Download FM 3-34.331

FM 3-34.331
Table 7-1. Correction Factors for Center-Wire Leveling
Difference in Intervals
Center-Wire Correction Value
0.000
0.0000
0.001
0.0003
0.002
0.0007
0.003
0.0010
C-FACTOR DETERMINATION
7-15. The determination of the C-factor may be performed as a part of leveling
or separately. In all cases, the C-factor determination must be recorded
separately from other recordings and must comply with all requirements for
note keeping. It is desirable to determine the C-factor under the same
conditions that the leveling will be performed, including the sight distance,
the slope of the ground, and the elevation of the line of sight above the ground.
7-16. Ensure that the circular bubble is carefully centered and that the
observed ends of the bubble in the level vial are in coincidence (when
applicable) before reading the three wires. If the C-factor is determined during
the first setup of the leveling, perform the following steps:
Step 1. Observe and record the foresight readings on the C-factor note sheet
after the regular foresight observations are recorded for the level line.
Step 2. Position the rear rodman to about 10 meters behind the level.
Step 3. Observe and record the rear-rod readings on the C-factor note sheet.
Step 4. Move the level to about 10 meters behind the front rod.
Step 5. Observe and record the front-rod readings on the C-factor note sheet.
Step 6. Observe and record the rear-rod readings on the C-factor note sheet.
7-17. The total correction for curvature and refraction (C&R) must be
determined for each far-rod reading using the distance from the instrument to
the far rod as the argument. Distances equal the product of the sum of the
intervals (for a single set of three-wire readings) times the stadia-interval
factor (SIF). The two corrections for C&R are algebraically added to the sum of
the mean wire readings for the distant rod. The maximum permissible
C-factor varies with the SIF. Instruments with a SIF of 1:100 may not have a
C-factor of greater than ±0.004. Instruments with a SIF of 1:200 may not have
a C-factor of greater than ±0.007. Instruments with a SIF of 1:333 may not
have a C-factor of greater than ±0.010. If the C-factor is determined to be
greater than what is permitted for the instrument’s SIF, the instrument must
be adjusted and the C-factor redetermined before performing differential
leveling. The notes for the C-factor determination become part of the
administrative notes for the leveling operation.
7-8 Differential Leveling