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Introduction to Surveying BPD LEVELLING PROCEDURES

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Collimation error ... Identifies whether the level has a collimation error. Allows the collimation error to be determined ... – PowerPoint PPT presentation

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Title: Introduction to Surveying BPD LEVELLING PROCEDURES


1
Introduction to Surveying (BPD)LEVELLING
PROCEDURES
  • Dr Philip Collier
  • Department of Geomatics
  • The University of Melbourne
  • p.collier_at_unimelb.edu.au
  • Room D316

2
Review of levelling
  • In this lecture we will cover
  • Equipment and procedures
  • Purposes of levelling
  • Some definitions
  • Reading a staff
  • Collimation error
  • Two-peg test
  • Booking and reduction
  • Adjustment
  • Errors
  • Applications

3
Equipment
  • Level
  • Tripod
  • Staff
  • Change plate
  • Staff bubble
  • 50 m tape measure (sometimes)

4
What is levelling?
  • A measurement process whereby the difference in
    height between two or more points can be
    determined

5
When do we level?
  • Typical examples include
  • To establish new vertical control (BM or TBM)
  • To determine the heights of discrete points
  • To provide spot heights or contours on a plan
  • To provide data for road cross-sections or
    volumes of earthworks
  • To provide a level or inclined plane in the
    setting out of construction works

6
Definitions
  • Level surface
  • A surface over which water will not flow
  • The direction of gravity is always normal to a
    level surface
  • Horizontal surface
  • A horizontal surface will be tangent to a level
    surface
  • Over short distances (lt100 m) the horizontal
    surface and the level surface will coincide

7
Definitions (cont.)
8
Reading an E-face staff
0.339
0.33
0.3
9
Collimation error
  • Occurs when the line of sight (as defined by the
    cross-hairs) is not horizontal
  • Leads to an incorrect staff reading

line of sight
error
horizontal line
10
Two-peg test
  • Identifies whether the level has a collimation
    error
  • Allows the collimation error to be determined
  • See the plane surveying web site for the two-peg
    test procedure

11
More definitions
  • Datum
  • A reference surface to which the heights of all
    points in a survey or on a site are referred
  • May be arbitrary or a national height datum
  • In Australia we have the Australian Height Datum
    (AHD)
  • The surface which defines the AHD is
    (approximately) Mean Sea Level (MSL)

12
More definitions
  • Reduced Level (RL)
  • The height of a point above the datum
  • Benchmark (BM)
  • A stable reference point of known RL
  • Usually used as the starting and finishing point
    when levelling
  • Temporary Bench Mark (TBM)
  • A point placed (e.g. peg, nail, spike) to provide
    a temporary reference point

13
More definitions
  • Backsight (BS)
  • Always the first reading from a new instrument
    station
  • Foresight (FS)
  • Always the last reading from the current
    instrument station
  • Intermediate sight (IS)
  • Any sighting that is not a backsight or foresight

14
More definitions
  • Change point (CP)
  • Location of the staff when the level is moved
  • Change points should be...
  • Stable
  • Well defined
  • Recoverable
  • e.g. sharp rock, nail, change plate, etc...

15
Rules for levelling
  • Always commence and finish a level run on a
    Benchmark (BM or TBM)
  • Keep foresight and backsight distances as equal
    as possible
  • Keep lines of sight short (normally lt 50m)
  • Never read below 0.5m on a staff (refraction)
  • Use stable, well defined change points

16
Levelling procedures
  • The example on the plane surveying web site
    shows
  • Observation procedures
  • Booking procedures
  • Reduction procedures

17
A sample loop
18
Booking the observations
CP 3
BM A
1.32
3.98
Kerb
Kerb
CP 1
CP 2
Post
19
Booking the observations
CP 3
BM A
2.56
Kerb
1.25
3.65
Kerb
0.67
CP 1
CP 2
Post
20
Booking the observations
CP 3
BM A
Kerb
Kerb
3.49
2.58
1.54
CP 1
CP 2
Post
21
Booking the observations
CP 3
BM A
Kerb
Kerb
2.64
3.79
CP 1
CP 2
Post
22
Reducing levels (Rise and Fall)
23
Reducing levels (Rise and Fall)
24
Reducing levels (Rise and Fall)
25
Reducing levels (Rise and Fall)
26
Reducing levels (Rise and Fall)
27
Reducing levels (Rise and Fall)
28
Reducing levels (Rise and Fall)
29
Reducing levels (Rise and Fall)
30
Reducing levels (Rise and Fall)
31
Reducing levels (Rise and Fall)
32
Reducing levels (Rise and Fall)
33
Reducing levels (Rise and Fall)
34
Reducing levels (Rise and Fall)
35
Reducing levels (Rise and Fall)
36
Loop misclosure
  • Misclosure
  • The amount by which the measured height
    difference (DHmeas) differs from the known height
    difference derived from the RLs of the starting
    and finishing benchmarks (DHknown)
  • Misclosure DHknown - DHmeas

37
An acceptable misclose?
  • Small misclosures 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 repeated
  • Misclosures can also result from errors in
    published BM levels and from BM instability

38
Testing the misclose
  • The amount of misclosure we are prepared to
    accept depends on the accuracy we are hoping to
    achieve
  • For routine levelling, the third order levelling
    standard is adopted
  • misclosure ? 12?k mm
  • where k is the length of the loop in km

39
Continuing the example
  • The misclosure is 30 mm
  • The length of the loop is 0.7 km
  • The misclosure limit is
  • 12?(0.7) 10 mm
  • The misclosure of 30 mm is too big
  • The loop must be repeated (or find the error)

40
Adjusting the misclose
  • Adjustment is carried out to ensure that the
    measured and known RLs of the closing benchmark
    agree
  • The misclosure is linearly distributed according
    to the number of set-ups
  • The adjustment per set-up for the previous
    example is (0.03/4)...

41
Adjusting the misclose
42
Adjusting the misclose
1(0.03/4)
43
Adjusting the misclose
2(0.03/4)
44
Adjusting the misclose
3(0.03/4)
45
Adjusting the misclose
4(0.03/4)
46
Adjusting the misclose
47
Errors in levelling
  • Collimation
  • Parallax
  • Change point instability
  • Instrument instability
  • Staff instability
  • Benchmark instability
  • Refraction

48
Errors in levelling
  • Staff reading and interpolation errors
  • Staff verticality
  • Instrument shading
  • Temperature on staff
  • Booking errors (e.g. using just 1 benchmark)
  • Earth curvature
  • Magnetic field effects on auto level

49
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

50
Establishing a new point
51
Measuring height differences
52
Profiles and cross-sections
53
Plotting contours
The RLs for points A, B and C have
been determined by levelling. We are now
required to determine the location of
the contours using a 0.5 m contour interval.
54
Plotting contours
55
Plotting contours
LINE AC ?HAC 2.905 - 1.100 1.805 DAC 14.14
m For the 1.5 m contour D 14.14(1.5 -
1.1)/1.805 3.13 For the 2.0 m contour D
14.14(2.0 - 1.1)/1.805 7.05 For the 2.5 m
contour D 14.14(2.5 - 1.1)/1.805 10.97
56
Plotting contours
LINE BC DHBC 2.905 - 2.510 0.395 DBC 10
m no contours cross this line
57
Plotting contours
58
Introduction to Surveying (BPD)LEVELLING
PROCEDURES
  • Dr Philip Collier
  • Department of Geomatics
  • The University of Melbourne
  • p.collier_at_unimelb.edu.au
  • Room D316
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