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Surveying a Level Loop
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Level Loop (or Closed Circuit)
BS = 7.13 ft
Elev. 100.00 ft
Instrument location (TYP)
Turning Point (TYP)
FS 4.26 ft
1
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Closed Circuit or Level Loop AUTO LEVEL READINGS
STADIA
PT(+)BS HI
(-)FS ELEV
TOP/BOT STADIA
DIST/Angle
BM X 100.00
7.13 107.13
TP-1 4.26 102.87
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Level Loop (or Closed Circuit)
Elev. 100.00 ft
BS = 1.75 ft
Turning Point Elev. 102.87
FS = 2.99 ft
2
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Closed Circuit or Level Loop AUTO LEVEL READINGS
STADIA
PT(+)BS HI
(-)FS ELEV
TOP/BOT STADIA
DIST/Angle
BM X 100.00
7.13 107.13
TP-1 4.26 102.871.75
2.99
104.62
101.63TBM
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Level Loop (or Closed Circuit)
Elev. 100.00 ft
BS = 0.94 ft
BS
= 5
.62
ft
BS = 9.12 ft
FS = 3.34 ft FS = 7.98 ft
FS =
6.0
1 ft
3
Elev. 101.63 ft
45
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Closed Circuit or Level Loop AUTO LEVEL READINGS
STADIA
PT(+)BS HI
(-)FS ELEV
TOP/BOT STADIA
DIST/Angle
BM X 100.00
7.13 107.13
TP-1 4.26 102.871.75
2.99
104.62
101.63TBM
TP-2 6.01 101.240.94
107.255.62
TP-3 7.98 94.209.12
102.18
POR 3.34 99.98
103.32
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Closure Error
The difference between a measured or calculated elevation and the true or established elevation.
Closure Error = Final Elev. – Initial Elev.
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Closed Circuit or Level Loop AUTO LEVEL READINGS
STADIA
PT(+)BS HI
(-)FS ELEV
TOP/BOT STADIA
DIST/Angle
BM X 100.00
7.13 107.13
TP-1 4.26 102.871.75
2.99
104.62
101.63TBM
TP-2 6.01 101.240.94
107.255.62
TP-3 7.98 94.209.12
102.18
POR 3.34 99.98
103.32
CLOSURE ERROR = - 0.02 ft - 100.00
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Allowable Error
• The allowable error depends on the type and purpose of the surveying.
• In this class, we will use
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Closed Circuit or Level Loop
AUTO LEVEL READINGS
STADIA
PT(+)BS HI
(-)FS ELEV
TOP/BOT STADIA
DIST/Angle
BM X 100.00 BS 8.63 / 5.63 300 ft
FS 5.69 / 2.83
BS 3.15 / 0.25 286 ft290 ft
FS 4.44 / 1.54BS 6.67 /4.57
290 ft210 ft
FS 7.08 / 4.94 BS 1.84 / 0.04
214 ft180 ft
FS 8.94 / 7.02 BS 10.42 / 7.82
192 ft260 ft
FS 4.58 / 2.10 248 ft
7.13 107.13
TP-1 4.26 102.871.75
2.99
104.62
101.63TBM
TP-2 6.01 101.240.94
107.255.62
TP-3 7.98 94.209.12
102.18
POR 3.34 99.98
103.32
CLOSURE ERROR = - 0.02 ft
- 100.00TOTAL = 2470 ft
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Level Loop (or Closed Circuit)
Elev. 100.00 ft
180 ft
210
ft
260 ft
Instrument location (TYP)
248 ft
192 ft
214
ft
3
Elev. 101.63 ft
45
300 ft
286 ft290 ft
290 ft
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Allowable Error
Survey data is acceptable
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Reducing Errors
• Check that bubble is centered for EVERY rod reading.
• Keep the rod plumb during each reading.• Establish equal BS and FS reading distances.
– Equal reading distances will cancel out any error caused by the line of sight not being parallel with the axis of the bubble tube.
– This is not always possible due to terrain.
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Common Errors
• Faulty rod reading (verify reading)
• Rod not fully extended (high reading)
• Tripod movement during reading
• Confusion between recording BS and FS entries in the field book
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Can You Top That?
According to NOAA, this survey party made 329 setups in 7 hours and 55 minutes working in an area of extreme grades in the Western United States in 1934.
Courtesy NOAA Photo Library http://www.photolib.noaa.gov/htmls/theb1809.htm
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Image Sources
National Oceanographic and Atmospheric Administration Photo Library http://www.photolib.noaa.gov/cgs/marks1.html