The error which is eliminated by changing the face is
1. Error due to the line of collimation not being perpendicular to the horizontal axis.
2. Error due to the horizontal axis not being perpendicular to the vertical axis.
3. Error due to the line of collimation not being parallel to the altitude level.
The correct answers are
Statement 1: Line of collimation not perpendicular to the horizontal axis (collimation
error). When the telescope is inverted, this error shifts the line of sight by an equal and
opposite amount, which cancels out when you average both face readings.
Statement 2: Horizontal axis (trunnion axis) not perpendicular to the vertical axis. This
causes the telescope to trace an inclined plane instead of a vertical plane. Changing the face
shifts the inclination to the opposite side, completely eliminating the error upon taking the
mean.
Statement 3: Line of collimation not parallel to the axis of the altitude level. This alters vertical angles and impacts leveling work. Just like the other structural geometry errors, it is neutralized by taking the mean of normal and inverted telescope face observations.
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