Environmental Engineering Reference
In-Depth Information
TABLE 4.5
(
Continued
)
Method
Range
Accuracy
Advantages
Limitations
Reliability
and
Precautions
Accuracy can
be improved
by using a
target with a
vernier and
by repeating
the sight
from the
opposite end
of the
baseline
Laser and
0 to 5 ft
0.005 ft
Faster than
Seriously
Good
photocell
off-
transit
affected by
detector
baseline
atmospheric
conditions
Triangulation
0.002 to 0.04 ft
Usable when
Requires
Good
direct
precise
measurements
measurement
are not
of base
possible.
distance and
Good for
angles. Good
tying into
reference
points outside
monuments
of
are required
construction
area
Photogrammetric
Can record
Weather
Good
1
00
1
000
50
50,
to
methods
hundreds of
conditions
potential
can limit use
movements at
one time for
determination
of overall
displacement pattern
Note
:
GPS becoming standard surveying practice
(Section 4.2.2).
Current precision about 1 cm.
a
From Cording, E. J. et al., Department of Civil Engineering, University of Illinois, Urbana, Vols. 1 and 2, 1975.
Water-Level (Hose-Level) Device
Application
Floor, wall, or column deflections inside structures undergoing settlements or heave may
be monitored with the water-level device. Its accuracy ranges from
0.001 to 0.5 in.
Procedure
Monument pins are set at a number of locations on walls, columns, and floors. Elevations
relative to some fixed point are measured periodically with the device, which consists of
two water-level gages connected by a long hose. In operation, one gage is set up at a
bench-mark elevation and the other is moved about from pin to pin and the elevation read
