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
DEA= (a – b) and DEB = (a’ – b’)
TDE = (DEA+ DEB)/2 = [(a – b) + (a’ – b’)]/2
Elev. BM2 = Elev. BM1 ±TDE
where:
DEA = the mean difference in elevation between A and B with level at left
DEB = the mean difference in elevation between A and B with level at right
TDE = the true mean difference in elevation between A and B
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Procedure:
- Set two hubs either on the 2nd or 3rd floors of the Velasco and
- Set up and level very carefully the instrument at point A at Velasco Building (3rd floor) near BM1.
- Take and record a series of three consecutive backsight rod readings on BM1 and six consecutive foresight rod readings on BM2.
- Move the instrument to point B at
- From the new set up, take and record three consecutive foresight rod readings on BM2 and six consecutive backsight rod readings on BM1.
- Tabulate observed and computed values accordingly. Refer to the accompanying sample format for the tabulation of field data.
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Instrument Set Up Near BM1
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Instrument Set Up Near BM2
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Station
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B.S.
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F.S.
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Station
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F.S.
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B.S.
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BM1
(a)
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BM2
(b’)
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BM2
(b)
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BM1
(a’)
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Sum
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Sum
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Mean
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Mean
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