How To Determine Elevations And Excavation Depths 2

How to Determine Elevations and Excavation Depths with A Transit Level 2

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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.

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