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

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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

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Equipment

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

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What is levelling?

• A measurement process whereby the difference in
  height between two or more points can be
  determined

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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

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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

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Definitions (cont.)
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Reading an E-face staff
0.339
0.33
0.3
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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
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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

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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)

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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

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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

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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...

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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

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Levelling procedures

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

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A sample loop
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Booking the observations
CP 3
BM A
1.32
3.98
Kerb
Kerb
CP 1
CP 2
Post
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Booking the observations
CP 3
BM A
2.56
Kerb
1.25
3.65
Kerb
0.67
CP 1
CP 2
Post
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Booking the observations
CP 3
BM A
Kerb
Kerb
3.49
2.58
1.54
CP 1
CP 2
Post
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Booking the observations
CP 3
BM A
Kerb
Kerb
2.64
3.79
CP 1
CP 2
Post
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Reducing levels (Rise and Fall)
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Reducing levels (Rise and Fall)
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Reducing levels (Rise and Fall)
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Reducing levels (Rise and Fall)
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Reducing levels (Rise and Fall)
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Reducing levels (Rise and Fall)
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Reducing levels (Rise and Fall)
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Reducing levels (Rise and Fall)
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Reducing levels (Rise and Fall)
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Reducing levels (Rise and Fall)
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Reducing levels (Rise and Fall)
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Reducing levels (Rise and Fall)
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Reducing levels (Rise and Fall)
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Reducing levels (Rise and Fall)
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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

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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

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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

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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)

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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)...

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Adjusting the misclose
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Adjusting the misclose
1(0.03/4)
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Adjusting the misclose
2(0.03/4)
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Adjusting the misclose
3(0.03/4)
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Adjusting the misclose
4(0.03/4)
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Adjusting the misclose
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Errors in levelling

• Collimation
• Parallax
• Change point instability
• Instrument instability
• Staff instability
• Benchmark instability
• Refraction

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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

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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

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Establishing a new point
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Measuring height differences
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Profiles and cross-sections
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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.
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Plotting contours
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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
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Plotting contours
LINE BC DHBC 2.905 - 2.510 0.395 DBC 10
m no contours cross this line
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Plotting contours
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Introduction to Surveying (BPD)LEVELLING
PROCEDURES

• Dr Philip Collier
• Department of Geomatics
• The University of Melbourne
• p.collier_at_unimelb.edu.au
• Room D316