Height of the instrument (HI)

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1. In surveying, the sum of the benchmark elevation and a rod reading at the benchmark, from which all subsequent readings are subtracted to establish elevations. The benchmark elevation may be established from older surveys, or may be arbitrarily assigned to provide a consistent basis for comparison.


Assuming the height of the instrument is known, elevation is set by means of level instrument.
Source: http://surveying.structural-analyser.com/chapter10/