Prismatic Compass

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 and preparing the maps. One of the important types of such survey instrument is the compass. Compass is a survey instrument designed for the measurement of direction concerning the magnetic meridian. Two types of compass i.e. prismatic compass and surveyors compass are most commonly used. 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. 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. 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.


Use of Prismatic Compass

It is commonly used in land survey. The simplicity of prismatic compass makes it applicable in preliminary surveying, road surveying, rough traversing etc where high accuracy is not required.


Components 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.

2. Pivot:

Pivot is the centrally located part that provides support to the freely suspended magnetic needle.

3. Lifting Pin and Lifting Lever:

Lifting pin is provided right below the sight vane. The lifting pin gets pressed as the sight vane is folded. The arrangement of lifting pin and lever help to lift the magnetic needle from the pivot point. This prevents the 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 north and south pole 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.

6. Prism:

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:

The 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.

10. Sunglasses:

Sunglasses can be used when a luminous object has to be bisected.

11. Reflecting Mirror:

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.


Least Count and Bearing

The least count of a prismatic compass is 30 minutes (30ꞌ). The compass calculates the bearing in whole circle bearing system. It determines the angle which the survey line makes with the magnetic north in the clockwise direction.


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