Magnetic compass survey

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Magnetic compass survey refers to the picking of details on the field by measuring angles. Also magnetic compass survey is a navigation and surveying instrument which is intensively used to find out the bearing of the traversing and included angles between them. The compass calculates bearing of lines with respect to magnetic needle. Magnetic compass survey is the most familiar compass type. It function as a pointer to magnetic north the local magnetic meridian, because the magnetized needle at its heart aligns itself with the horizontal components of the earth’s surface field exerts a torque on the needle pulling the north end or pole of the needle approximately towards the earth’s north magnetic pole and pulling the other towards the earth’s south magnetic pole. The needle is mounted on a low friction pirate point.
    According to Oxford Advanced Learner’s Dictionary defines survey as act of examining and recording the measurement, features of an area, of land in order to make a map or plan for it. Magnetic compass survey contain three types which are
                                                                  1.Prismatic compass survey
                                                                  2.Surveyor’s compass survey
                                                                  3.level compass survey
           Prismatic compass survey is a portable magnetic compass which can be either used as a hand instrument or can be fitted on a tripod. It contains prism which is used for accurate measurement of reading.
            Surveyor’s compass survey consists of a circular brass circle which is divided into 360degrees. The horizontal angle is measured using a pair of sight located on north-south axis of the compass.
             Level compass survey, the instrument has to be leveled if it is used as in hand, the graduated disc should swing freely and appears to be completely level in reference to the top edge of the case. If the tripod is used the usual l have a ball and socket arrangement for leveling purpose.
The above points were the types of magnetic compass survey.  But during the magnetic compass survey there are some errors which may occur during measurement these errors may be divided into three types which are
                             1.Instrumental error
                             2.Personal error
                             3.Natural error                     
      Natural errors these are errors which include earth’s curvature atmospheric refraction, variation in temperature, settlement of tripod as well as wind vibration.
      Personal errors these are errors which include mistakes in manipulation mistake in staff handling, mistake in reading the staff, errors in sighting as well as mistakes in recoding.
      Instruments errors these are errors which include error due to imperfect adjustment, error due to sluggish bubble, error due to movement, error due to defective as well as error to incorrect length of staff.
In order to overcome these errors there era several method should be used. The following are the methods to overcome errors in magnetic compass surveys;
Avoiding areas with metal elements like iron; during magnetic compass survey surveyors have to make sure that the area taking measurement is not having metal objects like iron because if it has those elements the compass may read in mistakes during measurement because iron is attracted to magnetic which may later disturb the compass and bring errors.
Check the accuracy of a compass; during magnetic compass measurement check the accuracy of a compass as one of the method to overcome errors this will help a surveyor to take the measurement correctly without making mistakes so that a surveyor may read exact points.
Hold a compass steadily when bearing are taken; when measurement a surveyor must hold a compass steadily and carefully so that it may give correct measurements because when you hold it in a bad way it may give oviously it may give wrong  measurements.
Ensure that a booker takes reading accurately by asking him to repeat all figure verbally; during measurement booker should listen well the points from the reader example when reader say 6 and the booker write 9 so the booker will have to be very carefully in taking and writing readings.

Generally I can say that magnetic compass survey have the advantages like it covers large areas, it avoids, accumulation of errors because a check can be done on every reading, it is a quick method. Apart from advantages there are disadvantage like transport problem to check bearing to all reading is very expensive for transport, human error involve in lacking observation because it is very difficult to hold the compass absolutely steal.

Reference
William, J, E.D.[1992]From sail to satellite; the origin and development of navigation science. Oxford university press.
Kreutz, Barbara, M.[1973] Mediterranean contribution to the medieval mariner’s compass’ technology and culture.
Agaubbins, DAVID, [2007]. Encyclopedia of Geomagnetism and paleomagnetic, springer press.
Admiralty, Great Bitain [1915]Admiralty manual of navigation ‘the magnetic compass[continued] the analysis and correction of the diviation’London.



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