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:
Important Point
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
Also, read: What Is Leveling | Leveling Methods
#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
Also, read: What is Chain Surveying (Principles, Procedure, Method, Instrument)
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. |
Surveyor Compass
A circumferentor, or surveyor’s compass, is an instrument used in surveying to measure horizontal angles.
A circumferentor consists of a circular brass box containing a magnetic needle, which moves freely over a brass circle, or compass divided into 360 degrees. The needle is protected by a glass covering.
Least Count of Prismatic Compass
The Least Count of Prismatic Compass is 30′ i.e. thirty minutes. It means that the instrument can show readings only in multiples of 30 minutes.
Prismatic Compass Survey Procedure
Procedure of Survey on Prismatic Compass.
- For taking readings with a prismatic compass, the same may be held in hand and turned till the ranging rod on the next station is bisected by the cross hair when seen through the eye-vane.
- The instrument should be held as nearly level as can be judged.
Surveyor Compass Vs Prismatic Compass
In a prismatic compass a mirror is provided with the sight vane. In a surveyor compass no mirror is attached to the objective vane. Sighting the object as well as reading the graduated circle can simultaneously be done without changing the position of the eye.
Prismatic Compass and Surveyor Compass
The surveyor’s compass is usually the larger and more accurate instrument, and is generally used on a stand or tripod. The prismatic compass is often a small instrument which is held in the hand for observing, and is therefore employed for less accurate work.
Prismatic Compass Least Count
Least count means the minimum value that an instrument can read which is 30 minutes in case of prismatic compass. It means compass can read only those observations which are multiples of 30 minutes, 5° 30′, 16° 00′, 35° 30′, for example.
Surveyors Compass
A surveyor’s compass is a tool used by surveyors to measure and mark out the direction and location of points on the ground. It typically consists of a compass needle that points to magnetic north, a sighting device for aiming at a target, and a rotating compass housing that can be adjusted to align with the target.
What Is Compass Surveying?
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.
Survey Compass
While all compasses measure horizontal angles with reference to magnetic north, a surveyor’s compass is equipped with vertical sights for taking aim at distant objects. The surveyor’s compass was originally a colonial instrument, designed for use in places such as Ireland and America where land was plentiful.
Surveying Compass
A surveying compass is a tool used by surveyors to measure horizontal angles during land surveying. It is also known as a theodolite or a transit. The compass typically consists of a telescope, a rotating graduated circle or vernier, and a base or tripod for stability.
Prismatic and Surveyor Compass
1 A prismatic compass is a navigating and surveying tool that is frequently used to determine the direction, waypoints, and bearing of travelling. Q. 2 What is a surveyor compass used for? Ans. 2 A surveyor’s compass is a tool for calculating horizontal angles and determining the bearing of a line of sight.
Surveyor Compass and Prismatic Compas
A surveyor’s compass, also known as a magnetic compass, is a tool that uses a magnetic needle to determine the direction of magnetic north. Surveyor’s compasses are used to measure angles and determine directions in the field. They are often used in conjunction with other tools, such as a measuring tape, to establish accurate measurements for mapping, land surveying, and construction projects.
Compass in Surveying
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.
The Prismatic Compass and Surveyor’s Compass
The prismatic compass, also known as a sighting compass, is a portable tool that is used for navigation and orientation purposes, such as hiking and orienteering. It consists of a prism that reflects the image of a compass dial, allowing the user to read the bearing and the target simultaneously. The prismatic compass is designed for ease of use and portability, and is typically less accurate than a surveyor’s compass.
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Suggested Read –
- What Is Leveling | Leveling Methods
- What Is EDM in Surveying | Type | Errors
- What Is Transit Theodolite | Theodolite Parts
- What Is Traversing in Surveying | Types | Method | Definition
- Total Station in Surveying | Operations | Advantage & Disadvantage | Types
- Principle of Plane Table Surveying Methods | Equipment | Error | Advantage | Limitation
Originally posted 2023-04-28 17:24:17.
Least count means the minimum value that an instrument can read .
• theodolite: least count is 20″
• Prismatic compass: least count is 30′
• least count of levelling staff is 5mm.
• dumpy level: least count 5mm as it is based on staff reading thats why least count is 5mm.
Thank for the post it help very much for student and for others….keep it up