What is compass surveying ?

Another type of survey instrument that forms the subject of this section is the compass.Here, we will explain the meaning, types of compass survey and also introduce and discus the concept of bearing.

Objectives 

· To introduce the students to the meaning and types of compass survey

· To enable students understand the concept of bearing.

Types and Meanings of Compass Survey

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 two major primary types of survey compass are: 

* Prismatic Compass

* Survey Compass

Compass surveys are mainly used for the rapid filling of the detail in larger surveys and for explanatory works. It does not provide a very accurate determination of the bearing of a line as the compass needle aligns itself to the earth’s magnetic field which does not provide a constant reference point.

* Prismatic Compass
This is an instrument used for the measurement of magnetic bearings. It is small and portable usually carried on the hand.

This Prismatic Compass is one of the Two Main kind of magnetic compass included in the collection
for the purpose of measuring magnetic bearings,with the other being he Surveyor's Compass.
The main difference between the two instruments is that the surveyor's compass is usually larger and more accurate instrument, and is generally used on a stand or tripod.

* The prismatic compass on the other hand is often a small instrument which is held in the hand for observing, and is therefore employed on the rougher classes of work. The graduations on this prismatic compass are situated on a light aluminum ring fastened to the needle, and the zero of the graduations coincides with the south point of the needle. The graduations therefore remain

stationary with the needle, and the index turns with the sighting vanes.  

Since the circle is read at the observer's (rather than the target's) end, the graduations run clockwise

from the south end of the Needle (0 to 360) whereas in the surveyor's compass ,the gradution run anticlockwise from the north.

* The prismatic attachment consists of a 45º reflecting prism with the eye and reading faces made slightly convex so as to magnify the image of the graduations. The prism is carried on a mounting which can be moved up and down between slides fixed on the outside of the case.

* The purpose of this up-and-down movement is to provide an adjustment for focusing. The image of the graduations is seen through a small circular aperture in the prism mounting, and immediately above this aperture is a small V cut on top of the mounting, over which the vertical wire in the front vane may be viewed. Using the V cut, the vertical wire and the station whose bearing is required are viewed in one line, the bearing is directly read off the graduated arc at the point immediately underneath the vertical wire.

* The mirror located in front of the forward vane slides up and down the vane, and is hinged to fold flat over it or to rest inclined at any angle with it. This mirror is used for solar observations, or for viewing any very high object, and is not a normal fitting to a compass. The two circular discs in front of the back vane are dark glasses which can be swung in front of the vane when solar observations

are being taken.

Components of Prismatic Compass

Components prismatic compass

Prismatic compass consists of a non-magnetic metal case with a glass top and contain the following:

* Cylindrical metal box: Cylindrical metal box is having diameter of 8to 12 cm. It protects the compass and forms entire casing or body of the compass. It protect compass from dust, rain etc.

* Pivot: pivot is provided at the center of the compass and supports freely suspended magnetic needle over it.

* lifting pin and lifting lever: A lifting pin is provided just below the sight vane.When the sight vane is folded, it presses the lifting pin. The lifting pin with the help of lifting lever then lifts the magnetic needle out of pivot point to prevent damage to the pivot head.

* Magnetic needle: Magnetic needle is the heart of the instrument. This needle measures angle of a line from magnetic meridian as the needle always remains pointed towards north south pole at two ends of the needle when freely suspended on any support.

* Graduated circle or ring: This is an aluminum graduated ring marked with 0o to 360o to measures all possible bearings of lines, and attached with the magnetic needle. The ring is graduated to half a degree.

* Prism : prism is used to read graduations on ring and to take exact reading by compass. It is placed exactly opposite to object vane. The prism hole is protected by prism cap to protect it from dust and moisture.

* Object vane:  object vane is diametrically opposite to the prism and eye vane.The object vane is carrying a horse hair or black thin wire to sight object in line with eye sight.

* Eye vane: Eye vane is a fine slit provided with the eye hole at bottom to bisect the object from slit.

* Glass cover: its covers the instrument box from the top such that needle and graduated ring is seen from the top.

* Sun glasses: These are used when some luminous objects are to be bisected.

* Reflecting mirror: It is used to get image of an object located above or below the instrument level while bisection. It is placed on the object vane.

* Spring brake or brake pin: to damp the oscillation of the needle before taking a reading and to bring it to rest quickly, the light spring brake attached to the box is brought in contact with the edge of the ring by gently pressing inward the brake pin

Temporary adjustment of prismatic compass 

The following procedure should be adopted after fixing the prismatic compass on the tripod for measuring the bearing of a line.

Centering : Centering is the operation in which compass is kept exactly over the station from where the bearing is to be determined. The centering is checked by dropping a small pebble from the underside of the compass. If the pebble falls on the top of the peg then the centering is correct, if not then the centering is corrected by adjusting the legs of the tripod.

Leveling : Leveling of the compass is done with the aim to freely swing the graduated circular ring of the prismatic compass. The ball and socket arrangement on the tripod will help to achieve a proper level of the compass.This can be checked by rolling round pencil on glass cover.

Focusing : the prism is moved up or down in its slide till the graduations on the aluminum ring are seen clear, sharp and perfect focus. The position of the prism will depend upon the vision of the observer.