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H.U. MINING ENGINEERING DEPARTMENT MAD 256 – SURVEYING. LEVELING P ROCEDURES. Equipment. Level Tripod Staff Change plate Staff bubble 50 m tape measure (sometimes). BS. FS. Difference in height H=BS-FS. What is levelling?.
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H.U. MINING ENGINEERING DEPARTMENT MAD 256 – SURVEYING LEVELING PROCEDURES
Equipment • Level • Tripod • Staff • Change plate • Staff bubble • 50 m tape measure (sometimes)
BS FS Difference in height H=BS-FS What is levelling? • A measurement process whereby the difference in height between two or more points can be determined
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
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 (<100 m) the horizontal surface and the level surface will coincide
direction of gravity horizontal surface level surface limit of practical coincidence (~100 m) Definitions (cont.)
Reading an “E-face” staff 0.339 0.33 0.3
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
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)
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
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
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...
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 < 50m) • Never read below 0.5m on a staff (refraction) • Use stable, well defined change points
Setup 4 CP 3 BM A Kerb Setup 3 Setup 1 Kerb Setup 2 CP 1 CP 2 Post A sample loop
Setup 1 Booking the observations CP 3 BM A 1.32 3.98 Kerb Kerb CP 1 CP 2 Post
Setup 1 Setup 2 Booking the observations CP 3 BM A 2.56 Kerb 1.25 3.65 Kerb 0.67 CP 1 CP 2 Post
Setup 1 Setup 3 Setup 2 Booking the observations CP 3 BM A Kerb Kerb 3.49 2.58 1.54 CP 1 CP 2 Post
Setup 4 Setup 1 Setup 3 Setup 2 Booking the observations CP 3 BM A Kerb Kerb 2.64 3.79 CP 1 CP 2 Post
Loop misclosure (Error) • Misclosure (Error) • The amount by which the measured height (RLmeasured) differs from the known height (RLknown) of the starting and finishing benchmarks Misclosure (Error) = RLmeasured – RLknown Error = 50.03 – 50.00 = 0.03 m or Error = Back – Fore = 10.01-9.98 = 0.03 m Error = Rise – Fall = 6.24-6.21 = 0.03 m
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
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 12k mm • where k is the length of the loop in km
Continuing the example • The misclosure is +30 mm (0.03 m) • 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)
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 example is Correction=(-0.03/4)
Adjusting the misclose =1*(-0.03/4)
Adjusting the misclose =2*(-0.03/4)
Adjusting the misclose =3*(-0.03/4)
Adjusting the misclose =4*(-0.03/4)
Errors in levelling • Collimation • Parallax • Change point instability • Instrument instability • Staff instability • Benchmark instability • Refraction
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
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
New point Benchmark RLNEW RLBM Datum Establishing a new point
DH3 DH2 DH1 Benchmark RLBM Datum Measuring height differences
Benchmark RLBM Datum Profiles and cross-sections
C B 2.905 2.510 1.100 A Plotting contours The RL’s 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.
