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Chapter 7 : Levelling (Terminology (Benchmark (BM) (A stable reference…
Chapter 7 : Levelling
Terminology
- The height of a point above the datum
- Height of reference point of a determined level
- A stable reference point of known RL
- Usually used as the starting and finishing point when levelling
- A fixed point on earth surface with known reduced level
- A reference surface to which the heights of all points in a survey or on a site area referred
- Temporary benchmark (TBM)
- A temporary reference point (e.g peg, nails, spike)
- The first reading from a new instrument station
- The last reading from the current instrument station
- Taken after intermediate sight (IS)
- A point between backsight & foresight
- Turning point (TP) or change point (CP)
- Location of the staff when the automatic level setup is moved
- A point which both backsight and foresight reading are taken
Definition
- A branch of surveying that measures or compares height/elevation of points on the surfaces of the earth
- Heights/elevations can be measured using total stations, handheld lasers and GPS devices
- Widely used in construction sites :arrow_forward: inexpensive, simple & inaccurate method for measuring heights
- Any method of measuring the heights of points above/below the ground using an agreed datum
Arithmetic check
- T BS - T FS = T R - T F = RL final - RL initial
(THE ANSWER MUST BE THE SAME)
- The value of (answer) is the misclosure error and need to be corrected
- Misclosure error :arrow_forward: amount by which the measured height differs from the known height
Purpose
- to find elevation of a given point with respect to the given or assumed datum
- to establish a point at a given elevation with respect to the given or assumed datum
Calculating correction
- Things that need to do the correction is
- setup = stations available
- error = misclosure error
- The correction for every setup is
Misclosure error / Setup
Accuracy in levelling
- Accuracy can be increased by
:check: repeating measurement
:check: taking reading with accuracy
:check: prevent errors
:check: using high quality instruments
- Jabatan Ukur & Pemetaan Malaysia (JUPEM) suggests the following way to assess accuracy
C = m (punca kuasa) KWhich is
C = allowable loop error misclosure (mm)
m = constant
K = total loop length (km)