Differential Leveling

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ABE 70FUNDAMENTALS OF SURVEYING

DEFINITION OF TERMS

BENCHMARK (BM) - a permanent or

temporary fixed point of reference whose

elevation is either known or assumed; should

be easily recognized and located where they

have the smallest likelihood of being disturbed

BACKSIGHT (BS) - a reading taken on a rod

held over a point of known or assumed elevation

Benchmark

Point 1

0.445 m

1.535 m

Mean sea level or datum

100 m MSL

BS

FS

DEFINITION OF TERMS

FORESIGHT (FS) - a reading taken on a rod held

over a point whose elevation is to be determined

Benchmark

Point 1

0.445 m

1.535 m


100 m MSL

BS

FS

DEFINITION OF TERMS

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BACKSIGHT DISTANCE (BSD) - measured from the

center of the instrument to the rod on which a backsight

is taken

FORESIGHT DISTANCE (FSD) - measured from the

center of the instrument to the rod on which a foresight

is taken

Benchmark

Point 1

0.445 m

1.535 m


100 m MSL

BS

FS

BSD FSD

DEFINITION OF TERMS TURNING POINT (TP) - an intermediate point

between two benchmarks upon which foresight

and backsight rod readings are taken

also referred to as change point (CP) and

usually numbered consecutively

BM 2

TP 1

BM 1

TP 2

DEFINITION OF TERMS

HEIGHT OF INSTRUMENT (HI) - elevation of the line

of sight of an instrument above or below a selected

reference datum

HI = ELEV + BS

Benchmark

Point 1

0.445 m

1.535 m


100 m MSL

BS

FS

HI

DEFINITION OF TERMS

DIFFERENTIAL LEVELING the process used in the establishment of

differences in elevation between two or more

points with respect to a datum or mean sea

level

DEFINITION OF TERMS

OBJECTIVES

To determine the elevation of

point(s) by differential leveling

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PROCEDURE FOR DIFFERENTIAL LEVELING

BM 1

BM 2

TP 1


BM 1

TP 2

BM 2

TP 1


BM 1

TP 2

BM 2

TP 1


BM 2

BM 1

BM 1

TP 2

BM 2

TP 1

LEVELING NOTES

STA sighted BS HI FS ELEV Remarks

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FIELD NOTES


BM1 1.100 100 concrete block supplementary BM (LAG-25)

TP1 0.750 drainage cover (metal)

FIELD NOTES


BM1 1.100 101.100 100 concrete block supplementary BM (LAG-25)

TP1 0.750 100.350 drainage cover (metal)

FIELD NOTES



TP1 1.245 0.750 100.350 drainage cover (metal)

TP2 0.350 big rock (with mark X)

FIELD NOTES



TP1 1.245 101.595 0.750 100.350 drainage cover (metal)

TP2 0.350 101.245 big rock (with mark X)

BM 2

BM 1




TP2 1.355 0.350 101.245 big rock (with mark X)

BM 2 0.575 top of fire hydrant

BM 2

BM 1




TP2 1.355 102.600 0.350 101.245 big rock (with mark X)

BM 2 0.575 102.025 top of fire hydrant

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∑ 3.700 1.675

Arithmetic check

DE 1 = Elev BM1 - Elev BM2

= 100.000 - 102.025

= 2.025 (negative/positive sign indicates the

position of BM2 with respect to BM1)

DE2 = ∑BS - ∑FS

= 3.700 - 1.675

= 2.025

To check the elevation:

100.000

+ 3.700 ∑BS

- 1.675 ∑FS

102.025

DETERMINING ACCURACY of the survey results:

BM 1

TP 2

BM 2TP 1

TP 3

TP 4






TP 3 0.475 101.755 1.250 101.280 land marker (concrete)

TP 4 0.700 101.340 1.115 100.640 side of the road (with mark X)

BM 1 1.300 100.040 concrete block supplementary BM(LAG-25)

Error of Closure (EC)

Allowable Error of Closure (AEC)

Error of closure (EC) = Computed Elevation - Known Elevation

= BM 1’ - BM 1

Allowable Error of Closure (AEC) = c √ k

where: c - constant (4, 8.4 or 12 mm if 1st, 2nd, or 3rd order)

k - total loop distance along the line of sight, km

Adjustment of the elevation of BM 2

Adj. Elev = Computed Elev - (a/k) (±EC)

where: a - distance from BM1 to BM2

If EC ≤ AEC, the survey is acceptable!

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Exercise No.

Title of the Exercise

Date and Time Weather condition

Name and Designation Signature

Objective of the exercise

List of Equipments

EXPECTED OUTPUT

1. Sketch

2. Tabulations

3. Computations

4. Results and Discussion

5. Summary and Conclusions

Documents

Differential Leveling