Instrument used for right angles
The following perpendicular equipmens are used to erect perpendicular from any point on the survey line.
1-Open cross staff
2- Optical square
Open cross staff
Open cross staff used for dropping perpendicular onto survey lines .This is the simple type of instrument. It is of the following three types .
1- Simple cross staff
2- French cross staff
3- Adjustable cross staff
1- Simple cross Staff
This is made of wood and metal.The open cross staff consists of four metals arms with vertical slits. The two pairs of arms are at right angles to each. other .the vertical slits are meant for sighting the object. The cross staff is mounted on a wooden pole of length 1.5 m and diameter 2.5 cm the pole is fitted with iron shoe.
French Cross Staff –
1. Definition
French Cross Staff is a simple surveying instrument used for measuring angles and setting out perpendicular or offset lines on the ground.
Construction
It is an octagonal (8-sided) staff, usually made of wood or metal.
On each face, there are slits (sighting holes).
The observer looks through these slits to align the line of sight.
The instrument is mounted on a staff or pole.
Uses
To set out right angles (90°)
To set out 45° angles
For taking offsets in chain surveying
For marking perpendicular lines in the field.
Features
Simple and lightweight
Easy to use in the field
Not highly accurate
Best suited for small surveys and rough work.
Adjustable Cross Staff .
1. Definition
An Adjustable Cross Staff is a surveying instrument used for setting out angles of any size, not just 90° or 45°. It is an improved type of cross staff compared to the French or simple cross staff.
Construction
It has a cylindrical head with two perpendicular arms (sighting slits).
The head can rotate on its socket, which is fixed on the staff.
A graduated circle (marked with degrees) is provided on the top of the cylinder.
The slits can be set at any required angle by rotating and reading the graduated circle.
Uses
To set out any angle in the field (not limited to 90° or 45°).
To measure and check angles between survey lines.
Useful in chain surveying and offset measurements.
Optical square
Definition
An Optical Square is a small surveying instrument used to set out right angles (90°) on the ground with the help of mirrors.
Construction
It is a metal box (cylindrical or square in shape).
Inside, there are two mirrors placed at 45° to each other:
Horizon glass → half silvered (upper part transparent, lower part reflective).
Index glass → fully silvered mirror.
The observer looks through a small slit or eyepiece.
Uses
To set out right angles (90°) during surveying.
For taking offsets in chain surveying.
To locate foot of perpendiculars from survey lines.
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