Circumferentor : 9 Parts, Uses, Advantages & Sources of Error

A circumferentor is an instrument used for the measurement of horizontal angles and the bearing of a line of sight.

The circumferentor is commonly referred to as the Surveyor Compass.

It essentially consists of a graduated horizontal circle, a pivoted magnetic needle, and a sighting device.

Originally, the circumferentor was used as a colonial instrument and was designed for use in countries such as America and Ireland where the land was plentiful.

A circumferentor essentially consists of a circular box with a magnetic needle that swings freely over a brass circle.

The brass circle is divided into 360 degrees. The measurement of the horizontal angle is accomplished using a pair of sights located on the north-south axis of the compass.

This instrument is equipped with vertical sights for aiming at a distant object and is leveled using a ball and socket mechanism and mounted over a tripod.

In circumferentor, graduations exist from 0 to 90 degrees; 0 being to north or south and 90 degrees being to east and west.

An angle of 20 degrees to the north direction to the east is written as N 20° E and an angle of 40 degrees to the east from the south is written as S 40 ° E. The reading is taken at the tip of the needle.

  1. Uses of Circumferentor  

The uses of circumferentor are as follows:

a. It is used to measure horizontal angles.

b. To determine the bearing of the surveying line of sight.

  2. Parts of Circumferentor  

A circumferentor essentially consists of a graduated circle or disc fitted to a box. A magnetic needle is provided which freely rotates over a brass circle.

Unlike a prismatic compass, it has a narrow slit at the viewing end. The sight of the line is fixed and the reading is directly taken from the top of the glass cover.

a. Graduated disc or ring:

It is fixed with the compass box. It moves along with the movement of the box but the magnetic needle in it remains stationary with poles of North-South.

The ring of the surveyor’s compass is divided into 4 quadrants and graduations from 0° to 90° in each quadrant. North and South points have 0° graduations while East and West points are marked at 90°.

Graduations are not inverted in the circumferentor. So, its readings can be directly taken from a glass cover having a narrow slit at the viewing end.

b. Magnetic needle:

The magnetic needle of the surveyor compass remains stationary in the North-South direction.

While taking readings, the ends of the needle act as an index.

c. Jewel bearing:

The Jewel bearing is placed at the center of the box to support the movement of the magnetic needle.

d. Pivot:

It is connected with a jewel bearing as a hinge.

e. Lifting lever:

It raises the magnetic needle to stick to the glass cover to stop its damp oscillations.

f. Circular box:

It is the circular part attached to the graduation circle.

g. Top glass:

The surveyor compass has a top covering made up of glass known as tough glass.

h. Eye vane and object vane:

The eye vane is used to observe the line of sight while on the object vane image of the object is formed.

i. Bubbling Tube

There is a bubbling tube for accurate leveling.

  3. Sources of Errors in Circumferentor  

The sources of errors in circumferentor are as follows:

✔ Local attraction (i.e. disturbance to the magnetic property of magnetic needle due to the presence of magnetic & electrical substances near the compass).

✔ Improper leveling and centering of the tripod.

  4. Precautions To Be Taken  

Here are a few precautions that need to be carried out:

✔ Stop the damp oscillations of a magnetic needle by pressing the knob containing the spring brake inside to observe readings.

✔ Local attractions should be avoided at the location.

✔ While taking readings from the top cover glass, it should be dust-free. While cleaning, take care of static force that may cause deflection of the magnetic needle.

  5. Advantages of Circumferentor  

The advantages of circumferentor are as follows:

✔ Easy to handle.

✔ Portable & Lightweight.

✔ It has fewer angle or position errors.

  6. Disadvantages of Circumferentor  

The disadvantages of circumferentor are as follows:

✔It is less precise compared to other advanced methods of surveying.

✔ It is easily subjected to various errors such as errors adjoining to magnetic meridian, local attraction, etc.

✔ Imperfect sighting of the ranging rods and inaccurate leveling also causes the error.