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Systematic / Instrumental Errors
Incorrect length of the rod.
When the bubble tube axis is not perpendicular to the standing axis of the
instrument
When the line of sight of the telescope is not parallel to the bubble axis.
If the line of sight of a level is not exactly horizontal when the bubble is
centered, but slopes either up or down, it will slope by the same amount for any
direction of the telescope. As long as the horizontal lengths of the BS and FS are
the same, from any given instrument position to the rod, the line of sight will
intercept the rod held on each point with exactly the same error in height. But since
one of the sights is a plus sight (+) and other a minus sight (-), the two errors will
cancel each other out in the leveling computation.

D. Checking For Mistakes
When the survey is complete, an arithmetic check is done; this simply assures
that no mistakes in addition or subtraction was made in the 'HI' and 'elevation.'
columns of the field notes. Sometimes, the line of levels is run back to benchmark
or the starting point. This is called a closed loop or level circuit. Any leveling
survey should close back either on the starting benchmark or on some other point
of known elevation, in order to provide a check against blunders.

Fig. 5.11 When the horizontal length of the foresight (plus) and backsight (minus) are the
same, the systematic error of adjustment of the level is cancelled

5.7 Trigonometric Leveling
Trigonometric leveling is an indirect procedure; the vertical distances are
computed from vertical angle and horizontal or slope distance data. It is also
applied for topo work over rough terrain or other obstacles.
The difference in elevation between two points can be determined by measuring
1. The inclined or horizontal distance between them
2. The zenith angle or the vertical angle to one point from the other
N.B: Zenith angles and Vertical angles are both measured in a vertical plane.

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