Introduction About pane table surveying
Plane table (PT) surveying is the method of serving in which field observa plotting proceeds simultanensly A plane table surveying is a graphical method of surveying, In this method, surveyor must be a good artist to represent various features on the surface of the earth. In plane table survey, the measurements of survey lines of the traverse and their plotting to a suitable scale are done simultaneously on the field Following are the cases in which the plane table survey is found to be useful:
• Compass survey cannot be carried out with success in industrial areas of the town Plane table survey will be the best alternative in such cases
• For preparing plans on a small scale, plane table survey proves to be speedy, easy and accurate.
• The city or town has expanded within two or three decades and it is required to plex the developed area on the previ.
1.1 Principle of Plane Table Surveying
Plane table surveying is based on the principle that Imes drawn during plotting always lay parallel to the corresponding lines actually present on the ground. Thus it is based on the principle of parallelism.
Fig: principle of plane table
Let us consider three ground stations A, B and C which on joining provides a tringle ABC This has been plotted on a sheet of paper at a scale by plane table surveying represented by also Here, the sides AB, BC and CA are plotted in such a way that they are parallel to the sides actually available on the ground. The plotted sides are ab, be and ca on a sheet.
•The principle is based on the fact that the lines joming the points on the plane table are made parallel to their corresponding lines joining the ground points while working at each station.
• The principle can be based understood by graphical reduction of a triangle to the given dimensions.
•The hase of the triangle is plotted on the desired scale and the base angle is directly plotted by turning the alidade at each end.
•The intersection of rays gives the desired location of the triangle vertex
1.2 Accessories Required for Plane Table Surveying
A plane table mounted on a tripod stand and a number of accessories are used dining plane table survey. The accessories cost of alidad, spint level, tough compass plumbing fork, plumb bob, drawing sheet, etc. Instruments accessories used in plane table surveying are listed below.
» Drawing board plane table (75cm×60cm)
» Alidade
» Plumbing fork
» Spirit level
» Trough compass and
» Drawing Paper and arrangements
Plane Table
The plane table consists of a drawing board with an arrangement for fixing on a tripod stand Figure shows a simple plane table.
Fig: plane table
Board Dimensions
Large board: 750×600 ×15mm
Medium board 600×500×15mm
Small board 500×400×15mm
Plane table stand is 1250 mm height from clamping head to shoe.
Plane table is meant for fixing a drawing sheet over it.
Alidade
The instrument which is used to develop a line of sight parallel to points joining ground points in plane table surveying is called alidade. There are two types of alidades:
a Plain alidade
b. Telescopic alidade Used for long sights.
Plain Alidade
It consists of a straight-edge ruler, made of a metal or wood, in which one of the edges perpendicular to its ends, fitted with hinges at their bases, known as sight vanes. These are kept folded down on the ruler when not in use. One of the sight vanes is provided with a narrow slit having three holes. This is used as eye vane. The other, used as object vane, is open and carries a hair or thin wire at its centre Thus, the line passing through the slit of the eye vane joining the thin wire of the object vane and passing beyond known as the line of sight of a plane alidade. A string is fitted at the top of the vanes and is used for mclined sight In some alidade, a compass needle as well spirit level gets fitted in a box engraved at its base. However, the plam alidade is ou very accurate The figure below shows a plain alidade.
Fig : plane alidade
Telescopic Aldade
It consists of a telescope as an arrangement for sighting (similar to that presem in the upper part of a theodolite). The telescope is fitted with a stadia diaphragm and can t used as tachometer also for computations of horizontal distance and vertical elevation The line of sight of the telescope is aligned along the fiducial edge in this instrumen the object is sighted through the telescope and the distance is scaled off in that directi along the fiducial edge Figure shows a telescopic alidade.
Fig: Telescopic Alidade
. Plumbing Fork
A plumbing fork is a U-shaped piece of metal or wooded frame. The end of one of th arm is pointed and the other arm is having an arrangement for hanging a plumb bob. The frame is constructed in such way that the tip of the pointed arm and the plumb line lie in the same vertical line. At the time of use, the pointed arm is placed on the table and the other arm, with a plumb bob attached, is kept below the table Plumbing fork with a plumb bob is used in large scale surveying for Centering of plane table and for transferring of ground point.
Fig: plumbing Fork
. Spirit Level
It consists of a flat based tube with a small bubble either circular or tubular in shape. It is used to check the level of plane table by placing it on the board in two positions at nght angles to each other When the bubble tube remains in the centre at any point on the table is considered to be properly leveled Figure shows a spirit level.
Fig: Spirit Level
. Trough Compass
A trough compass consists of a long, narrow rectangular box, covered with glass. Inside the box, at its centre, there is a magnetic needle resting on the prvot. At the extremities of the trough compass, there are graduated scales with zero at the centre and marking up to 5 on either side of the zero line Figure shows a Trough compass. The trough compass is used for marking the magnetic north line on the drawing sheet of the plane table. In this case, the magnetic needle point to 0-0 of the graduated scale and a line drawn parallel to the edge of the trough compass is along the magnetic meridian A trough compass is also used to orient the plane table with respect to the magnetic meridian.
Fig: Trough compass
Drawing Arrangements
A drawing paper of good quality and well-seasoned is to be used for plotting It sho be able to withstand the rubbing of the alidade. The effect of changes in humidity at temperature of the atmosphere should be minimum thus reduction in the expansion contraction of the paper, and minimum alteration in the scale of the map and distortion in the plan. A sharp, hard (41 or 61) pencil is to be used for drawing lines Soft pencils an uned for lines to be erased. A rubber eraser with good quality is used for crasing the fines
1.3. Working Operations of Plane Table Surveying
At each station the plane table is required to get set up before carrying out any plotting
work It basically consists of the following operations:
1 Fixing of the plane table
3 Centering of the plane table
2. Leveling of the plane table
4 Transferring of ground point
5 Onentation of the plane table 6 Sighting the ground station
1. Fixing of the Plane Table
In this operation, the top of the topod stand is fixed in level by eye estimation convenient height with its legs uniformly spread and shoes fixed firmly into ground. The board is fixed to the tripod head by tightening the clamping screw
2. Leveling the Plane Table
The top of the table is leveled by moving the legs of the tripod. The level of pl table is first judged by eye estimation. Further, it is checked by keeping spirit lest at different positions on the table and if required. legs are further adjusted.
Procedures
» Set up plane table at the convenient height (approx 12m) by spreading the legs to keep approximately leveled, ensuring that the locanon occupied station is roughly centered over its ground
» Rotate the plane table about its vertical axis till its longer edge is parallel to the line joining the shoes of any two legs of tripod and place the third leg pointing fowards the observer in between his her legs.
» Place a spirit level on the table such that its longitudinal axis is parallel to the longer edge By the help of third leg, bring the bubble of spint level at centre Place the spin level perpendicular to its previous position, with the help of third leg, moving leg forward or backward, bring the bubble of the spirit level at centre
» Rotate the plane table top through 180 check the bubble remained central in all positions.
» Repeat the above steps if found necessary.
3. Centering of the Plane Table
In this operation, the pointed end of the upper arm of plumbing fork is held at the (already) plotted position of the station and the table is shifted and adjusted in such a way that the tip of the plumb bob points exactly over the ground station.
4. Transferring of Ground Point
In this operation, the tip of the plumb bob is placed exactly over the ground point which is required to be transferred. The point corresponding to the tip of the pointed arm on the table provides the transferred position on paper
There are two types of operations involved in centering of plane table.
» The centering with reference to already plotted position of the station In this case the pointed end of the upper arm of a U-fork is held at the plotted position of the station and the table is shifted and adjusted in such a way that the tip of the plumb bob points exactly over the ground station
» The centering with reference to ground location of the station In this case, the up of the plumb bob attached to a U-frame is placed exactly over the ground poust which is required to be transferred The point corresponding to the tip of the pointed arm of the U-frame provides the transferred position on the table However, the accuracy with which centering is being carried out depends upon the scale of plotting.
5. Orientation of Plane Table
The objective of this operation is to maintain the orientation of the table constima all the stations to any particular plane table surveying. Le the four edges of the plane table will always be in the same direction at all the stations Thus, all le plotted on the plane table sheet will maintam parallelism to their corresponding lines on the ground.
During orientation, the leveling of the plane table generally gets disturbed a usually carried out with leveling simultaneously iteratively The orientation of plane table can be carried out by:
A. Onentation by using a trough compass
B. Orientation by backsighting
A .Orientation by using Trough Compass
In this method, the edge of the trough compass is placed along the magnet mendian (drawn at the starting station) and the plane table is rotated till the needle points to zero-zero of the scale Once it is achieved, the table is said be oriented and this clamped. This method of orientation is not very accurat and also may get affected by local attraction and is generally used for small scale survey.
Procedure
» Suppose A and B are two stations.
» Temporary adjustment is done.
» A compass is placed on the right-hand top comer of the sheet in sock way that the needle coincides with 0-0 mark. This line represents the north The table is then clamped.
» With the alidade touching the point a, the ranging rod at B is bisected and any is drawn. The distance AB is measured and plotted.
» The table is shifted and centred over B so that point b is just over B. Now the trough compass is placed exactly on north line drawn previously.
» Table is then turned clockwise or anticloskwise until the needle cond exactly 0-0 mark of compass.
B. Orientation by Backsighting
In this method, the fiducial edge of alidade is laid along a ray drawn from sa previous station to the present station and the plane table is then rotated till the Iine of sight of alidade bisects exactly the ranging rod placed at previous station. The plane table is then clamped and said to be oriented. In this method, the level of the plane table has to be maintained identical in both the stations. This method is accurate and is always preferred.
Procedure
» Suppose A and B are two stations on ground.
» Plane table is setup over A.
» With the alidade touching a on sheet over table, the ranging rod at B is bisected and a ray is drawn. The distance AB is measured and plotted to any suitable scale. So, the point b represents station B.
» The table is shifted and set up over B. Now the alidade is placed along the line ba and ranging rod at A is bisected by turning the table clockwise or anticlockwise.
6. Sighting the ground station.
After the plane table is accurately centered and leveled over the ground station, the fiducial edge or working edge of alidade is kept touching the plotted location of instrument station. The ground control point is sighted through the sight vane so that the station, the thread of the object vane and the silt hole of the vane, are in the wame straight line. The sighting operations are required for sighting all the stations or details whose positions are either known or to be surveyed.
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