July 10, 2024

Compass Surveying: Prismatic Compass Vs Surveyor Compass

pristamic compass

Essentially, a compass is an instrument that is widely used to find direction. However, this small gadget with a tiny magnetic needle pointing towards the magnetic meridian can also be used to measure angles between objects indirectly. The two main instruments used in the series of magnetic bearing measurements are Prismatic Compass and Surveyor’s Compass.

Now, you must be wondering, what do these gadgets do? How is a Prismatic compass different from Surveyor’s Compass? Well, rest assured and read through the article to know everything about Compass Surveying and its types.

Everything you need to know about Compass Surveying & Its Types

Compass surveying is a form of surveying that involves taking measurements of an object.

In simpler terms, compass surveying refers to the act of determining the position of an object by taking two types of measurements (1. Angular measurements that are taken by compass, and 2. Linear measurements are determined using tape or chain).

Both the Prismatic compass and Surveyor’s compass come under Magnetic compass, which is a common type of Compass Survey.

Advantages of Compass Surveying

  1. Runs without electricity.
  2. Easy to handle and operate.
  3. Minimum human resources required.
  4. Suitable for retracting the old survey.

Disadvantages of Compass Surveying

  1. Less accuracy
  2. Chances of instrumental error
  3. Chances of human error

Prismatic Compass Survey Vs Surveyor Compass Survey

prismatic compass detail view

1. Prismatic Compass

The Prismatic compass was invented back in the year 1814. The purpose of this compass is to, take accurate measurements at a faster speed or pace. It consists of a small circular box of about 100m and can be carried in hand or mounted on a tripod as needed.

The best part about this compass is that the user doesn’t have to move his head around to read the circle and also it is very compact. However, it cannot be used in the presence of a magnetic field. Also, long-distance surveys cannot be done using this compass.

The following are the main components of the Prismatic Compass:

  • Circular Box
  • Needle
  • Magnetic needle
  • Prism
  • Prism cap
  • Lifting lever
  • Glass cover
  • Agate cover
  • Reflecting mirror
  • Coloured glass
  • Brake pin
  • Spring break
  • Focusing screw

The Least Count of the Prismatic Compass is 30′ (30 minutes) which implies that it is limited to reading only those observations that are multiples of 30 minutes. Additionally, the Prismatic Compass has two adjustments:- Temporary and Permanent.

surveying compass

2. Surveyor’s Compass

Surveyor’s Compass or Circumferentor is an instrument used in compass surveying for measuring horizontal angles. The most common use case would be Mine Surveying.

The Surveyor’s compass was invented in 1667, many years before Prismatic Compass. The purpose of this compass is to determine the horizontal direction of a line concerning the direction of a magnetic needle.

Usually, this compass is made up of brass and has no prism in it, instead, the graduated ring is attached directly to the circular box and the edge bar needle freely floats over the pivot.

Following are the main components of the Surveyor’s Compass:

  • Circular Metal
  • Glass Cover
  • Eye Vane
  • Guide Vane
  • Bubble Tube
  • Magnetic Needle
  • Fixed Graduated Plane
  • Adjustment Screw
  • Fixed Graduated Plane

Surveyor’s compass has four types of adjustments i.e. Adjustments of a Pivot Point, Adjustments in Levels, Adjustments of Needle, and Adjustments of Sight vanes. Also, the Least Count of the Surveyor’s Compass is 15′( 15 minutes).

surveyor compass detail view

Difference Between Prismatic Compass & Surveying Compass

The Surveyor’s compass is usually bigger as compared to the Prismatic compass and requires a tripod to work. Additionally, Surveyor’s compass gives more accurate results as compared to the Prismatic compass.

We’ve listed below a detailed table that highlights the key differences between Prismatic and Surveyor’s compass.

SIPrismatic CompassSurveyor’s Compass
1The magnetic needle and the graduated attached when the prism and box in rotation.The magnetic needle would be free when the dial is attached to the box.
2Readings are taken with the help of a prism.Readings are taken with naked eyes.
3The initial reading starts from the south end of the instrumentThe initial reading starts from the north end of the instrument.
4Has a mirror with sight vaneNo mirror
5East & West position constantEast & West positions interchange
6Can be used without the TripodTripod is necessary
7Measures the whole circle bearing ( W.C.B)Measures reduced bearing
8Graduations are marked from 0° to 360°Graduations are marked from 0° to 90°

Final Thoughts

Both instruments are quite essential to carry out a survey and at the same time possess some key differential points. While one is used to get more accuracy, the other one is handy to use and gives out speedy measurements. We hope that all your queries regarding this topic are answered.

Thank you for reading this blog.

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