3b. bec 102 levelling-correction & contour

7/27/2019 3b. BEC 102 Levelling-Correction & Contour

1/36

3b. LEVELLING

Errors, Corrections & Contour

1


2/36

OUTLINE (Previous chapter:3a)

Levelling When do we level?

Definitions/Terminology

Levelling Instrumentation

Method of Levelling

Rules for Levelling

Levelling Procedures (Two peg test, reducing levels

using rise and fall method)

2


3/36

OUTLINE (Now...)

Misclosure for Reduce Levels (Rise & Fall) Correction or Adjusting the Misclosure

Errors in Levelling

3


4/36

Levelling procedures & errors

Two-peg test(Determining for collimation errors)

4


5/36

Error on reducing levels (Rise and Fall)

Back Inter Fore Rise Fall RL Comment

1.32 50.00 BM A

2.56 3.98 2.66 47.34 CP 1

1.25 1.31 48.65 Kerb3.65 2.40 46.25 Post

3.49 0.67 2.98 49.23 CP 2

2.58 0.91 50.14 Kerb

2.64 1.54 1.04 51.18 CP 3

3.79 1.15 50.03 BM A

10.01 9.98 6.24 6.21

(0.03) (0.03) (0.03) 5


6/36

An acceptable misclose (error)?

Misclose=the amount by which the measuredheight

difference (Hmeas) differs from the known height difference

derived from the RLs of the starting and finishing

benchmarks (Hknown)

Smallmisclosures in closed level loops are expected

because of the accumulation of errors

If the misclosure is small, it can be adjusted

If the misclosure is large, the loop (or part of it) must

be repeated6


7/36

Adjusting the misclose

Adjustment is carried out to ensure that themeasured and known RLs of the closing benchmark

agree

The misclosure is linearly distributed according to the

number ofset-ups

The adjustment per set-up for the previous example

is (0.03/4)...

7


8/36

Adjusting the Errors

0.03/4

8


9/36


Measured RL Point Adjustment Adjusted RL

50.00 BM A

47.34 CP 1

48.65 Kerb

46.25 Post

49.23 CP 2

50.14 Kerb

51.18 CP 3

50.03 BM A9


10/36



50.00 BM A 0.000 50.000

47.34 CP 1 0.008 (0.0075) 47.332

48.65 Kerb

46.25 Post

49.23 CP 2

50.14 Kerb

51.18 CP 3

50.03 BM A

=1*(0.03/4)

10


11/36



50.00 BM A 0.000 50.000

47.34 CP 1 0.008 47.332

48.65 Kerb 0.015 48.635

46.25 Post 0.015 46.235

49.23 CP 2 0.015 49.215

50.14 Kerb

51.18 CP 3

50.03 BM A

=2*(0.03/4)

11


12/36



50.00 BM A 0.000 50.000

47.34 CP 1 0.008 47.332

48.65 Kerb 0.015 48.635

46.25 Post 0.015 46.235

49.23 CP 2 0.015 49.215

50.14 Kerb 0.023 50.117

51.18 CP 3 0.023 51.157

50.03 BM A =3*(0.03/4)12


13/36



50.00 BM A 0.000 50.000

47.34 CP 1 0.008 47.332

48.65 Kerb 0.015 48.635

46.25 Post 0.015 46.235

49.23 CP 2 0.015 49.215

50.14 Kerb 0.023

51.18 CP 3 0.023

50.03 BM A 0.030 50.000

=4*(0.03/4)

13


14/36



50.00 BM A 0.000 50.000

47.34 CP 1 0.008 47.332

48.65 Kerb 0.015 48.635

46.25 Post 0.015 46.235

49.23 CP 2 0.015 49.215

50.14 Kerb 0.023 50.117

51.18 CP 3 0.023 51.157

50.03 BM A 0.030 50.00014


15/36

Errors in levelling

Equipment Error Field Error

Reading and Booking

15


16/36

Errors in the Equipment

Collimation Error - Line of sight not horizontal

Correctreading

Actual/observed

reading

Two-peg test(Determining for collimation errors)

16


17/36



Correct

reading

Actual/observed

reading

Size of error depends onsight length

17


18/36



Keep sight lengths from each instrument position the same

Check collimation error using Two-Peg Test

Parallax

Focus eyepiece on cross-hairs, then focus telescope on staff

Check prism is not stuck (every reading)

18


19/36


Staff Errors

Zero error - base may be worn - doesnt matter as long assame staff is always used

May not be extended properly

Tripod Errors

Must be stable

19


20/36

Field Errors

Staff not vertical

Use pond bubble on staff

Rock staff and take minimum reading

Unstable equipment

Watch out for soft ground under tripod or staff

Dont touch (or kick!) tripod

20


21/36

Reading and Booking Errors

Keep sightings short to estimate mm on staff accurately

Double check all readingsWrite clearly

WeatherWind causes level to vibrate, heat causes shimmer

Carry out calculation checks

21


22/36

Applications of levelling

Point heights (relative to a datum) Height differences (independent of datum)

Longitudinal sections and cross sections

Data for volume calculations

Contouring

Setting out

22


23/36

Establishing a new point

Benchmark

New point

RLBM

Datum

RLNEW

23


24/36

Measuring height differences

Benchmark

RLBM

Datum

H1

H2

H3

24


25/36

Profiles and cross-sections

RLBM

Benchmark

Datum25


26/36

Contour Map

40m

50m

60m

26


27/36

What is Contour?

Imaginary line on the ground joining point of equalaltitude or elevation.

Contour line shown on maps or plans as a line.

Contour line mostly used for indicate the elevation

directly.

27


28/36

What is Contour?

5

10

15

20

25

5

1015

20 25

ContourInterval: 5 m

28


29/36

USES OF CONTOUR MAP Alignment of a highway or railway line.

For selecting the most suitable site variousengineering projects.

Determination of the capacity of the reservoir area.

Quantities of earth work can be computed byselecting suitable sections.

Location of various strategic points which are veryimportant in military operations.

29

METHODS OF LOCATING


30/36

METHODS OF LOCATING

CONTOURS

DIRECT METHOD

INDIRECT METHOD

Squares or Gridding Method

Radial Method

30


31/36

INTERPOLATION OF CONTOURS

Arithmetical Calculation

By Graphical Method

31


32/36

1.100

2.9052.510

A

B C

Plotting contours (Arithmetical

Calculation)

The RLs for points A, B and C have beendetermined by levelling. We are now

required to determine the location of thecontours using a 0.5 m contour interval.

32


33/36

Plotting contours

LINE ABHAB = 2.51 - 1.10 = 1.410DAB = 10 m

1.100

2.510

A

B

A

B

1.41

10 m

1.4

9.93

For the 2.5 m contour :

D = 10*(2.5 - 1.1)/1.41 = 9.93

0.9

6.38

For the 2.0 m contour :

D = 10*(2.0 - 1.1)/1.41 = 6.38

0.4

2.84

For the 1.5 m contour:

D = 10*(1.5 1.1)/1.41 = 2.84

33


34/36

Plotting contours

LINE BCDH

BC= 2.905 - 2.510 = 0.395

DBC = 10 m

no contours cross this line

1.100

2.9052.510

A

B C

34


35/36

1.100

2.9052.510

A

B C

Plotting contours

35


36/36

THANK YOU

36

3b. bec 102 levelling-correction & contour

Documents