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.
#1. Prismatic Compass Survey:
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.
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 30′ i.e. thirty minutes. It can read only those observations, which are multiples of 30 minutes.
Adjustments of Prismatic compass:
Two different types of adjustments:
- Temporary adjustment
- Permanent adjustment
#2. Surveyor Compass.
Compass surveying is a type of surveying in which the directions of surveying lines are determined with a magnetic compass, and the length of the surveying lines are measured with a tape or chain or laser range finder. The compass is generally used to run a traverse line.
or
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
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
- Adjustments in Levels
- Adjustment of a Pivot Point
- Adjustment of Sight Vanes
- Adjustment of Needle
Prismatic Compass Survey Vs Surveyor Compass.
| Sr.No. | Detail | Surveyor Compass | Prismatic Survey |
| 1 | Magnetic Needle | The needle is of edge bar type | The needle is a broad needle. |
| 2 | Graduated Ring | 1. 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 |
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. |
| 3 | Reading 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. |
| 4 | Tripod | The instrument cannot be used without a tripod. | The instrument can be held in hand also while making the observations. |
| 5 | Vanes | The eye vane consists of the small vane with a small slit. | The eye vane consists of a metal vane with a large slit. |
FAQ: Prismatic Compass vs. Surveyor Compass
What Is a Prismatic Compass and How Does It Work?
A prismatic compass is a surveying instrument used to measure angles and directions. Invented in 1814, it consists of a circular box with various components like a needle, prism, graduated ring, and sighting slit. It can be used as a hand instrument or mounted on a tripod. The prismatic compass provides quick and rough survey results, particularly useful in conditions where high accuracy is not required.
What Are the Main Components of a Prismatic Compass?
The main components of a prismatic compass include the 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, reflecting mirror, brake pin, and spring brake.
What Is the Least Count of a Prismatic Compass?
The least count of a prismatic compass is 30 minutes (30′). This means it can only read measurements that are multiples of 30 minutes.
What Types of Adjustments Are Necessary for a Prismatic Compass?
There are two types of adjustments for a prismatic compass:
- Temporary Adjustment: Made before each use to ensure accuracy.
- Permanent Adjustment: Made to correct any inherent inaccuracies in the compass.
What Is a Surveyor Compass and Its Primary Use?
A surveyor compass is a surveying instrument used to determine the horizontal direction of a line with reference to a magnetic needle. It is used to run traverse lines and is known for its accuracy. The surveyor compass is typically made of brass and was first used in colonial times.
What Is the Least Count of a Surveyor Compass?
The least count of a surveyor compass is 15 minutes (15′), allowing for finer measurements compared to the prismatic compass.
What Types of Adjustments Are Necessary for a Surveyor Compass?
Adjustments for a surveyor compass include:
- Adjustment of Levels: Ensures the compass is level.
- Adjustment of Pivot Point: Ensures the needle pivots correctly.
- Adjustment of Sight Vanes: Aligns the sighting vanes.
- Adjustment of Needle: Ensures the needle is properly calibrated.
How Does the Reading System Differ Between a Prismatic and a Surveyor Compass?
- Prismatic Compass: Readings are taken using a prism which allows simultaneous sighting and reading.
- Surveyor Compass: Readings are taken directly by looking through the top glass, which does not allow for simultaneous sighting and reading.
Can a Prismatic Compass Be Used Without a Tripod?
Yes, a prismatic compass can be held in hand while making observations, though it can also be mounted on a tripod for stability.
What Are the Key Differences in the Graduated Rings Between the Two Types of Compasses?
- Surveyor Compass: The graduated ring is attached to the box and rotates with the line of sight, with engravings that read directly.
- Prismatic Compass: The graduated ring is attached to the needle and does not rotate with the line of sight, with engravings that are read through a prism.
