site surveying levelling errors - trent global · best practice guide for levelling ... levelling...
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SITE SURVEYING
LEVELLING ERRORS
Known datum
TBM required here
A
B
CD E
F
GHI
Instrument positions Staff positions
1.003
1.524
1.645
2.134 2.835
2.163 2.884
1.332 1.058
2.272
Example 1
BS IS FS H of I RL Remarks
100.000 OBM
B
D
F - TBM
H
OBM
1.003
Notice there are no intermediate sights since we only want to know the level at F.
1.524 1.645
2.134 2.835
2.163 2.884
1.332 1.058
2.272
101.003
99.479 101.124
98.990 101.825
99.662 102.546
101.214 102.272
100.000
Accuracy
Allowable Misclosure = ±5n mm
Must always close a survey onto a known point
where n = number of Instrument
Positions (FS readings)
Sum BS – Sum FS = First RL – Last RL
If more than is allowable, the levelling should be repeated.
If less than, the misclosure is spread equally between the CP values
BS IS FS H of I RL Remarks
100.000 OBM
B
D
F - TBM
H
OBM
1.003
1.524 1.645
2.134 2.835
2.163 2.884
1.332 1.058
2.272
101.003
99.479 101.124
98.990 101.825
99.662 102.546
101.214 102.272
100.000
∑ =9.425 ∑ =9.425
Example 1 accuracy check
∑ B.S - ∑ F.S = 9.425 – 9.425 = 0m
Last RL – First RL = 100.000 – 100.000 = 0m
Best Practice Guide for levelling
•If the level is knocked over or bumped during use always
recheck it before carrying on
•Always carry the level upright on its tripod
•Ensure that the person holding the staff understands the
importance of what they are doing and knows how to do the
job properly
•Always check that the level is reading accurately before you
use it by doing the two peg test
•Do not let the instrument get wet as they take a long time to
dry out and you will not be able to use it if it ‘mists’ up
Levelling Errors
Cumulative Errors
• Staff not vertical - the staff can be used with a spirit
level. The staff can be rocked slightly backwards and
forwards slowly, with the lowest position sighted being
recorded. This method is used in windy conditions as it is
difficult to keep the staff still.
•Height of collimation of the instrument is not truly vertical.
Keep BS and FS equal lengths where possible. A two
peg test can be carried out before starting the survey and
any adjustments can be applied if necessary
Gross Errors
•Level incorrectly adjusted - constantly recheck the
spirit level on the instrument.
•Staff moved position at the change point - carefully
swivel staff on the same spot through 180.
•Staff not properly extended - ensure button has clicked
into the aperture.
•Reading off stadia lines instead of axial line - ensure
familiarity of different cross hairs.
•Booking in wrong column or with wrong figures - great
care needed when booking figures
Random Errors
•Soft ground under staff - use a staff base plate.
•Wind - extra care required to ensure staff vertically in
wind conditions.
•Thermal expansion of staff - most staffs have a
temperature compensation figure printed on the staff,
though this effect is negligible
Checking Accuracy of Level – Two Peg Test
Level of tolerance depends on the work you are doing, for a land survey 5mm is
acceptable, but for setting out holding down bolts it would need to be no greater
than 2mm
reading at A - reading of B = reading of A - reading of B
from IP1 from IP1 from IP2 from IP2
Sightings equal distance
Sightings unequal distance
Example 2
wall
building
ic3
ic1 ic2 step
gully
OBM
IP1
IP2
IP3
CP2
CP1
Remarks
OBM 35.215m AOD
ic1 (square)
Top of wall
ic2 (round) cover
CP1 pavement
ic3 (square) cover
Step to building
Gulley, bldg corner
CP2 pavement
OBM 35.215m AOD
BS IS FS H of I RL Remarks
35.215 OBM 35.215m AOD
ic1 (square)
Top of wall
ic2 (round) cover
CP1 pavement
ic3 (square) cover
Step to building
Gulley, bldg corner
CP2 pavement
OBM 35.215m AOD
1.724
1.855
1.232
1.658
1.597 1.754
1.585
1.843
1.954
2.081 1.797
1.602
36.939
35.084
35.707
35.281
35.342 37.096
35.511
35.253
35.142
35.015 36.812
35.210
∑=5.275 ∑= 5.280
∑ B.S - ∑ F.S = 5.275 – 5.280 = -0.005m
∑ B.S = 5.275m
∑ F.S = 5.280m
Arithmetical Check ∑ B.S - ∑ F.S = Last RL – First RL
Last RL – First RL = 35.210 – 35.215 = - 0.005m = 5mm
Allowable Misclosure = ±5n mm
= ±5√3 =±9mm
The 5mm can be redistributed equally amongst the change points
BS IS FS H of I RL Remarks
35.215 OBM 35.215m AOD
ic1 (square)
Top of wall
ic2 (round) cover
CP1 pavement
ic3 (square) cover
Step to building
Gulley, bldg corner
CP2 pavement
OBM 35.215m AOD
1.724
1.855
1.232
1.658
1.597 1.754 +2
1.585
1.843
1.954
2.081 1.797 +3
1.602
36.939
35.084
35.707
35.281
35.342 +2 37.098
35.513
35.255
35.144
35.017 +3 36.817
35.215
∑=5.280 ∑= 5.280
Example 3
BS IS FS H of I RL Remarks
OBM A 40.000mAOD
OBM A
40.000mAOD
OBM B
40.500mAOD
TBM
0.505 1.290
0.290 2.459 1.482 0.868 1.492
2.538 1.518
2.664 0.220
0.505
1.290 0.290
2.459 1.482
0.868
1.492 2.538
1.518 2.664
0.220
40.000 40.505
CP1
CP2
TBM
CP3
CP3
OBM B 40.500mAOD
39.215 39.505
37.046 38.528
37.660
37.036 39.574
38.056 40.720
40.500
BS IS FS H of I RL Remarks
OBM A 40.000mAOD 0.505
1.290 0.290
2.459 1.482
0.868
1.492 2.538
1.518 2.664
0.220
40.000 40.505
CP1
CP2
TBM
CP3
CP3
OBM B 40.500mAOD
39.215 39.505
37.046 38.528
37.660
37.036 39.574
38.056 40.720
40.500
∑=7.479 ∑=6.979
∑BS - ∑FS = 7.479 - 6.979 = 0.500m
First RL – Last RL = 40.500 – 40.000 = 0.500m
How would you spread the error?
You wouldn’t it is not within the allowable misclosure, you would redo the survey
Example 4
1.200 2.100
2.800 2.400 1.800 2.000 1.700
OBM A
100.000
1.300
OBM B
100.900 IS1
IS2 CP1 IS3
IS4
BS IS FS H of I RL Remarks
Example 4
1.200 2.100
2.800 2.400 1.800 2.000 1.700
OBM A
100.000
1.300
OBM B
100.900 IS1
IS2 CP1 IS3
IS4
BS IS FS H of I RL Remarks
OBM A 100.000mAOD
∑4.100 ∑3.200 0.900 Difference of 0.900
2.800
2.400
1.800
1.300 1.200
1.700
2.100
2.000
100.000 102.800
IS1
IS2
CP1
IS3
IS4
OBM B 100.900mAOD
100.400
101.000
101.600
101.200
102.900
100.800
100.900
Example 5
B
C
DA1
BS
2.400
A2
IS
2.000
A3
IS
1.900
A4
FS
1.400
B1
BS
2.800
B2
IS
2.000
B3
IS
1.400
B4
FS
2.600
C1
BS
1.300
C2
IS
2.500
C3
FS
3.000
D1
BS
0.600
D2
IS
1.700
D3
FS
2.100
A
Level positions
Staff positions
Staff positions where
level is moved
TBM 28.000
D3 26.000
B
C
OBM
BS
1.280
A1
IS
2.250
A2
IS
2.405
A3
FS
3.000
B1
BS
2.850
B2
IS
2.675
B3
IS
2.025
B4
FS
1.950
C1
BS
2.050
C2
IS
1.850
TBM
FS
1.535
A
Level positions
Staff positions
Staff positions where
level is moved
Example 6
OBM 32.500
TBM 32.195