Plane table surveying is a type of surveying which can be done very fast. The necessary equipment of the plane table survey is mentioned below. Also, brief discussion and photographs of major instruments are given below.
General Plane Table Survey Equipment
Some basic surveying instruments are required for plane table surveying. Followings are the general plane table survey equipment.
- Plane table
- Alidade for sighting (telescopic or simple)
- Plumb bob and plumb fork
- Spirit level
- Ranging rods
- Drawing sheet and drawing tools
- Paper clips or screws
Details of Major Plane Table Survey Equipment
Alidade compass has two edges. One straight and other one is beveled. The straight edge is made of brass or gunmetal and works as a ruler. The beveled edge is called the fiducial edge. It consists of two vanes at the ends. The vanes are hinged and can be folded when the alidade is not in use.
One of the vanes is the sight vane or the eye vane. It has three equidistant narrow silts. The surveyor looks through these silts towards the object or station. The other vane which is known as object vane is open and a fine thread is stretched between the top and bottom silt.This thread is used to establish a horizontal line of sight parallel to the ruler. The problem with simple alidade compass is that it can be used only when the elevations of the objects are low.
While the plane alidade only took horizontal sights, this telescopic alidade is used to take inclined sights. It is used to increase the accuracy of the sight taken. A small spirit level tube is provided with the telescope so that the alidade can be leveled with the working station. A scale is mounted on the horizontal axis. Lines are drawn along the straight ruler. It is also mounted with a vertical circle that gives us the angle of the object with the horizontal axis.
Plumbing Fork and Plumb Bob
Plumbing fork is a U-shaped metal frame that has two different types of arms. One is horizontal (upper hand) and the other is inclined at a certain angle(lower hand).The upper arm has a pointer at the end while the lower arm has a hook from which plumb bob is suspended.
When the plumbing fork is kept on the table it has to pass through a particular point (the pointer in the other hand). The plumb bob helps in centering.
Compass is of two types-
- Prismatic Compass
- Surveyor’s Compass
Both the compasses are used to navigate the surveying terrain. These are strictly used to determine the bearing of the land that has been traversed.
Both prismatic and surveyor’s compass has following parts:-
- Magnetic needle,
- Graduated circle,
- Vanes for line of sight, and
- A box to keep them in place
When plane table surveying is carried out it is really essential to know whether the working station (surface) is horizontal or vertical. Spirit level is used for this purpose. A bubble (mercury is usually used) is put inside the tube. The tube has an upward curvature which makes the bubble remain in center unless the surface is inclined.
A ranging rod is an instrument extensively used for marking the position of different surveying stations/pegs and for taking sights of those stations. It is also used for ranging for straight lines.