Height of the instrument (HI)
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.
Here is a surveyor's tool which will be used to determine the height of the instrument by adding the benchmark elevation and a rod reading at the benchmark.