Compass Surveying | Types of Compass | Advantages & Disadvantages of Compass Surveying with Example | In this Article we are discussing what is compass chain surveying , the definition of chain surveying, how many types of chain in surveying,
Compass Surveying or Compass survey – The framework of the compass survey consists of a series of connected lines, the length and direction of which are measured by a chain and compass respectively, The process is known as compass surveying.
What is Compass Surveying?
Compass surveying is the branch of surveying in which directions of survey lines are determined with compass and the lengths of the lines are measured with a tape or a chain.
In compass surveying the direction of survey line called the bearing of line is defined as the angle made by the line with the magnetic meridian.
In practice compass is generally employed to run a traverse. Traverse consists of series of straight lines connected together to form an open or closed traverse.
Define Compass in Compass Surveying
Definition – The commonly used instrument for compass surveying is Compass. A compass is small instrument which consists essentially of magnetic needle, a graduated circle and a line of sight.
When the line of sight is directed towards a line, the magnetic needle points towards magnetic meridian and the angle which the line makes with the magnetic meridian is read at the graduated circle.
Compass consists of cylindrical metal box of about 8-12 cm diameter in the center of which is a pivot carrying a magnetic needle which is already attached to the graduated aluminum ring. The ring is graduated and is read by reflecting prism. Diametrically opposite to the prism is the object vane hinged to the box side carrying a horse hair with which the object in the field is bisected.
TYPES OF COMPASS in COMPASS SURVEYING
There are two forms of compass as under -:
- Prismatic compass
- Surveyors compass
- Level Compass
Prismatic compass is a portable magnetic compass which can be either used as a hand instrument or can be fitted on a tripod. It contains a prism which is used for accurate measurement of readings. The greatest advantage of this compass is both sighting and reading can be done simultaneously without changing the position.
Major parts of a Prismatic Compass are:
- Magnetic needle
- Graduated ring
- Adjustable mirror
- Sliding arrangement for mirror
- Object vane
- Eye vane
- Metal box
- Glass cover
- Horse hair
The two compass are almost same except few differences so far as their construction is concerned. Prismatic compass uses WCB (0⁰-360⁰) circular ring while as the surveyors compass uses Quadrantal Bearing (0⁰-90⁰) circular ring system.
Besides bearing system, the former has graduated ring attached to magnetic needle as the result of which when compass box and sight vane is rotated the needle remains stationary on the other hand in surveyors compass graduated ring being attached to compass box moves with it as the box is rotated.
ADJUSTMENT OF COMPASS (WORKING)
Working of compass involves three steps:
- OBSERVING THE BEARING
Centering involves to align the compass is such a way the Centre is placed vertically over the station point. It is done with the help of tripod stand and is checked by dropping a small pebble below the Centre of compass.
Levelling is done so that the graduated ring swings quite freely. It is done with the help of ball and socket arrangement and can be checked by rolling a round-pencil on the compass box
Observing the bearing once centering and levelling has been done, raise or lower the prism until the graduations on the ring are clearly visible when looked through the prism. Afterwards turn compass-box until the ranging rod at the station is bisected by horse-hair of objective vane. At this position note down the reading.
Whenever in traversing compass is used for making angular measurements, it is known as compass traversing or compass surveying.
In compass traversing, the compass is used to determine the direction of survey lines of the framework of the traverse for measuring the angles which these lines make with the magnetic meridian.
The process of chaining and offsetting is the same as in chain surveying and running the check lines is not necessary.
Compass traverse may be closed or open. Close traverse starts from one traverse station and closes either on same or on another traverse station whose location is already known. On the other hand an open traverse starts from one station and closes at other station whose location is neither known nor established.
EQUIPMENT USED IN COMPASS SURVEYING
- PRISMATIC COMPASS
- MEASURING TAPE
- RANGING RODS
- PLUMB BOB
- CROSS STAFF
MEASUREMENT OF INCLUDED ANGLE
In a compass (prismatic) which uses Whole Circle Bearing (0⁰ to 360⁰) system the included angle is given by
Included angle = F.B of next line — B.B of previous line at same station
Compass surveying is used in case of rough surveys where speed and not accuracy is main consideration. One of the biggest disadvantages of compass is that magnetic needle which gives bearing of lines is disturbed from its normal position in presence of materials such as iron-pipes , current carrying wires , proximity of steel structures , transmission lines etc. called sources of local attraction.
Principle of Compass Surveying
Principle of compass surveying lies in the fact that the readings or a graduated circle are the angles which a line makes with the magnetic meridian.
Instruments for the direct measurements:-
For angle :- Theodolite, Sextant
Bearing and Angles:-
The bearing of a line is its directions relative to a given meridian.
A meridian is any direction such as True Meridian and Magnetic Meridian and Arbitrary Meridian.
Types of Bearing :-
Whole Circle Bearings.
Quadrantal bearings or Reduced Bearings.
Whole Circle Bearings (WCB) – In this system, the bearing of the line is measured with magnetic North or South in the clockwise direction.
Quadrantal Bearings – In this system, the bearing of the line is measured eastward or westward from North or South whichever is near.
Fore & Back Bearing:-
. If the bearing of a line AB is measured from A towards B , it is known as forward bearing or Fore Bearing.
. If the bearing of the line AB is measured from B towards A it is known as backward bearing or back bearing.
Difference between Prismatic and Surveying Compass :-
|S.No.||Item||Prismatic Compass||Surveyor’s Compass|
|1.||Magnetic Needle||Broad type.||Edge Bar type.|
|2.||Graduated Card||Based on WCB.||Based on QB or Reduced Bearing.|
|3.||Sighting Vanes||(i)The Object vane consists of metal vane with a vertical hair.(ii)The eye vane consists of a small metal vane with slit.||(i)The Object vane consists of metal vane with a vertical hair.(ii)The eye vane consist of metal vane with fine slit.|
|4.||Readings||Reading is taken through prism provided at the eye slit.||Reading is taken directly.|
|5.||Tripod||Tripod may or may not be provided.||Instrument cannot be used without a Tripod.|
Local Attraction :-
Local Attraction denotes the influences which prevents the needle from pointing to the Magnetic North in a given locality. It may be due to electric wired , steel structures, railroads, underground iron pipes, etc.
The Local attraction can be detected by observation of fore bearing and back bearing of each line and finding the difference.
e.g. B.B – F.B = 180 then both stations free from local attraction. No observation errors occur.
Magnetic Declination :-
It is the horizontal angle between the true meridian and the magnetic meridian shown by needle at the time of observation. If the magnetic meridian is to right side(or Eastern Side) of the true meridian than declination is said to be Eastern or positive declination. If it is downward west of true Meridian then it is called negative or westward Meridian.