of the Velasco and La Salle Buildings and call these the BM1 and BM2, respectively. The elevation of BM2 is to be determined while the elevation of BM1 is known or assumed to be 10.000 meters above see level. Also, imagine that the football field between the Velasco and La Salle Buildings is an intervening deep ravine or wide river.
2. Set up and level very carefully the instrument at point A at Velasco Building (3^rd floor) near BM1.
3. Take and record a series of three consecutive backsight rod readings on BM1 and six consecutive foresight rod readings on BM2.
4. Move the instrument to point B at La Salle Building (3^rd floor) which is on the opposite side of the imaginary ravine and near BM2, set up, and level the instrument carefully.
5. From the new set up, take and record three consecutive foresight rod readings on BM2 and six consecutive backsight rod readings on BM1.
6. Tabulate observed and computed values accordingly. Refer to the accompanying sample format for the tabulation of field data.

Instrument Set Up Near BM1			Instrument Set Up Near BM2
Station	B.S.	F.S.	Station	F.S.	B.S.
BM1 (a)			BM2 (b’)
BM2 (b)			BM1 (a’)
Sum			Sum
Mean			Mean