procedure to the left, making readings on all objects until the initial or first station is observed once
again. This terminates the position. Recheck the pointing on the initial object, and increase the seconds
and degree setting for the next position. Repeat the same procedure for all remaining positions.
(b) When all 16 positions are complete, calculate the mean of all observations. Reject any of
the positions that are not within ... 4 seconds of this mean. Reject on sight any position that is obviously
bad (one which is 3 or 4 seconds above or below any other direction in the set) and do not include it in
the trial mean. This eliminates the possibility of an abnormal direction causing a good direction to be
rejected. You may reject a direction at the time of observation due only to a kicked tripod, wrong light,
or flaring light. All rejected positions must be reobserved using the required circle and micrometer
settings for those positions. Observe only the objects involved in the rejected positions, since it is
unnecessary and timewasting to reobserve positions that are within the limits.
(c) Make every possible effort to obtain 16 positions within the limits of retention of ... 4
seconds from the mean. However, it is permissible to use a minimum of 12 good positions for first-
order work if it is impossible to obtain the full 16 positions. Once you reject an observation, it remains
rejected even though a new mean computed from the reobservation would bring it within the retention
(4) For second-order, Class I triangulation supplemental to or adjoining first-order triangulation,
observe with a T-3, and use the same procedures as described for first-order, Class I (in this case, the
limits of retention are ...5 seconds from the mean.) For Class II, 8 positions within 5 seconds of the mean
are recommended to avoid reoccupations, especially when the stations are not easily accessible or when
the observer is somewhat inexperienced. When recording for Class II, show the degrees and minutes for
all odd-numbered stations.
h. Vertical Circle Reading. The vertical circle (Figure 3-6, page 3-19) is read in exactly the same
manner as the horizontal circle. To read the vertical circle, turn the changeover knob (Figure 3-4, page
3-16 ) counterclockwise as far as it will go to bring the vertical circle into view in the reading
microscope. A coincidence of the graduations is made as described in the horizontal-circle reading
section. The collimation level is centered exactly. Due to the method of graduation, the same figures
will appear upright and inverted. The plate circle (from the numbered value farthest left to the same
figure inverted on the right) and double coincidence drum values are read as described in paragraph 3-
13, f (1), page 3-18, and totaled as 123, 34' 27.4" (as shown in Figure 3-6). This constitutes one
reading. For a reverse reading, plunge the telescope, reverse the instrument, and read the circle.
i. Vertical-Angle Computation. A vertical-angle computation is unique to this instrument. The
vertical circle is set so that a perfectly level line reads 90, in the direct and reverse positions.
(1) To differentiate between direct and reverse positions, the nomenclature circle left (direct) and
circle right (reverse) are used normally. This refers to the