INSTRUMENTS FOR PLANE TABLE SURVEYING
The following instuments are utilized in plane table survey:
- Plane table
- Alidade for sighting
- Plumbing fork and plumb
- Spirit level
- Drawing paper with a rainproof cover
1) Plane Table :
Three distinct sorts of tables (board and tripod) having devices for levelling the plane table and controlling its orientation are in common use :
i) The Traverse Table :
The traverse table consists of alittle drafting board mounted on a light-weight tripod in such how that the board are often rotated about the vertical axis and may be clamped in any position.
The table is levelled by adjusting tripod legs, usually by eye-estimation.
ii) The Johnson Table :
This consists of a drafting board usually 45 x 60 cm or 60 x 75 cm. the top consists of a ball-and-socket joint and a vertical spindle with two thumb screws on the underside.
The ball-and-socket joint is operated by the upper thumb screw.
When the upper screw is free, the table could also be tilted about the ball-and-socket for levelling.
The clamp is then tightened to repair the board during a horizontal position.
When the lower screw is loosened, the table could also be rotated about the vertical axis and may thus be oriented.
iii) The Coast Survey Table :
This table is superior to the above two types and is usually used for work of high precision.
The levelling of the table is completed very accurately with the assistance of the three foot screws.
The table are often turned about the vertical axis and may be fixed in any direction very accurately with the assistance of a clamp and tangent screw.
2) Alidade :
A plane table alidade is a straight edge with some sort of sighting device. Two types are used :
i) Plain Alidade :
Plain Alidade is employed for ordinary work.
It generally contains a metal or wooden rule with two vanes at the ends.
Fig. Simple Alidade
The two vanes or sights are hinged to fold down on the rule when the alidade isn’t in use.
One among the vanes is given a narrow slit while the opposite is open and carries a hair or thin wire.
Both the slits thus provide a particular line of sight which may be made to undergo the thing to sighted.
The alidade are often rotated about the purpose representing the instrument station on the sheet in order that the road of sight passes through the thing to be sighted.
A line is then drawn against the working edge (known because the fiducial edge) of the alidade. it’s essential to possess the vanes perpendicular be the surface of the sheet.
The alidade isn’t considerably suitable on hilly area since the inclination of the road of sight is restricted.
A string joining the tops of the two vanes is usually provided to use it when sights of considerable inclination need to be taken.
ii) Telescopic Alidade :
The telescopic alidade is employed when it’s required to require inlined sights.
Also the accuracy and range of sights are increased by its use.
Fig. Telescopic Alidade
It essentially consists of a small telescope with a level tube and graduated arc mounted on horizontal axis.
The horizontal axis rests on a A-frame fitted with verniers fixed in position within the same manner as that during a transit.
All the parts are finally supported on an important rule, one side of which is employed because the working edge along which line could also be drawn.
The inclination of the road of sight are often read on the great circle.
The horizontal distance between the instrument and therefore the point sighted are often computed by taking stadia readings on the staff kept at the purpose .
The elevation of the purpose also can be computed by using usual tacheometric relations.
Sometimes, to facilitate calculation work, a Beaman stadia arc could also be provided as an additional.
Thus, the observer can very quickly and simply obtain truth horizontal distance from the plane table to a levelling staff placed at the purpose and therefore the difference in elevation between them.
An equivalent geometric principle apply to the alidade on the transit, but the adjustments are somewhat modified in accordance with the lower degree of accuracy required.
3) Plumbing Fork and Plumb Bob :
The plumbing fork utilized in large scale work, is supposed for centering the table over the purpose or station occupied by the plane table when the plotted position of that time is already known on the sheet.
Also, within the beginning of the work, it’s meant for transferring the bottom point on to the sheet in order that the plotted point and therefore the ground station are within the same vertical line.
The fork consists of a hair pin-shaped light metal frame having arms of equal length, during which a plumb-bob is suspended from the top of the lower-arm.
The fitting are often placed with the upper arm lying on the highest of the table and therefore the lower arm below it.
The table being centred when the plumb-bob hangs freely over the bottom mark and therefore the pointed end of the upper arm coincides with the equivalent point on the plan.
4) Spirit level :
A small level could also be used for ascertaining if the table is correctly level.
The extent could also be either of the tubular variety or of the circular type, essentially with a flat base in order that it are often laid on the table and is actually level when the bubble is central.
The table is levelled by placing the extent on the board in two positions at right angles and getting the bubble central in both positions.
5) Trough Compass :
The compass is employed for orienting the plane table to north .
The compass used with a plane table may be a trough compass during which the longer sides of the trough are parallel and flat in order that either side are often used as a ruler or laid right down to coincide with a line drawn on the paper.
6) Drawing paper :
The sheet used for plane tabling must be of superior quality in order that it’s going to have minimum effect of changes within the humidity of the atmosphere.
The changes within the humidity of the atmosphere produces expansion and contraction in several reactions and thus alter the size and warp the map.
To beat this difficulty, sometimes sheets are mounted with their grains at right angles and with a sheet of muslin between them.
Single sheet must be seasoned previous of the utilization by exposing it alternatively to damp and a dry atmosphere.
For work of high precision, fibre glass sheets or paper a backed with sheet aluminium are often used.