The first step of any construction work is surveying and thus it is an important branch of civil engineering. In general, surveying can be defined as the art of determining the relative position of various features above, on or beneath the surface of the earth using different instruments. One such important survey instrument is a compass.
Compass is a survey instrument designed for the measurement of direction concerning the magnetic meridian. Two types of the compass (i.e. prismatic compass and surveyor compass) are most commonly used.
The prismatic compass has been briefly discussed below.
Prismatic Compass may be defined as the survey instrument that is used for determining the bearing of traversing and included angles between them, waypoints and direction. This compass is used for measuring the angles and distances with the aid of tape.
As the name itself implies, the prismatic compass essentially consists of a prism for taking the observations correctly. The prismatic compass was invented by Charles Schmalcalder and patented in 1812. The manufacturing of prismatic compass began only after 1900.
A prismatic compass is portable. It can be used as a hand instrument or can be fitted on a tripod.
One of the most advantageous features of such a compass is that it facilitates both sighting and reading simultaneously. Due to this reason, it is extensively used in land surveying.
1.2. Instruments Used in Prismatic Compass Survey
Some instruments that are used in the prismatic compass survey are:
i. Prismatic compass with tripod stand
ii. Measuring Tape
iv. Ranging Rods
v. Plumbing Fork or Plumb Bob
vi. Field Book
1.3. Uses of Prismatic Compass
a. It is commonly used in land surveys.
b. The simplicity of prismatic compass makes it applicable in preliminary surveying, road surveying, rough traversing etc where high accuracy is not required.
c. It is used to find out angles between bearings of a line.
1.4. Parts of Prismatic Compass
A prismatic compass consists of a non-magnetic metal case fitted with a glass top. The main parts of a prismatic compass can be briefly described as follows:
1. Metal Box:
The compass is enclosed in a cylindrical metallic box. The diameter of the box usually varies from 8 to 12cm. It serves as a protective casing and protects against dust, rain etc.
Pivot is the centrally located part that provides support to the freely suspended magnetic needle.
3. Lifting Pin and Lifting Lever:
A lifting pin is provided right below the sight vane. The lifting pin gets pressed as the sight vane is folded. The arrangement of the lifting pin and lever help to lift the magnetic needle from the pivot point. This prevents damage to the pivot head.
4. Magnetic Needle:
The magnetic needle is the main part of a prismatic compass.
It measures the angle of a line from the magnetic meridian as the needle always points towards the north and south poles at the two ends of the needle when freely suspended.
It is regarded as the heart of a prismatic compass.
5. Ring or Graduated Circle:
The graduated circle consists of an aluminium ring that measures the bearing. It is marked from 0 degrees to 360 degrees and is attached to the magnetic needle.
Prism is used to take the exact readings and is placed exactly opposite to the object vane. The hole of the prism is protected from dust and rain by a prism cap.
7. Object Vane:
The object vane is placed diametrically opposite to the prism and eye vane.
The main purpose of object vane is to sight the object in line with the eyesight. It consists of horsehair or black wire.
8. Eye Vane:
Eye vane is a fine silt-like part provided to bisect the object from silt. It consists of an eye hole at the bottom.
9. Glass Cover:
A glass cover is provided to cover the instrument box. A provided glass cover protects the instrument and is transparent which helps in taking the readings.
Sunglasses can be used when a luminous object has to be bisected.
11. Reflecting Mirror:
A reflecting mirror is directly placed on the object vane. It is used to get the image of an object located below or above the instrument level.
12 Spring Break:
It is also known as the break pin. Spring Break is provided on the compass to damp the oscillation before a reading is taken.
1.5. Least Count and Bearing
The least count of a prismatic compass is 30 minutes (30ꞌ). The compass calculates the bearing in the whole circle bearing system. It determines the angle which the survey line makes with the magnetic north in the clockwise direction.
What do you mean by Whole Circle Bearing?
In this system, the bearing of a line is measured from the true north or magnetic north in a clockwise direction. The value of a bearing may vary from 0⁰ to 360⁰, utilizing the whole circle of graduation.
Prismatic compass is graduated on the WCB system. This type of measuring bearings from the north direction is adopted in India and UK.
Referring to the figure:
W.C.B of lines OA=θ1
W.C.B of lines OB=θ2
W.C.B of lines OC=θ3
W.C.B of lines OD=θ4
1.6. Handling of Prismatic Compass
A prismatic compass can be handled with a hand or using a tripod stand over the station point of the survey line.
While handling with the hand; keep turning the compass until the bisecting image of the ranging rod has been seen in the mirror through sight vanes.
Set up the prismatic compass with tripod stand by arranging a ball and socket to carry spindle and then screw the compass over the station point to adjust centring, levelling and focusing of the prism.
1.7. Adjustment of Prismatic Compass
To take an accurate reading of the bearings of a survey line, the prismatic compass is adjusted over the station in two categories. They are:
a. Temporary adjustment
b. Permanent adjustment
a. Temporary adjustment:
It includes centring, levelling and focusing of the prism that is placed over the station in the survey line for accurate measurement of the bearing.
Centring is the process of bringing the vertical axis of the prismatic compass over the station mark. It is done by utilizing the plumb bob.
Levelling of the prismatic compass is done by ball and socket arrangement by eye judgement.
Levelling should be done such that the dial moves freely and does not touch the rim of the bob.
Focusing the prism
To get accurate readings from obtained images and sharp graduation, the surveyor slides the prism of the compass up or down until it is focused in one place.
b. Permanent adjustment:
There may be many obstacles like weather, internal part damages of the compass, etc. causing difficulties for temporary adjustment. At that time, we have to adjust the compass permanently.
Permanent adjustment of prismatic compass includes:
• Adjustment in levels
• Adjustment of the pivot point
• Adjustment of sight vanes
• Adjustment of needle
1.8. Sources of Errors in Prismatic Compass
Some sources of errors in this compass are:
a. Local attraction
When the magnetic substances are present near the compass; it affects the magnetic property of the compass.
b. Improper levelling of the compass.
c. Improper centring.
1.9. Advantages of Prismatic Compass
a. Due to the presence of prism, accurate readings can be taken.
b. As it has lightweight and easy to carry, it can be used in hand as well as a tripod.
1.10. Precautions To Be Taken While Using Prismatic Compass
a. Keep electronics & metallic objects away from the compass to avoid free rotation of magnetic needle ( to avoid local attraction).
b. The surveyor should stand in the right direction while taking the readings.
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