In compass survey, the direction of the survey line is measured by the use of a magnetic compass while the lengths are by chaining or taping. Where the area to be surveyed is comparatively large, the compass survey is preferred, whereas if the area is small in extent and a high degree of accuracy is desired, then chain survey is adopted. However, where the compass survey is used, care must be taken to make sure that magnetic disturbances are not present.
The compass the two major primary types of survey compass are: the prismatic compass and surveyors compass Prismatic Compass Invented in 1814, the prismatic compass consists of a small circular box of about 100m. It can either be used as a hand instrument or mounted on a tripod; and is very useful in a situation where rough surveys are needed i.e. where the accuracy of the survey is not the main consideration but the speed. The main parts of a prismatic compass are: compass box, lifting lever, needle, Agate cap, Glass cover, magnetic needle, graduated ring, prism, prism cap,sighting slit, lifting pin, coloured glasses, focusing screw, object hair – vane; horse hair, reflecting mirror, brake pin and spring brake.
Prismatic compass is useful for filling in details in a survey and in places where the ground does not allow the use of chaining. It is used by the military for reconnaissance survey, might motility and for sketching along roads or rivers. However, while making observation with a prismatic compass care must be taken to avoid local attractions. Also, keys, pins and other metallic substances must not be brought near the compass.
Surveyor’s Compass Similar to the prismatic compass but with few modifications, the surveyors compass is an old form of compass used by surveyors hence the name. it is used to determine the magnetic bearing of a given line and is usually used in connection with the chain or compass survey
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