Instruments for Setting out Right Angles
Cross staff:
The
cross staff is used for
cross staff is used for
a)
Finding out foot of the perpendicular from a given point to a line
b) Setting right angle at a given point on a line
Finding out foot of the perpendicular from a given point to a line
b) Setting right angle at a given point on a line
There are three forms of cross staff
- Open cross staff
- The French and
- The Adjustable
- Optional Square
- Prism square, the first one is
commonly use.
1. Open Cross Staff:
It
is a simple. It consists of two parts: – 1) the head; 2) the leg. The common
type of cross staff consists of 4 metal arms with vertical slits for sighting
through.
is a simple. It consists of two parts: – 1) the head; 2) the leg. The common
type of cross staff consists of 4 metal arms with vertical slits for sighting
through.
The
head is fixed to the top of an iron stand about 1 .2 to 1.5 m long this is
driven in to the ground.
head is fixed to the top of an iron stand about 1 .2 to 1.5 m long this is
driven in to the ground.
For
setting perpendiculars lines, one pair of opposite arms is aligned with the
chain line. It is specially used for setting off and marking contour lines.
setting perpendiculars lines, one pair of opposite arms is aligned with the
chain line. It is specially used for setting off and marking contour lines.
To find the foot of perpendicular from a given point to a given
lines:
lines:
To
take offset, the cross staff is held vertically on chain where the offset is
likely to occur, and turn until one pair of opposite slits directed to
arranging rod at the forward end of the chain line, the offset is taken is
bisected. If not the cross staff is moved forward or back word the chain line
until the line of sight through the pair of slits at right angle the chain
lines, does bisect the point.
take offset, the cross staff is held vertically on chain where the offset is
likely to occur, and turn until one pair of opposite slits directed to
arranging rod at the forward end of the chain line, the offset is taken is
bisected. If not the cross staff is moved forward or back word the chain line
until the line of sight through the pair of slits at right angle the chain
lines, does bisect the point.
In setting out a right angle at point on the chain line:
The
cross staff is held vertically over the given point on the chain and turn until
the ranging road at either end of the chain line is bisected the line of slight
through 1 pair of the opposite slits. Then the line through the other pair of
slits which is at right angles to the chain line through the other pair of
slits which is at right angles to the chain line. Line may be marked by a
ranging rod at inconvenient point on the sighted.
cross staff is held vertically over the given point on the chain and turn until
the ranging road at either end of the chain line is bisected the line of slight
through 1 pair of the opposite slits. Then the line through the other pair of
slits which is at right angles to the chain line through the other pair of
slits which is at right angles to the chain line. Line may be marked by a
ranging rod at inconvenient point on the sighted.
2. French Cross Staff:
The
octagonal form cross staff is the French cross staff. It consists of an
octagonal brass tube with slits on all eight sides. If has an alternate
vertical slit and an opposite vertical window with a vertical horse hair or
affine wire on each of the four sides. These are used for setting our right
angles. On the other side are vertical slits, which are at 45 degree to those
previously mentioned, for setting out angles of 45 degree.
octagonal form cross staff is the French cross staff. It consists of an
octagonal brass tube with slits on all eight sides. If has an alternate
vertical slit and an opposite vertical window with a vertical horse hair or
affine wire on each of the four sides. These are used for setting our right
angles. On the other side are vertical slits, which are at 45 degree to those
previously mentioned, for setting out angles of 45 degree.
The
base carries a socket so that it may be fitted on the pointed staff when the
instrument is to be used. The sight being too close (only 8 cm apart) it is
inferior to the open type.
base carries a socket so that it may be fitted on the pointed staff when the
instrument is to be used. The sight being too close (only 8 cm apart) it is
inferior to the open type.
3. Adjustable Cross Staff:
It
consists of a brass cylindrical tube about 8 cm in diameter and 1 cm in deep,
and is divided in the centre. The upper cylinder can be rotated relatively to
the lower one by a circular rack and pinion arrangement actuated by screw. Both
are provided with sighting slits. The lower part is graduated to degrees and
sub divisions, while the upper one carries a vernier. Thus it may be use for
setting out angles of any magnitude. It has a magnetic compass at the top,
which may serve to take the bearing of the line.
consists of a brass cylindrical tube about 8 cm in diameter and 1 cm in deep,
and is divided in the centre. The upper cylinder can be rotated relatively to
the lower one by a circular rack and pinion arrangement actuated by screw. Both
are provided with sighting slits. The lower part is graduated to degrees and
sub divisions, while the upper one carries a vernier. Thus it may be use for
setting out angles of any magnitude. It has a magnetic compass at the top,
which may serve to take the bearing of the line.
4. Optional Square:
An
optional square is an compact hand instrument used in setting out right angles
with greater accuracy than a cross staff. It consists of a circular box about 5
cm in diameter and 1.25 cm deep in which two mirrors are fitted at right
angles to the plane of instruments . The mirror ‘h’ called the horizontal glass
which is half silvered and half uncovered. The mirror ‘I’ is known as index
glass is known as wholly silvered. There are three openings on the side of box
at e, b and ,c. the opening e is pin whole for eye , b is small rectangular
slot placed opposite to pin hole and c is large rectangular slot placed at
right angle to line joining e & b
optional square is an compact hand instrument used in setting out right angles
with greater accuracy than a cross staff. It consists of a circular box about 5
cm in diameter and 1.25 cm deep in which two mirrors are fitted at right
angles to the plane of instruments . The mirror ‘h’ called the horizontal glass
which is half silvered and half uncovered. The mirror ‘I’ is known as index
glass is known as wholly silvered. There are three openings on the side of box
at e, b and ,c. the opening e is pin whole for eye , b is small rectangular
slot placed opposite to pin hole and c is large rectangular slot placed at
right angle to line joining e & b
The
surveyor simply turns the optical square upside down which throws the aperture
for the object on that side.
surveyor simply turns the optical square upside down which throws the aperture
for the object on that side.
5. Prism Square:
It
is a modern instrument and is a very use full for setting out right angles. It
is based on the same principles as the optical square and is used in same
manner. It requires no adjustment, since the angle between reflecting surface
of the prism (45 degree) is fixed. It is unaffected by dust & can be used
in poor light. For taking offset to an object the observer holds the instrument
in his hand & slights directly over the prism at ringing of station.
is a modern instrument and is a very use full for setting out right angles. It
is based on the same principles as the optical square and is used in same
manner. It requires no adjustment, since the angle between reflecting surface
of the prism (45 degree) is fixed. It is unaffected by dust & can be used
in poor light. For taking offset to an object the observer holds the instrument
in his hand & slights directly over the prism at ringing of station.
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