(2) To check for gross errors in the measurement of the six rod readings, compute each separate
observation. The average of the results of the six separate computations will serve as a numerical check.
If any of the six individual stadia constant values is more than 0.002 from the mean of these six values,
redo the rod readings or remeasure the distance. Any tendency for the values to vary in one direction is
good evidence that there is some error in distance between the point 0.2 meter behind the center of the
instrument and the first of the rod points. It is possible that all six measured distances could be in error
by an even 10 meters.
(3) There are two stadia constants for the N-3 level--1/100 and 1/300. The constant depends
upon the spacing of the crosshairs in the type of reticle installed in the instrument. The fraction constant
1/100 means that the distance between the lower and upper crosshairs is 1/100 of the focal length of the
instrument, and the constant 1/300 means that the distance is 1/300 of the focal length of the instrument.
The stadia constant determined in Figure 4-5, page 4-11 was 1/310 or 0.300. (Remember, this value is
not the decimal equivalent of 1/300 but the determined value of the table.) The crosshairs for a 1/100
constant are spaced to represent a horizontal distance of about 100 meters for every meter intercepted on
the rod between the lower and upper crosshairs. This is a ratio of 1:100. For a 1/300 constant, the closer
spacing of the crosshairs represents a horizontal distance of about 300 meters for every meter
intercepted on the rod. The latter spacing has the advantage that, at longer distances over uneven terrain,
all three hairs can be observed on the rod. After the stadia constant has been determined, any rod
intercept (the distance between the upper and lower crosshairs) during the day's work may be multiplied
by that decimal constant to obtain the stadia distance.
b. Determine the C Factor and Instrument Adjustment. The most common cause of instrument
error is due to tubular level adjustment. When in perfect adjustment, the axis of the tubular level is
parallel to the line of sight. The line of sight on the telescope is horizontal when the bubble is centered.
When it is not in adjustment, the line of sight will fall above or below the horizontal when the bubble is
centered. This type of error becomes evident when, over the course of several setups, it is not possible
to obtain equal lengths of backsights and foresights. This type error can be minimized or eliminated by
first determining the C-factor correction, in which C represents the coefficient of the collimation error.
The actual inclination error is a function of the distance and the C factor.
(1) The C factor should be computed before each day of leveling is begun and any time the
instrument is jolted. Set two turning pins about 75 meters apart and at about the same elevation. Set the
level between the two pins 10 meters from the rear point. With the instrument carefully leveled, read the
rods on both points, using all three hairs. Record the results. Move the instrument 10 meters from the
front point and repeat the procedure. The bull's-eye bubble on the rod must be accurately centered while
the rods are read.
(2) All figures and computations should be recorded on a DA Form 5820, as shown in Figure 4-
6. The circled numbers shown on the figure correspond to the procedures for completing the required
entries and computations.