Establishing
A
Benchmark Before Measuring Elevations
Before an elevation can
be measured,
a
benchmark must be established. The benchmark is your reference point.
It is a
stable object that will not move during the construction phase.
Benchmarks are are assigned an elevation high enough so that no point
on the construction site is below zero. The benchmark elevation
is usually 100.000 metres (M) as shown in figure 7 below.
Figure 7
Determining Foundation
Wall Height
The following procedure can be used to determine the height of the
foundation wall as shown in figure 5. Determine the height of the
instrument. The height of the instrument is the height of the
line of site above the benchmark (B.M.). Place the measuring rod over
the benchmark and take a first reading. Add the reading to 100.000
metres. This is the height of the instrument (H.I.). Next, place the
measuring rod on top of the foundation wall and swing the transit
level to take a second reading. Subtract the reading taken at the
foundation wall from the height of the instrument. This gives the
elevation at the top of the foundation wall. Record the measurements
and calculations in your log book. See the measurements below.
B.M.
= 100.000
First Reading
= + 0.932
H.I.
= 100.932
Second
Reading
=  1. 616
Foundation Elevation = 99.316
Establishing
Elevations At
Different Instrument Locations
Elevations at different
instrument
locations can be established. Move
the transit to take the measurements as
indicated by figure 7, and set up a log book similar to the Elevation
Log
below.
Determine
the elevation of a station by adding the backsite to the benchmark to
determine the height of the instrument. Subtract the foresite
measurement from the height of the instrument to determine the
elevation. Use the elevation of the station as if it was a new
benchmark and
repeat the process. For example in figure 3, add B.S. 1.769 M to the
B.M.100.000 at B.M, to get the H.I.101.769 M. Subtract F.S. 2.338 M
from
H.I. 101.769 M to get the elevation 99.431 M at station A. Add B.S.
3.112 M to the elevation at A 99.431 M to get H.I. 102.543 M . Subtract
F.S.0.964 M from H.I. 102.543 M to get the elevation of 101.579 M at
station B.
Establishing
Elevations At The
Same Instrument Location
Elevations at the same
instrument
location can be established. See
figure 7 and the Elevation Log. The elevation at station B can be
used to determine the
elevations at
stations C and D because the instrument is not being moved to take the
measurements. Add the B.S. 3.455 M to the elevation 101.579 M
at
station B to get the H.I. 105.034 M. Subtract the foresight 3.346 M
from
the H.I. 105.034 M to get the elevation 101.688 M at station C. Add the
B.S.
3.455M to the elevation 101.579 M at station B to get the
H.I. 105.034 M as you did in the station C procedure. Subtract the
foresight 3.659 M from the H.I. 105.034 M to
get the elevation 101.375 M. at station D. Record the measurements and
calculations in your log book.
Elevation
Log
Measurements are in Metres
Stations
(+)
B.S.
H.I.
()
F.S.
Elevation
B.M







100.000

100.00

1.769 
101.769 


A



2.338 
99.431 
99.431 
3.112 
102.543 


B



0.964 
101.579 
101.579 
3.455 
105.034 


C



3.346 
101.688 
101.579
(El. at B)

3.455 
105.034 


D



3.659 
101.375 





Totals

8.336
C


6.648
C

101.688
C

Checking Elevation
Calculations
If
you add the total B.S
(up to C) to
100.000, then subtract the total
F.S. from 100.000, the answer should equal the elevation at C. By
ignoring C and totaling, adding B.S. and subtracting F.S. up to D, the
answer should equal the elevation at D.
Elevations
and
Excavation Depth Links
1.Elevations
and
Excavation Depths: Measure Elevation Differences With A
Transit.
2.Elevations and
Excavation Depths: Establish Benchmark Before Elevations.
3.Elevations
and
Excavation Depths: Determine Foundation Excavation
Depth.
Written
and maintained by
Ronald
Hunter
All
images and text are copyright Ronald Hunter 2005 to 2011
All
rights reserved
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