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Examoogle • User • info@examoogle.com • EE-2024-8821
Chapter 1 of 12 • Page 1 of 248🔒 Protected PDF • Watermarked
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__________ Instrument is used to sight to an object.
Trough compass
Spirit level
U-fork with plumb bob
Alidade
Alidade
An alidade is a sighting instrument used in plane table surveying to draw rays toward an object or station. It consists of a straight edge and a sighting mechanism (either simple vanes or a telescope) to establish the line of sight.
An alidade is a sighting instrument used in plane table surveying to draw rays toward an object or station. It consists of a straight edge and a sighting mechanism (either simple vanes or a telescope) to establish the line of sight.
L=Dtan(α) — where L is the height difference, D is the horizontal distance, and α is the vertical angle measured by a telescopic alidade.
The alidade is placed on the drawing board and rotated until the target object is visible through the sights. Once aligned, a pencil line is drawn along the fiducial edge of the alidade onto the drawing sheet, representing the direction of the object from the station point.
Simple alidade uses two vertical vanes (object vane and sight vane) to define the line of sight.
Telescopic alidade is used for higher precision and measurement of vertical angles.
The fiducial edge of the alidade is always kept in contact with the station point marked on the drawing sheet.
Allows direct plotting of rays in the field
Simplifies graphical triangulation
Simple alidade is limited to short-range sighting
Requires stable table mounting to avoid errors
Plane table surveying for topography mapping
Setting out points from a base line
Spirit level is used for leveling the plane table (ensuring it is horizontal).
Trough compass is used for orienting the plane table with respect to magnetic north.
U-fork with plumb bob is used for centering the plane table over the ground station.
D is correct — The alidade is the specific instrument used to define the line of sight when plotting directions on a plane table.
In surveying exams, remember that the Alidade performs the 'sighting' (direction), whereas the Trough Compass provides the 'orientation' (bearing).