|written 4.6 years ago by||• modified 3.4 years ago|
|written 4.6 years ago by|
It is a fixed reference point whose elevation with respect to some datum is known. It is noted that any leveling work is first started from BM.
Types of Bench Mark
- GTS benchmark
- Permanent benchmark
- Arbitrary benchmark
- Temporary benchmark
1) GTS benchmark (great trigonometrical survey benchmark)- This benchmark is established by the survey of India department at a large interval all over the country. The values of reduced level the relevant position and the number of the benchmark are given in a catalogue published by this department. See in the fig below.
2) Permanent benchmark- This is a fixed point or mark established by different government department like PWD railway, irrigation etc. The RLs of this point is determined with reference to the GTS benchmark and are kept on permanent point like the plinth of a building, parapet of a bridge or culvert and so on. Sometimes they are kept from underground pillar as shown in fig.below
3) Arbitrary benchmark- when small ordinary levelling work is to be carried out or when the permeant benchmark is not nearby the place where the survey is to be carried out then to start the levelling work any prominent object like plinth or step of building etc. is chosen as the benchmark and its elevation is assumed arbitrarily.
4) Temporary benchmark- The survey for big project normally continues for a number of the day. Hence at the end of the day when the levelling work for that day has to be stopped. Then any permanent object is chosen on which the work is stopped and can be started further on the next day. Then these types of benchmark chosen are known as the temporary benchmark.With reference to this temporary benchmark, the work is carried forward on the next day and so on.