site surveying procedures bueed1006 · levelling • the level is used is used for two main...
TRANSCRIPT
Levelling Recap
EQUIPMENT
• Automatic level
– Consist of a telescope that is fixed rigidly to
a spindle and is supported on three foot
screws
– The level is supported by attachment to a
tripod.
– Tripod
• level is supported by attachment to the
tripod
– Staff – metric rule
Telescopic
staff/stave
2.5
2.6
metres
decimetres
centimetres
Each centimetre block = 10mm.
These are the only part of the
readings that need estimating
What are the main applications of levelling?
Levelling
• The level is used is used for two main
purposes:
1. To establish the reduced level, which
is the height of a point relative to the
original starting datum, of existing
features and ground
2. To establish datums that can be used
for the vertical control of building work
Datum Terminology
• Ordnance Datum (O.D.)
– lowest point in the British Isles
– Newlyn in Cornwall = 0.000m
E.G. 34.239 is 34metres and 239mm
above the mean sea level at Newlyn
• Ordnance Bench Mark (O.B.M.)
– Marks made by ordnance surveyors on
permanent structures
– Usually cut into stone or brick in the shape
shown below
Ordnance
Level
OBM
Survey Reference System
SLA introduced the Co-ordinated Cadastre system.
The new system defines property boundaries by co-
ordinates, rather than bearings and distances.
SLA has established the land survey control network as the
infrastructure to support survey work conducted by all
Registered Surveyors. This allows all land lots in
Singapore to be geographically linked to each other and to
the whole of Singapore.
Survey Reference System
We maintain 2 types of survey control:
•Horizontal Control Network
•Vertical Control Network
Horizontal Control Network
There are two categories of horizontal control: Primary and
Secondary control network. As a whole, they are known as the
Integrated Survey Network (ISN).
Survey Reference System
Vertical Control Network
The vertical control network consists of physical points on the
ground known as Precise Levelling Benchmarks (PLBM). There
are about 400 PLBMs located at 1 km intervals along major roads.
PLBM information is used to determine heights of locations in
Singapore.
Survey Reference System
Positioning with Global Navigation Satellite System
(GNSS)
SLA maintains a network of continuously operating satellite
reference stations on the ground, known as Singapore Satellite
Positioning Reference Network (SiReNT) . SiReNT is a
nation-wide infrastructure established and maintained by SLA
that supports real-time and post-processed high precision
satellite survey and positioning applications. With SiReNT, the
establishment of new control points is seamless and easy.
• Temporary Benchmark (T.B.M.)
• A TBM is a level that is transferred
from the OBM to a location that is
more practical for the work that is
required i.e. nearer.
• TBMs should be located:
– For excavations, the TBM should be
located away from the excavation line, but
not be too far away
– Not located on a wall that is going to be
demolished during the works
– Not located on a pavement by the site if
deliveries are to be made
• Examples of how a TBM can be fixed
include:
– A mark on a wall
– A bolt cast into a concrete slab
– An existing feature such as a step
TBM
Level Reading Definitions
• There are only three types of reading that can be taken.
– Backsight readings
– Foresight readings
– Intermediate sight readings
Change Point
• When you can no longer see all the points that need to be picked up – you move the instrument
• To continue the link – the staff remains in place
• Staff then has a FS reading taken and a BS reading taken on it’s single position
Booking Sheet
BS IS FS H.o.I. R.LEVEL REMARKS
2.000 50.000 OBM 1.500
2.000
First reading to book is the Back Sight
Last reading to book
is the Fore Sight
If any detail is being picked up
An Intermediate Sight is used
Must always have A known start point
Booking Sheet
BS IS FS H.o.I. R.LEVEL REMARKS
2.000 50.000 OBM 1.500
1.500 2.000
At a change point – the staff doesn’t move So two readings are booked on the same line (same staff = same line)
O.B.M.
50.000m
A B C D E F
1st set up 2nd set up 3rd set up
point point point
BS IS IS FS
2.000 2.500 2.250 1.500
BS IS FS
3.000 2.500 2.750
BS FS
1.750 3.500
Booking Information
Have a go at booking this information
Example Booking
Buildings
Known datum
TBM required here
A
B
CD E
F
GHI
Booking Sheet BS IS FS H of
I R.L. Remarks
1.003 BS Reading on known datum
Level is at A 1.645 1.524 FS Reading at B when level at A
BS Reading at B when level at C 2.835 2.134 FS Reading at D when level at C
BS Reading at D when level at E 2.884 2.163 FS Reading at F when level at E
BS Reading at F when level at G 1.058 1.332 FS Reading at H when level at G
BS Reading at H when level at I 2.272 FS Reading onto known datum
when level at I
Calculating the Reduced Level
• RISE AND FALL METHOD
• HEIGHT OF COLLIMATION METHOD
• The Height of the Instrument is calculated every time the instrument is set up and used to calculate the reduced level.
b
A
if the RL at A is 50.000m and the staff reading at b is 1.200m, then the height of the instrument is 51.200m
Calculations • Once you have checked the accuracy of you
readings you can move on to do the calculations.
• There are only two formulae that you need:
Height of Instrument =
Reduced level + Backsight
Reduced Level =
Height of Instrument – Foresight or Intermediate Sight
Example
Back
Sight
Intermediate
Sight
Fore
Sight
H of I R.L. Remarks
1.003 25.444 BS Reading on
known datum
1.645 1.524 at B
2.835 2.134 at D
2.884 2.163 at F
1.058 1.332 at H
2.272 FS Reading
onto known
datum
Example
Back
Sight
Intermediate
Sight
Fore
Sight
H of I R.L. Remarks
1.003 26.447 25.444 BS Reading on
known datum
1.645 1.524 26.568 24.923 at B
2.835 2.134 24.434 at D
2.884 2.163 at F
1.058 1.332 at H
2.272 FS Reading
onto known
datum
B
C
OBM
BS
1.551
A1
IS
2.018
A2
IS
2.155
A4
FS
1.550
B1
BS
2.573
B2
IS
1.347
B3
IS
3.002
B5
FS
2.056
C1
BS
1.863
C2
IS
1.432
C3
FS
2.637
A
Level positions
Staff positions
Staff positions where
level is moved
A3
IS
1.096
B4
IS
2.675
D
E
D1
BS
2.425
D2
FS
1.288
E2
FS
2.039
E1
BS
2.068
F
F1
BS
1.811
OBM
FS
2.721
Book the information
Accuracy
Allowable Misclosure = ±5n mm
Must always close a survey onto a known point
where n = number of Instrument Positions (FS readings)
CHECK 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.o.I. R.LEVEL REMARKS
0.824 39.220 OBM1
1.628
0.790
0.380
2.154 1.224
2.336
2.757
2.555 0.461
2.275
0.436
0.227
0.716
0.652
0.233 OBM2
BS IS FS H.o.I. R.LEVEL REMARKS
0.824 40.044 39.220 OBM1
1.628 38.416
0.790 39.254
0.380 39.664
2.154 1.224 40.974 38.820
2.336 38.638
2.757 38.217
2.555 0.461 43.068 40.513
2.275 40.793
0.436 42.632
0.227 42.841
0.716 42.352
0.652 42.416
0.233 42.835 OBM2
5.533 1.918 42.835
1.918 3.615 39.220 3.615