(4) The program of observation is arranged in order to avoid all unnecessary changing of focus.

At the first station, the observer should read the near rod before taking the first set of 25 observations.

At the second station, the observer should make the near rod reading between the two sets of

observations. The day's program for each observer is as follows: read the near rod and take a set of

observations; move to the second station, take a set of observations, read the near rod, and take a set of

observations; return to first station, take a set of observations, and read the near rod.

(5) Observations should begin and end at the same time. A faster observer should continue

reading until the slower observer has 25 readings. Both observers should stop at the same time.

(6) The DE for each set of 25 observations per observer is computed, and the comparable sets

from both observers are averaged. Sets 1 and 2, and sets 3 and 4 are then paired for a mean. The mean

of pairs must agree within the AE for the length of section, as previously discussed under the criteria for

first- and second-order leveling. The means of pairs are combined for a mean of sets (one day's

observations), which must agree with the second day's mean of sets, within the AE. The two days'

observations (mean of sets) are averaged to obtain the difference in elevation between the TBMs.

stations in a triangulation system can be determined by measuring the vertical angles between the

stations. Apply the fundamentals of trigonometry, and use these vertical angles to compute the

differences in elevations. This process is known as *trigonometric leveling*.

a. Trigonometric leveling is usually a part of triangulation or traverse work, with the transit or

theodolite used to determine the vertical angle and the measured distance. The determinations by

trigonometric leveling are checked at reasonable intervals by a connection with lines run by instrumental

leveling. In order to provide starting and check elevations, a complete scheme of vertical-angle

observations should be carried through all triangulation.

b. This complete scheme consists of a continuous series of vertical angles measured through the

main scheme of triangulation, observations made on each line over which horizontal angles are observed

(the observations over each line to be made in both directions if both ends of the line are occupied), and

observations of vertical angles upon all supplementary and intersection stations corresponding to the

horizontal angles measured upon such stations.

c. As you recall, a vertical angle, as measured with a transit, is the angle measured vertically up or

down from a horizontal plane of reference. When the telescope is pointed in the horizontal plane (level),

the value of the vertical angle