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Prismatic compass (surveying)
A prismatic compas is a navigation and surveying instrument which is extensively used for determining course, waypoints (an endpoint of the leg of a course) and direction, and for calculating bearings of survey lines and included angles between them.〔(【引用サイトリンク】title=What is a prismatic Compass? )〕 Compass surveying is a type of surveying in which the directions of surveying lines are determined with a magnetic compass, and the length of the surveying lines are measured with a tape or chain or laser range finder.〔(【引用サイトリンク】title=What is Prismatic compass survey? )〕 The compass is generally used to run a traverse line. 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.
==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" (35 degrees 45 minutes 30 seconds) are some examples which cannot be read by prismatic compass. The compass will give approximate values of these angles pertaining to its least count.〔(【引用サイトリンク】 title=Surveying )〕
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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