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In order to measure the magnetic bearing of a line, the theodolite should be provided with ___.
extra telescope
spirit level
compass
tabular or trough compass
tabular or trough compass
To measure the magnetic bearing of a line with a theodolite, it must be equipped with a tubular or trough compass. This attachment allows the surveyor to orient the horizontal circle of the theodolite with the magnetic north.
To measure the magnetic bearing of a line with a theodolite, it must be equipped with a tubular or trough compass. This attachment allows the surveyor to orient the horizontal circle of the theodolite with the magnetic north.
Bearing=╬╕measuredтАЛ+╬┤
╬┤ represents the index correction or local magnetic declination.
The trough compass consists of a magnetic needle suspended on a pivot, enclosed in a rectangular box (trough). When the needle is free, it aligns itself with the magnetic meridian. By rotating the theodolite until the needle coincides with the zero-markings at the ends of the trough, the horizontal circle is aligned to Magnetic North, allowing the measurement of bearings of other lines relative to this datum.
The trough compass is specifically designed to be attached to the standards or the plate of a theodolite.
It helps in determining the magnetic meridian at any survey station.
Unlike a circular compass, a trough compass provides higher precision for orienting the instrument.
The magnetic bearing is the horizontal angle measured clockwise from the magnetic north.
Allows for orientation of the survey in magnetic north without separate compass equipment.
Increased accuracy compared to integrated circular compasses due to longer needle sensitivity.
Susceptible to local attraction from metallic objects or power lines.
Needle requires frequent calibration if the pivot point becomes dull.
Reconnaissance surveys.
Traverse surveying where magnetic orientation is required.
Determining the magnetic declination at a specific site.
A standard trough compass has a graduated arc only at the ends, limiting its use for orientation rather than independent bearing measurement.
Option B (spirit level) is used for leveling the instrument, not for direction orientation.
Option A (extra telescope) is typically used for specific engineering tasks like tunnel alignment or steep sight measurements.
D is correct тАФ The theodolite must be fitted with a tabular or trough compass to align its horizontal circle with the magnetic meridian.
Always remember that in surveying, the 'Trough Compass' is used to identify the Magnetic North, whereas 'Spirit Levels' are strictly for establishing a horizontal reference plane.