The image for Defining and Benchmarking the Job depicts a chiselled benchmark.
The word benchmark has its origins among surveyors who chiselled these marks in stone to indicate levels and heights as reference points from which the constructions could be calculated.
An angle-iron was placed within the cuts to form a “bench” on which to place a levelling rod. This allowed the levelling rod to be accurately repositioned for reference in the future.
The ‘Fundamental Benchmark’ was worked out as the Mean Sea Level many years ago. The height of each individual benchmark is calculated relative to the heights of those nearby creating a network that extends to a fundamental benchmark.
Many large scale maps still show the position and height of benchmarks although these are rapidly being replaced by Global Positioning Systems or GPS.