Test: Theodolite Traversing - Civil Engineering (CE) MCQ

Magnetic bearing:

• The magnetic bearing of a line is the horizontal angel which it makes with the magnetic north. Since magnetic meridian changes gradually with time and thus magnetic bearing also gets changed with time.
• The magnetic bearing is determined from either the prismatic compass or from surveyor’s compass.
• In order to measure the magnetic bearing of a line, the theodolite  should be provided with a simple compass.

Trough compass: 

• It is used in plane table surveying for aligning the plane table along the north direction, not for measuring the angles

Dip

• It is the inclination of the magnetic needle with the horizontal. The dip is zero at the equator and the needle will remain horizontal.
• At a place near 70° north latitude and 96° west longitude, the dip will be 90°. This area is called the magnetic north pole. Similarly, near the south magnetic pole, the dip is 90°.​

Declination

• It is the angle between the magnetic and geographic meridians or the angle in the horizontal plane between magnetic north and true north.

Different magnetic lines are as follows

Important Points
Isobar or pressure bulb 
It is a stress contour or a line that connects all points below the ground surface at which the vertical pressure is the same. 

There are three methods of prolonging a straight line. Those are named as first method, second method and third method.

Permanent adjustments:
The permanent adjustment are made to establish the fixed relationships between the fundamental lines of the instrument, and once made, they last for long time. The following are the permanent adjustments in transit Theodolites:

• Adjustment of the Horizontal Plate Levels.
• Collimation Adjustment
• Horizontal Axis Adjustment
• Adjustment of the Telescope Level or the Altitude level.
• Vertical Circle Index Adjustment

Temporary adjustments
The temporary adjustments are made at each set up of the instrument before starting taking observations with the instrument. The following are the temporary adjustments in transit Theodolite:

• Setting up the Theodolite over the station.
• Levelling up the Theodolite.
• Elimination of the parallel.
• Focussing.

∴ From above Elimination of parallax is not permanent adjustment of theodolite.

Set instrument at A and take sight on B. Establish intermediate points C, D, E etc. In the line of sight. It possible only in case both ends are inter visible.

Prismatic compass:

• The prismatic compass consists of a circular box about 85 to 110 mm in diameter.
• At the center of the metal box, a needle and pivot are provided.
• The graduations are in degrees to 30 minutes and from 00 to 3600 in the clock wise direction.
• A glass cover is fitted over the box to protect the needle from dust. The compass is fitted to a tripod stand.

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.

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.

Graduation Circle

• This is an aluminum graduated ring labeled from 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 lever raise.

Spring Break

• In order to dampen the vibration of the needle once obtaining a measurement to putting it to resting easily.

Object Vane

• The object vane holds horsehair or black thin wire to see the object in line with the sight of the object.

Reflecting Mirror

• It is used to obtain a picture of an object positioned higher or lower surface of the device when bisecting.

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.

Dip

• It is the inclination of the magnetic needle with the horizontal. The dip is zero at the equator and the needle will remain horizontal.
• At a place near 70° north latitude and 96° west longitude, the dip will be 90°. This area is called the magnetic north pole. Similarly, near the south magnetic pole, the dip is 90°.

Declination

• It is the angle between the magnetic and geographic meridians or the angle in the horizontal plane between magnetic north and true north.

Explanation