7910

• UCREFRP Catalog Number: 7910
• Author: Bovee, K. D. and R. T. Milhous.
• Title: Hydraulic Simulation In Instream Flow Studies
• USFW Year: 1978.
• USFW - Doc Type: Theory And Techniques, Instream Flow Information Paper No. 5.
• Copyright Material: NO

4. Turning Point
A turning point (TP) is a temporary benchmark upon which fore-
sight and backlight readings are taken to continue the line of
levels. Turning points should be solid and not likely to
change elevation while the instrument is being moved. The rod
should remain on the TP while the instrument is moved, unless
the TP is a stake or some other object which would allow the
rod to be reset at exactly the same place for both plus and
minus sites. The rod should not be placed on the ground when
making-a turn, as it may settle before a new HI is determined.
Figure A-5 shows the principle involved in using a turning
point.
0

BS HI =103.03
0 3.03
SM
ELEV. =100.00
5.85
FS BS
1.83
TP
ELEV. = 97.18
_ 103.03-5.85
110
HI=99.01=97.18+1.83
/\
Figure A-5. Principle involved in using a turning point.
Differential' Leveling
One of the most common applications of differential leveling is to
run a circuit of levels to determine the elevations of cross section
headstakes relative to a previously established benchmark. Unless
otherwise instructed, the elevation of only one headstaice per transect
is normally required. Once t~e elevations of transect headstakes is de-
termined, the headstake acts as a benchmark for subsequent measurements.
The most important concept of differential leveling is level loop
closure or "return check." Elevations are determined for each headstake
on one side of the raver, moving the level as required to make sights.
After the last headstake has been measured, the instrument is moved, if
only picked up and then releveled., Then the same headstakes are re-
surveyed on a r
deviates somewh
texts. ,Hcwever
g Ives ad=-,I
may be pc_sible
the i nstrument
eturn survey back to the benchmark. This procedure
at from the classic procedures expounded in surveying
experience has shown that this procedure for closure
closure precision for the designated purpose. While it
to sicht all of the headstakes from one setup position,
must be moved at ;east once during the level loop, to
1-