COMPASS SURVEYING

A compass is an instrument designed to measure the magnetic bearing of limes. in compass surveying, every bearing may be observed directly from the magnetic meridian established at each station by the floating needle. compass surveying is used in a variety of situation similar to those of plane table surveying that is to say:

• Detailing:

            Compass surveying provides one of the quickest methods of fixing the position of detail or points especially where high accuracy of positioning is not needed. in this situation, the magnetic bearing and distance from a station to the detail or point are measured in the field. Using these data, the position of the detail is located during plotting in the office.

• For Referencing:

              In very high order surveys, it is always required to witness the position of survey marks, (beacons, pillars or bench marks) to enable these marks to be relocated at any time required in the future. this function has always been done using magnetic compass or theodolites, to refer the position of such marks to some prominent or permanent features in the area. In a similar situation chain or plane table surveys are whenever possible referred to the magnetic meridian of one of the stations of the survey.

• Reconnaissance Survey:

              In this case, it is used as an alternative method of obtaining information required to evaluate the requirements of a survey or a triangulation network. More importantly, it is used in reconnaissance or temporary survey to gather quick information required foe decision making. For example, if one wants to acquire a piece of land for development purpose, a compass survey may have to be carried out to compute the approximate area, so that the purchaser knows the price to pay for the land and also satisfy himself that is suitable for his needs.

• Large Scale Surveys:

            A wide range of large scale surveys of small areas are conducted by compass surveying especially where high accuracy is not pre-requiste. indeed, it is possible to carry out a range of professional work of the architect, engineer,planner, lawyers, etc. using compass surveys. in each case, what matters is the degree of accuracy expected of the work in relation to those obtainable from compasses.

WORKING PRINCIPLES OF THE COMPASS

  Compass surveying is based on the principle of surveying working from whole to path. the method employed is the traverse method of surveying. In this case however, no base line is established, but the bearings of all survey lines are observed, in effect, each line observed can be regarded as a base. when all the lines enclosing a particular parcel of land are observed and plotted, the survey is completed. Details measured relative to stations are also plotted. the data of observation is a very important parameter since the position of the magnetic poles is not fixed in relation to the true north and south poles. the north magnetic pole tends to spin more than the south magnetic pole. Because of this, the magnetic declination at any point is ever changing. at least, there are four variations that affect the magnetic declination. these variation are : secular, diurnal, periodic and irregular variations.

PROCEDURE IN COMPASS SURVEYING

• Reconnaissance.
• Survey Stations.
• Accuracy of Measurement.
• Measurement Procedure.
• Field Observation.
• Reduction of Observation.
• Plotting of the Survey.

EQUIPMENT USED IN COMPASS SURVEYING

1. The prismatic compass or Compass Theodolite.
2. Tape.
3. Ranging Poles.
4. Other Chain survey equipment. 

THE ADVANTAGES OF COMPASS SURVEYING

       The advantages of compass surveying includes the following:

1. The instrument is simple, light and easy to carry, and requires a shot time of training before expertise is acquired.
2. The speed of field operation is comparatively very high.
3. each magnetic bearing observed is independent of previous observation so that errors do not accumulate.

THE DISADVANTAGES OF COMPASS SURVEYING

       The disadvantages of compass surveying include the following:

1. The accuracy of magnetic bearing is about 1: 350 and so not enough; hence the scope of compass surveying is limited. 
2. Magnetic readings may be influenced temporarily by variations and local attraction.

SOURCES OF ERRORS IN COMPASS SURVEYING

• Instrumental Error.
• Observation Error.
• Local Attraction Errors.
• Magnetic Variations.
• Parallax Effects.