Prismatic Compass Survey Vs Surveyor Compass. | Least Count of Prismatic Compass & Surveyor Compass
Civil Survey

Prismatic Compass Survey Vs Surveyor Compass. | Least Count of Prismatic Compass & Surveyor Compass

Compass survey

Prismatic Compass Survey is part of the Compass survey. A compass survey is a branch of a survey. This position of an object is located using angular measurements determined by a compass and linear measurements using a chain or tape.

Two different types of a Compass as per below

  • Prismatic Compass.
  • Surveyor Compass.

Prismatic Compass Survey.Compass

This compass was invented in 1814; this compass consists of a small circular box of about 100m. This prismatic compass can either be used as a hand instrument or mounted on a tripod; and is very useful in a situation where rough surveys are needed, i.e., where the accuracy of the survey is not the main consideration but the speed.

The main parts of a prismatic compass.

This compass box, lifting lever, needle, Agate cover, Glass cover, magnetic needle, graduated ring, prism, prism cap, sighting slit, lifting pin, colored glasses, focusing screw, object hair -vane; horsehair, reflecting mirror, brake pin, and spring break.

This Prismatic compass is useful for filling in details in a survey and in places where the ground does not allow the use of chaining.

The military also uses it for reconnaissance surveys, might motility, and for sketching along roads or rivers.

However, while observing with a prismatic compass, care must be taken to avoid local attractions.

Also, keys, pins, and other metallic substances must not be brought near the compass.

Least Count of Prismatic Compass.

Least Count of Prismatic Compass is 30ꞌ.  It can read only those observations, which are multiples of 30 minutes.

Adjustments of Prismatic compass.

Two different types of adjustments:

  1. Temporary adjustment
  2. Permanent adjustment

Also, read: What Is Leveling | Leveling Methods

Surveyor Compass.

A  surveyor’s compass was originally a colonial instrument designed for use in places such as Ireland and America, where land was plentiful.

The first reference to an instrument of this sort appeared in William Folkingham, Art of Surveying (London, 1610).

The earliest extant example was made in Dublin in 1667 and is now in the Museum of the History of Science, Oxford.

Surveyor Compass is used for determining the horizontal direction of a line with reference to the direction of a magnetic needle, and it is considered as an accurate instrument for surveying.

Most of the surveyor’s compasses are made of brass

Surveyor’s-compass-National-Museum-of-American-History

A surveyor’s compass. As illustrated in the figure, the graduated ring is directly attached to the box, and not with a needle. The edge bar needle freely floats over the pivot.

Therefore, the graduated ring or card isn’t oriented in the magnetic meridian, as was the case at the prismatic compass. The object vane is similar to like a prismatic compass.

The eye vane consists of as a simple metal vane using a fine slit. Since no prism is provided, the object is to be sighted first with the eye and object vanes, and the reading is then taken against the North end of the needle by looking vertically through the top glass.

Least Count of Surveyor Compass.

The Least Count of Surveyor Compass is 15ꞌ.

Adjustments of Surveyor Compass.

As below type of adjustment of Surveyor Compass

  1. Adjustments in Levels
  2. Adjustment of a Pivot Point
  3. Adjustment of Sight Vanes
  4. Adjustment of Needle

Also, read: What is Chain Surveying (Principles, Procedure, Method, Instrument)

Prismatic Compass Survey Vs Surveyor Compass.

Sr.No.Item.Surveyor Compass.Prismatic Survey
1Magnetic NeedleThe needle is of edge bar typeThe needle is a broad needle.
2Graduated Ring1. The graduated ring is attached to the box and rotates along with the line of sight.

2. The graduations have 0° at N and S., and 90° at E and W. The letters E and W are interchanged from their true positions to read the bearing in its proper quadrant (below fig.). As the graduated ring is attached to the box, it moves with the sight. If the hearing of a line in the first quadrant is to be measured, since the letters E and W are reversed from their natural positions, the proper quadrant NE will be read
3. The graduations are engraved erect, since the graduated ring is read directly.
1. The graduated ring is attached with the needle and does not rotate with the line of sight.
2. The graduations have 0° at S, 90° at W, 180° at N and 270° at E (as below fig.). When the needle points north, the reading under the prism should be zero. It is so because the prism is placed exactly opposite the object vane, i.e. on the observer's side, and the south end will be under the prism while the needle points north. Hence, the Zero is placed at the south end then the ring is graduated clockwise from it
3. Graduations are engraved inverted since the graduated ring is read through the prism.
3Reading System 1. The readings are taken directly by seeing through the top of the box glass.
2. Sighting and reading cannot be done simultaneously.
1. The readings are taken with the help of a prism, provided at the eye vane.
2. Sighting and reading can be done simultaneously.
4TripodThe instrument cannot be used without a tripod. The instrument can be held in hand also while making the observations.
5VanesThe eye vane consists of the small vane with a small slit. The eye vane consists of a metal vane with a large slit.

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