A backsight (BS) is a rod reading taken on a point of known or assumed elevation. It is the vertical distance between the line of sight through the instrument and the point of known or assumed elevation on which the rod is set. When a Philadelphia or similar type rod is used, the vertical measurement will be in units of decimal feet.

The backsight reading is used to establish height of the instrument, see Figure 15.1. A backsight will always be taken on a bench mark or a turning point.


Backsight reading

Not to scale

FIGURE 15.1. Backsight rod reading.

FIGURE 15.2. Determining instrument height.

A backsight on a turning point is used to continue a survey beyond the starting instrument position. The word "backsight" has nothing to do with the direction in which the instrument is pointed. It is important to remember that there is only one backsight for each setup of the instrument.

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