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Which of the following statements is/are incorrect about the Prismatic Compass? A. The needle is broad but it does not act as an index. B. The graduated ring is attached with the needle. This does not rotate along with the line of sight. C. The readings are taken directly seeing through the top of the glass.

Which of the following statements is/are incorrect about the Prismatic Compass? A. The needle is broad but it does not act as an index. B. The graduated ring is attached with the needle. This does not rotate along with the line of sight. C. The readings are taken directly seeing through the top of the glass. Correct Answer Only C

Explanation:

Out of the following Statements, only A, B are correct. 

The compasses used in survey are generally two types: Surveyors compass and Prismatic compass. The basic difference between them is given below:

Parameter

Surveyor’s Compass

Prismatic Compass

Magnetic Needle

Edge bar needle is used and it act as index

Broad needle is used and it is hidden below the aluminium ring and does not act as index.

Graduation

Graduated ring is attached to the box and ring rotates with box.

Graduations vary from 0° to 90° in the four quadrants. Zero points are marked with N and S.

Graduated ring attached to the needle and remains stationary.

Graduations vary from 0° to 360° clockwise with zero at south end. 

Sighting Vanes

Readings are taken by directly looking through the glass and reading the north end of needle.

Readings are taken through the vertical side of the prism provided at the eye vane.

Reading

Sighting and reading are done separately and that too from different positions. After sighting the object, the observer move around and takes the reading at North end of needle.

Sighting and reading are done simultaneously.

Support Requirement

Tripod is compulsory.

Tripod may or may not be used.

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