Temporary Adjustments Of Theodolite on:
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Temporary adjustments are a set of operations which are performed on a
Leveling of an instrument is done to make its vertical axis adjusted with respect to the apparent force of gravity at the station.
For two spirit vials at right angles:
# Bring one of the level tubes parallel to any two of the foot screws, by rotating the upper part of the instrument.
# The bubble is brought to the centre of the level tube by rotating both the foot screws either inward or outward. The bubble moves in the same direction as the left thumb.
# The bubble of the other level tube is then brought to the centre of the level tube by rotating the third foot screw either inward or outward. n step 1 itself, the other plate level will be parallel to the line joining the third foot screw and the centre of the line joining the previous two foot screws.# Repeat step 2 and step 3 in the same quadrant till both the bubble remain central.
# By rotating the upper part of the instrument through 180°, the level tube is brought parallel to first two-foot screws in reverse order. The bubble will remain in the centre if the instrument is in permanent adjustment.Otherwise, repeat the whole process starting from step 1 to step 5.
The same principle applies for a bulls-eye level:
# Bring the level parallel to any two of the foot screws, by rotating the upper part of the instrument.
# The bubble is brought to the centre of the level tube by rotating both the foot screws either inward or outward.
# Rotate the upper part of the instrument through 180°, so the level is over the remaining foot screw. The bubble will remain in the centre if the instrument is in permanent adjustment. If not adjust this screw to halve the error. Then rotate back through 180° and check the error. Adjust those screws to halve the residual error. Continue until the bubble is always central on the ring.
theodolite
A theodolite () is a precision optical instrument for measuring angles between designated visible points in the horizontal and vertical planes. The traditional use has been for land surveying, but it is also used extensively for building and i ...
to make it ready for taking observations. These include its initial setting up on a tripod or other stand, centering, levelling up and focusing of eyepiece.
Initial setting
The initial setting operation includes fixing the theodolite on a
tripod
A tripod is a portable three-legged frame or stand, used as a platform for supporting the weight and maintaining the stability of some other object. The three-legged (triangular stance) design provides good stability against gravitational loads ...
, along with approximate levelling and centering over the station mark. For setting up the instrument, the tripod is placed over the station with its legs widely spread so that the centre of the tripod head lies above the station point and its head approximately level (by eye estimation). The instrument is then fixed with the tripod by screwing through the trivet.
The height of the instrument should be such that observer can see through
telescope
A telescope is a device used to observe distant objects by their emission, absorption, or reflection of electromagnetic radiation. Originally meaning only an optical instrument using lenses, curved mirrors, or a combination of both to observe ...
conveniently. After this, a plumb bob
A plumb bob, plumb bob level, or plummet, is a weight, usually with a pointed tip on the bottom, suspended from a string and used as a vertical reference line, or plumb-line. It is a precursor to the spirit level and used to establish a vertic ...
is suspended from the bottom of the instrument and it should approximately align with the station mark.Centering
A tribrach with an optical plummet (the black cylinder pointing to the left lower corner of the image)
Centering means bringing the
Exact centering is done by using the shifting head of the instrument. During this, first the screw-clamping ring of the sliding head is loosened and the upper plate of the shifting head is slid over the lower one until the plumb bob is exactly over the station mark. After the exact centering, the screw clamping ring is tightened. This can be done by means of a forced centering plate or tribrach. An optical or laser plummet is normally used for the most accurate setting. The centering and levelling of the instrument is interactive and iterative; a re-levelling may change the centering, so error each is eliminated successively until negligible.
vertical axis
A Cartesian coordinate system (, ) in a plane (geometry), plane is a coordinate system that specifies each point (geometry), point uniquely by a pair of number, numerical coordinates, which are the positive and negative numbers, signed distance ...
of the theodolite exactly over the station mark.
Levelling
Leveling of an instrument is done to make its vertical axis adjusted with respect to the apparent force of gravity at the station.
For two spirit vials at right angles:
# Bring one of the level tubes parallel to any two of the foot screws, by rotating the upper part of the instrument.
# The bubble is brought to the centre of the level tube by rotating both the foot screws either inward or outward. The bubble moves in the same direction as the left thumb.
# The bubble of the other level tube is then brought to the centre of the level tube by rotating the third foot screw either inward or outward. n step 1 itself, the other plate level will be parallel to the line joining the third foot screw and the centre of the line joining the previous two foot screws.# Repeat step 2 and step 3 in the same quadrant till both the bubble remain central.
# By rotating the upper part of the instrument through 180°, the level tube is brought parallel to first two-foot screws in reverse order. The bubble will remain in the centre if the instrument is in permanent adjustment.Otherwise, repeat the whole process starting from step 1 to step 5.
The same principle applies for a bulls-eye level:
# Bring the level parallel to any two of the foot screws, by rotating the upper part of the instrument.
# The bubble is brought to the centre of the level tube by rotating both the foot screws either inward or outward.
# Rotate the upper part of the instrument through 180°, so the level is over the remaining foot screw. The bubble will remain in the centre if the instrument is in permanent adjustment. If not adjust this screw to halve the error. Then rotate back through 180° and check the error. Adjust those screws to halve the residual error. Continue until the bubble is always central on the ring.
Focusing
To obtain an accurate clear sighting, the cross hairs should be in focus; adjust the eyepiece to do this. ; Focusing of eyepiece lens: For focusing of the eye piece, point the telescope to thesky
The sky is an unobstructed view upward from the surface of the Earth. It includes the atmosphere and outer space. It may also be considered a place between the ground and outer space, thus distinct from outer space.
In the field of astronomy, ...
or hold a piece of white paper in front of telescope. Move the eye-piece in and out until a distinct sharp black image of the cross-hairs is seen. This confirms proper focusing.Text book on surveying by prof. C.L Kochher on Page no. 342. Retrieved on 16 August 2014
To clearly view the object being sighted focus the objective lens.
; Focusing of objective lens: It is done for each independent observation to bring the image of the object in the plane of cross hairs. It includes following steps of operation: First, direct the telescope
A telescope is a device used to observe distant objects by their emission, absorption, or reflection of electromagnetic radiation. Originally meaning only an optical instrument using lenses, curved mirrors, or a combination of both to observe ...
towards the object for observation. Next, turn the focusing screw until the image of the object appears clear and sharp as the observer looks through properly focused eye-piece. If focusing has been done properly, there will be no parallax i.e., there will be no apparent movement of the image relative to the cross hairs if the observer moves his eye from one side to the other or from top to bottom.
See also
*Permanent adjustments of theodolite The permanent adjustments of theodolites are made to establish fixed relationship between the instrument's fundamental lines. The fundamental lines or axis of a transit theodolite include the following:-
* Vertical axis
* Axis of plate levels
* Axi ...
* Tape (surveying)
A tape measure or measuring tape is a flexible ruler used to measure length or distance.
It consists of a ribbon of cloth, plastic, fibre glass, or metal strip with linear measurement markings. It is a common measuring tool. Its design all ...
* Ranging rod
A ranging rod (or range rod) is a surveying instrument used for marking the position of stations, and for sightings of those stations, as well as for ranging straight lines. Initially these were made of light, thin and straight bamboo, or of well ...
s
* Survey camp
Survey Camp is an army tradition that was discontinued in the later part of twentieth century but was reinstated in 2002 across the universities of the world with a whole new structure. It is the civil engineering training course for two weeks usu ...
* Local attraction While compass surveying, the magnetic needle is sometimes disturbed from its normal position under the influence of external attractive forces. Such a disturbing influence is called as ''local attraction''. The external forces are produced by source ...
References
{{reflist Surveying Civil engineering Engineering academics