In any particular set up of an instrument height of instrument, which is the elevation of the line of sight, is constant. The elevation of unknown points can be obtained by subtracting the staff readings at the desired points from the height of instrument. This is the basic behind the height of instrument method for reduction of level.

With reference to Figure 13.2 and Table 13.2, when the instrument is at I₁, the staff reading observed at A is 2.365m. The elevation of the line of sight i.e., the height of instrument is 102.365m obtained by adding the elevation of A (100.0m) with the staff reading observed at A (2.365m). The elevation of S₁ (101.130m) is determined by subtracting its foresight reading (1.235m) from the the height of instrument (102.365m) when the instrument is at I₁ . Next, the instrument is set up at I₂. S₁ is considered as a point of known elevation and backsight reading ( 0.685m) is taken . The height of the instrument (101.815 m) is then calculated by adding backsight reading ( 0.685m) with the elevation (R.L.) of point S₁ (101.130m). Foresight is taken at S₂ and its elevation (98.245m) is determined by subtracting the foresight (3.570m) from the height of the instrument (101.815 m). In this way, elevation of points are calculated by Height of instrument method.

Table 13.2 Level book note for Height of instrument method

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