What Is Prismatic Compass Surveying?
Prismatic compass is compact and flexible, normally borne by hand. This prismatic compass is among the two major types of magnetic compasses used in the series for the function of magnetic bearing measurement, the exception being the magnetic compass.
The circle is read as observing the conclusion of the graduation run clockwise from the south end of the needle. The mirror located in front of the forward vane slides up and down the vane, folding flat over it, or resting inclined at any angle with it.
This mirror is used to see single objects or to view some very high object and is normally fitted to a compass. The prismatic adapter consists of a 45-degree translucent prism with ice and a strongly convex reading mask to magnify the image of the graduation.
The prism is installed and can be rotated up and down between both the slides around the outside of the case. The goal of this up and down movement is to have a concentration change.
The graduation image is seen through a small circular aperture in the prism mounting and immediately above this aperture is a small v cut on top of the mounting over which the vertical wire in the front aperture can be viewed.
The prismatic compass is mostly found in a compact instrument which would be kept in the hand for observation and is thus used in rougher working groups.
The graduated upon that prismatic compass is located on a lightweight aluminum ring affixed to the needle, as well as the graduation zero corresponds with the graduation point only at the south of the needle, which means that the needle stays stationary as well as the index is rotated with the sighting valves.
Parts of Prismatic Compass
- Cylindrical Metal Box: The cylindrical box with a size of 8 to 12 cm covers the compass and then the whole casting or body of the compass.
- Eye Valve: The eye valve is fine silt with an eye hole in the bottom to bend the object out of the silt.
- Cover of Glass: It fills the instrument box from the top so that the needle as well as the graduated ring can be seen from top.
- Graduation Circle: This is an aluminum graduated ring labeled of 0° to 360° to calculate all potential line bearings and connected with a magnetic needle.
- Lifting Pin and Lifting Lever: Below the viewing vane. Lift pin pressed while the sight vane is folded. The magnetic needle was raised from the pivot point with the aid of the lever raise.
- Prism: Prism is being used to recognize the graduation on the ring and also to read the same reading by compass. It’s positioned in the opposite direction of the object vane. The prism hole is covered by a prism cap to shield it from dust and moisture.
- Spring Break: In order to dampen the vibration of the needle once obtaining a measurement to putting it to resting easily, the light spring break connected to the box comes into contact only with a knob of the ring by softly squeezing the brake pin word.
- Object Vane: The vane of the object is diametrically opposed to the prism and eye valves. The object vane holding horsehair or black thin wire to see the object in line with the sight of the object.
- Sunglasses: Used whenever some luminous objects have to be bisected. Mounted in front of the eye slit and also in a batch of 3 or 4 shades of various colors to give a sharp image of the artifacts.
- Reflecting Mirror: Often used to obtain a picture of an object positioned higher or lower surface of the device when bisecting. Located directly opposite to the vane object. The prism hole is sealed by a prism cap to secure it against dust including moisture. Mounted mostly on the vane of the subject.
- Pivot: The pivot is offered at the middle of the compass which supports a loosely attached magnetic needle.
- Magnetic Needle: The magnetic needle is the core of the device. That needle determines the angle of the line from either the magnetic meridian since the needle is often pointing to the north-south pole at the opposite edges of the needle while freely placed on some support.
Uses of Prismatic Compass
- Prismatic compass, liquid, can always be used separately or in combination with other angle measuring instruments in the orientation of a chart or a plane table and in the survey or traversing process.
- Prismatic compass is Commonly used for rough surveys where the key concern is speed and not precision.
- Geologists and surveyors are commonly used to make precise measurements of degree and angle.
- A prismatic compass is an instrument used to calculate the bearing of a magnetic meridian.
Advantages of Prismatic Compass
- Viewing the object and also understanding the circle can be achieved concurrently without shifting the location of the eye.
- The circle is interpreted at the reading where the hairline seems to be sliced.
- They’re lightweight and compact.
- They have fewer settings to repair it on a station.
- The error made in a single survey doesn’t really impact other lines.
- It is necessary to retrace surveys.
Disadvantages of Prismatic Compass
- Cannot be performed in environments considered to have magnetic content.
- Demand expertise, especially about rocks, to recognize the presence of iron ore.
- It is difficult to keep the compass perfectly still while taking the bearing, so it is hard to totally avoid errors.
- Prismatic compass surveying is not useful for long distances.
Applications of Prismatic Compass
- Such a mirror can be used for solar analysis or for observing some very high object and is not a standard compass fitting.
- The two circular discs at the front of the back of the vane are darkened glasses that can be swing at the front of the vane whenever solar observation is completed.
Method of Using a Prismatic Compass
Following tools are essential for leading a prismatic compass survey.
- Prismatic compass with tripod stand.
- Field Book.
- Ranging Rods.
- Measuring Tape.
- Plumbing Fork.
The provisional modifications typically monitored for prismatic compass are:
- Focusing Prism.
- In centering, the compass must be secure to the stand then usually finished the station.
- To the center, the compass poles of the tripod stand must be progressed inner-outer or in the perimeter way.
- To check centering aligned may be used a pebble from the center of the compass.
- In prismatic compass, surveying perfects the leveling is not essential nonetheless it must be satisfactory to document allowed interruption of the magnetic needle.
- Afterward centering bubble must be guaranteed in the central of the circle providing for it in the level.
3. Focusing the prism
- In the prismatic compass to focus the prism on the graduated circle, it’s extra is slid up and down till the reading are obviously noticeable.
Prismatic Compass Survey Is Accepted on in Two Methods:
1. Open Traverse Survey.
A traverse is supposed to be an open traverse once the traverse starts at one point and dismisses at a different point. Open traverse is also named as unclosed traverse. It is appropriate for surveying roads, coastal lines, etc.
2. Closed Traverse Survey.
A traverse is supposed to be a closed traverse once the traverse designed as a closed circuit. In this circumstance, together with starting then dismissing points of the traverse accord with each other. It is appropriate for the survey of limitations of ponds, sports grounds, forests, etc.
Methods of Prismatic Compass Survey
Like the Plane table survey, following methods are used for conducting the prismatic compass survey:
- Radiation or Radial Line Method.
- Intersection or Sketch Method.
- Traverse (Open and Closed) Method.
- Resection Method.
In the circumstance of compass traversing, together with linear in addition, angular dimensions of traverse lines are occupied by means of chain besides prismatic compass correspondingly. Mutually for bearing and back bearings are measured and essential alterations for local attraction are practical. If any closing error is attained through the plotting of traverse, then the Bowditch rule is useful for the alteration of error.
Principle of Compass Surveying
Traversing that includes a sequence of linked lines the magnetic bearing of the lines are measured by a prismatic compass then the distance of the area measured by a chain. Such a survey ensures not need the preparation of a system of triangles.
Working of the Prismatic Compass
- When the compass needle is freely suspended. It still points to the north. Thus, all angles determined with a prismatic compass are with respect to the north (magnetic meridian).
- The horizontal angle of the line of a survey with relation to the magnetic meridian in the clockwise direction is called the bearing of the line.
- Usually, when using a compass, it is fixed on a light tripod that has a vertical spindle in the ball and socket configuration on which the compass is screwed.