Bench mark
Also called: BM · Topic: Topographical Maps: Relief Representation and Interpretation · NCERT: Class 11, Ch 5 "Topographical Maps"
Meaning
A bench mark (BM) is a permanent mark fixed by surveyors on a building, pillar or rock. It records the exact height of that spot above mean sea level. Surveyors use it as a reference point for levelling, which means measuring the heights of nearby places by comparing them with a known height. On topographical maps it is shown as "BM" followed by its height.
Example
Surveyors may cut a bench mark into the stone pillar of a railway station building in the Ganga plain near Varanasi, Uttar Pradesh. The same height is printed beside "BM" on the toposheet. Later surveyors can start from it to measure the heights of nearby fields and roads.
Don't confuse with
- Spot height: this is just a dot with a number on the map that gives the height of a point. Nothing is fixed on the ground. A bench mark is a real, permanent mark on a structure.