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.
Types of Compass
Two different types of Compass are as follows.
- 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.
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
Difference Between Prismatic Compass and Surveyors Compass
|Sr.No.||Detail||Surveyor Compass||Prismatic Survey|
|1||Magnetic Needle||The needle is of edge bar type||The needle is a broad needle.|
|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.|