Prismatic compass (surveying)

A prismatic compass is a surveying instrument which is extensively used for calculating bearings of survey lines and included angles between them. The compass calculates bearings of lines with respect to magnetic north. The included angles can then be calculated using suitable formulas in case of clockwise and anti-clockwise traverse respectively. For each survey line in the traverse, surveyors take two bearings that is fore bearing and back bearing which should exactly differ by 180° if local attraction is negligible. The name Prismatic compass is given to it because it essentially consists of a prism which is used for taking observations more accurately.[1]

Least count

Least count means the minimum value that an instrument can read which is 30 minutes in case of prismatic compass. It means compass can read only those observations which are multiples of 30 minutes, 5° 25', 15° 55', 35° 45' 30" are some examples which cannot be read by prismatic compass. The compass will give approximate values of these angles pertaining to its least count.[2]

Bearings

The compass calculates the bearings in whole circle bearing system which determines the angle which the survey line makes with the magnetic north in the clockwise direction. The included angles can be calculated by the formulas F-P ±180 in case of anti-clocwise traverse and P-F ±180 in case of clockwise traverse, where 'F' is the fore bearing of forward line in the direction of survey work and 'P' is the fore bearing of previous line.[3]

Parts

The essential parts of the prismatic compass are listed below:-

See also

References

  1. ↑ Dr. B.C Punmai. Surveying Vol. 1. Retrieved 14 December 2014.
  2. ↑ Cl Kochher. "Surveying". Retrieved 14 December 2014.
  3. ↑ Cl Kochher. "Surveying". Retrieved 14 December 2014.
  4. ↑ Cl Kochher. "Surveying". Retrieved 14 December 2014.
  5. ↑ "sites google.com". Retrieved 14 December 2014.