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Temporary Adjustment of Prismatic Compass - Compass Traversing, Surveying and Levelling | Surveying and Levelling Notes- Agricultural Engg - Agricultural Engineering PDF Download

TEMPORARY ADJUSTMENT OF PRISMATIC COMPASS (FIELD PROCEDURE OF OBSERVING BEARING)

The following procedure should be adopted while measuring the bearing by prismatic compass.

  1. Fixing the compass with tripod stand The tripod stand is placed at the required station with its legs well apart. Then the prismatic compass is held by the left hand and placed over the threaded top of the stand. After this, the compass box is turned clockwise by the right hand. Thus the threaded base of the compass box is fixed with the threaded top of the stand.
  2. Centering Normally, the compass is centred by dropping a piece of stone from the bottom of the compass box. Centring may also be done with the aid of a plumb bob held centrally below the compass box.
  3. Levelling: Levelling is done with the help of a ball-and-socket arrangement provided on top of the tripod stand. This arrangement is loosened and the box is placed in such a way that the graduated ring rotates freely without touching either the bottom of the box or the glass cover on top.
  4. djustment of prism: the prism is moved up and down till the figures on the graduated ring are seen sharp and clear.
  5. Observation of bearing: After centering and leveling the compass box over the station, the ranging rod at the required station is bisected perfectly by sighting through the slit of the prism and horsehair at the sight vane.

At this time the graduated ring may rotate rapidly. The brake pin is pressed very gently to stop this rotation. When the ring comes to rest, the box is struck very lightly to verify the horizontality of the ring and the frictional effect on the pivot point. Then the reading is taken from the graduated ring through the hole in the prism. This reading will be magnetic bearing of the line.

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FAQs on Temporary Adjustment of Prismatic Compass - Compass Traversing, Surveying and Levelling - Surveying and Levelling Notes- Agricultural Engg - Agricultural Engineering

1. What is a prismatic compass and how is it used in compass traversing?
Ans. A prismatic compass is a surveying instrument used to measure horizontal angles. It consists of a magnetic needle that aligns with the Earth's magnetic field, and a prism that allows the user to read the bearing directly. In compass traversing, the prismatic compass is used to measure the bearings of the survey lines, which are then used to calculate the angles and distances between points.
2. How do you temporarily adjust a prismatic compass?
Ans. To temporarily adjust a prismatic compass, follow these steps: 1. Place the compass on a level surface and make sure it is not affected by any magnetic objects. 2. Rotate the compass until the magnetic needle aligns with the north-south direction. 3. Read the bearing on the compass dial. 4. Repeat the process at least three times, ensuring that the readings remain consistent. 5. Take the average of the readings to obtain the temporary adjustment for the compass.
3. What is the purpose of compass traversing in surveying?
Ans. Compass traversing is a surveying technique used to determine the relative positions of points on the Earth's surface. It is particularly useful in areas where the terrain is rugged or obstructed, making it difficult to use other surveying methods. By measuring the bearings of survey lines using a prismatic compass, compass traversing allows surveyors to calculate the angles and distances between points, creating a comprehensive map of the area.
4. What is the role of levelling in agricultural engineering?
Ans. Levelling plays a crucial role in agricultural engineering as it helps in creating and maintaining a well-drained and leveled field surface. It ensures that water flows evenly across the field, preventing waterlogging and facilitating proper irrigation. Levelling also helps in determining the slopes and gradients of the land, which is essential for efficient land use planning, soil conservation, and crop management.
5. What are the potential errors in compass traversing and how can they be minimized?
Ans. Some potential errors in compass traversing include magnetic declination, local attraction, and errors in reading the compass. To minimize these errors, surveyors can: - Correct for magnetic declination by adjusting the compass readings accordingly. - Identify and account for local attraction by comparing compass readings with known reference points. - Take multiple readings with the prismatic compass to reduce errors in reading and record the average value for greater accuracy. - Ensure the compass is properly leveled and not affected by nearby magnetic objects. - Regularly calibrate and maintain the prismatic compass to ensure its accuracy.
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