CROSS-STAFF AND OPTICAL
SQUARE
SQUARE
A. Cross-staff
The cross-staff is a
simple instrument for setting out right angles. There are three types of
cross-staves.
simple instrument for setting out right angles. There are three types of
cross-staves.
- Open
- French
- Adjustable
The open cross-staff is
commonly used.
commonly used.
Open cross-staff
The open cross-staff
consists of four metal arms with vertical slits. The two pairs of arms (AB and
BC) are at right angles to each other. The vertical slits are meant for
sighting the object and the ranging rods. The crossstaff is mounted on a wooden
pole of length 1.5m and diameter 2.5 cm. The pole is fitted with an iron shoe.
consists of four metal arms with vertical slits. The two pairs of arms (AB and
BC) are at right angles to each other. The vertical slits are meant for
sighting the object and the ranging rods. The crossstaff is mounted on a wooden
pole of length 1.5m and diameter 2.5 cm. The pole is fitted with an iron shoe.
For setting out a
perpendicular on a chain line, the cross-staff is held vertically at the
approximate position. Suppose slits A and B are directed to the ranging rods
(R, R1) fixed at the end stations. Slits C and D are directed to the
object (O). Looking through slits A and B, the ranging rods are bisected. At
the same time, looking through slits C and D, the object O is also bisected. To
bisect the object and the ranging rods simultaneously, the cross staff may be
moved forward or backward along the chain line
perpendicular on a chain line, the cross-staff is held vertically at the
approximate position. Suppose slits A and B are directed to the ranging rods
(R, R1) fixed at the end stations. Slits C and D are directed to the
object (O). Looking through slits A and B, the ranging rods are bisected. At
the same time, looking through slits C and D, the object O is also bisected. To
bisect the object and the ranging rods simultaneously, the cross staff may be
moved forward or backward along the chain line
B. Optical Square
An optical square is
also used for setting out right angles. It consist of a small circular metal
box of diameter 5 cm and depth 1.25 cm. It has a metal cover which slides round
the box to cover the slits. The following are the internal arrangements of the
optical square.
also used for setting out right angles. It consist of a small circular metal
box of diameter 5 cm and depth 1.25 cm. It has a metal cover which slides round
the box to cover the slits. The following are the internal arrangements of the
optical square.
1. A horizon glass H is
fixed at the bottom of the metal box. The lower half of the glass is unsilvered
and the upper half is silvered.
fixed at the bottom of the metal box. The lower half of the glass is unsilvered
and the upper half is silvered.
2. A index glass I is
also fixed at the bottom of the box which is completely silvered.
also fixed at the bottom of the box which is completely silvered.
3. The angle between the
index glass and horizon glass is maintained at 450.
index glass and horizon glass is maintained at 450.
4. The opening ‘e’ is a
pinhole for eye E, ‘b’ is a small rectangular hole for ranging rod B, ‘P’ is a
large rectangular hole for object P.
pinhole for eye E, ‘b’ is a small rectangular hole for ranging rod B, ‘P’ is a
large rectangular hole for object P.
5. The line EB is known
as horizon sight and IP as index sight.
as horizon sight and IP as index sight.
6. The horizon glass is
placed at an angle of 1200 with the horizon sight. The index
glass is placed at an angle of 1050 with the index sight.
placed at an angle of 1200 with the horizon sight. The index
glass is placed at an angle of 1050 with the index sight.
7. The ray of
light from P is first reflected from I, then it is further reflected from
H, after which it ultimately reaches the eye E
light from P is first reflected from I, then it is further reflected from
H, after which it ultimately reaches the eye E
Principle
According to the
principle of reflecting surfaces, the angle between the first incident ray and
the last reflected ray is twice the angle between the mirrors. In this case,
the angle between the mirrors is fixed at 450. So, the angle between
the horizon sight and index sight will be 900.
principle of reflecting surfaces, the angle between the first incident ray and
the last reflected ray is twice the angle between the mirrors. In this case,
the angle between the mirrors is fixed at 450. So, the angle between
the horizon sight and index sight will be 900.
Setting up the
perpendicular by optical square
perpendicular by optical square
1.
The observer should
stand on the chain line and approximately at the position where the
perpendicular is to be set up.
The observer should
stand on the chain line and approximately at the position where the
perpendicular is to be set up.
2.
The optical square is
held by the arm at the eye level. The ranging rod at the forward station B is
observed through the unsilvered portion on the lower part of the horizon glass.
The optical square is
held by the arm at the eye level. The ranging rod at the forward station B is
observed through the unsilvered portion on the lower part of the horizon glass.
3.
Then the observer looks
through the upper silvered portion of the horizon glass to see the image of the
object P.
Then the observer looks
through the upper silvered portion of the horizon glass to see the image of the
object P.
4.
Suppose the observer
finds that the ranging rod B and the image of object P do not coincide. The he
should move forward or backward along the chain line until the ranging rod B
and the image of P exactly coincide
Suppose the observer
finds that the ranging rod B and the image of object P do not coincide. The he
should move forward or backward along the chain line until the ranging rod B
and the image of P exactly coincide
5.
At this position the
observer marks a point on the ground to locate the foot of the perpendicular.
At this position the
observer marks a point on the ground to locate the foot of the perpendicular.
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