Topic: “Plane table surveying”… Presented by Patel Riya :1305001060340 Radhwa Aarti:130500106041 Tandel Hetal:130500106051 Lad Parth:130500106029 B.E(Civil):3 Semester Topic: “Plane table surveying”… Guided by: Jaimin sir Krunal sir Referance: 1

Sigma Institute Of Engineering… 2

plane table surveying Plane tabling is a graphical method of survey in which the field observations and plotting are done simultaneously. It is simple and cheaper than the theodolite survey. It is most suitable for small scale maps. Equipment and accessories for plane tabling  (a)Equipments: 1)Plane table 2)tripod 3)alidade (b)Accessories: 1)Trough compass 2)Spirit level 3)U-fork with plumb bob 4)Water proof cover 5)Drawing paper 6)Pins 7)Drawing accessories

. (1)Plane table: The drawing board is made from well-seasoned wood with its upper surface exactly plane. It is normally rectangular in shape with 75 cm*60 cm. (i)Simple plane table (ii)Johnson plane table (iii)Coast survey plane table (2)Tripod: The plane table is mounted on a tripod. The tripod is generally of open frame.

(b)Accessories: (1)Trough compass: (3)Alidade: Alidade is used for draw straight line in the direction of lie of sight and for ordinary work. By telescopic alidade angle can be measured in a vertical plane. Two types of alidade (i)Simple alidade (ii)Telescopic alidade (b)Accessories: (1)Trough compass: The trough compass is required for drawing the line showing magnetic meridian on the paper. (2)Spirit level: A spirit level is used for ascertaining if the table is properly level and level the plane table.

(3)U-fork with plumb bob: U-fork plumb bob is used for centering the table over the point or station occupied by the plane table. (4)Water proof cover: An umbrella is used to protect the drawing paper from rain. (5)Drawing paper: Drawing paper is used for plotting the ground details.

Equipment/Accessories used in plane tabling: Plane table: Drawing paper is fixed on the plane table for plotting details.it provides horizontal plane. Tripod: It is used to support the plane table. Alidade: to draw straight line in the direction of line of sight. Trough compass: To establish magnetic north on the drawing paper and to orient the table in the direction of magnetic meridian. Spirit level: to level the plane table. U-fork with plumb bob: for centering the table over the point. Drawing accessories: for plotting details on the drawing paper.

ADVANTAGES The plan is drawn by surveyor himself while the area to be surveyed is before his eyes. Therefore, there is no possibility of omitting the necessary measurements. The surveyor can compare the plotted work with actual featuresof the area. It is simple and cheaper then the theodolite survey. It is most suitable for small scale maps. No great skill is required to produce a satisfactory map and the work may be entrusted to a subordinate. It is useful in magnetic areas where compass may not be used. The mistakes in writing field books are eliminated. It is the most rapid method of surveying. Inaccessible points can be easily located by intersection.

DISADVANTAGES It is not intended for very accurate work. It is not suitable in monsoon. It is essentially a tropical instrument. Due to heaviness, it is inconvenient to transport. Since there are so many accessories, there is likelihood of these being lost.

PRINCIPLE OF PLAN TABLE SURVEY “All the lines drawn on the drawing sheet are parallel to the corresponding lines on the ground, if the table is in properly oriented position”

MOTHOD OF SETTING UP THE PLANE TABLE The following processes are involved in setting up the plane table over the station ; Fixing the plane table on the tripod Levelling Centering Marking the north line orientation

the process of setting up the plane table on the plotted position of the ground station exactly over the station . Is known as centering. It is done by U- fork and plumb bob.

4. Orientation the process by which the positions occupied by the board at various survey stations are kept parallel Is known as the Orientation .

METHODS OF PLANE TABLING There are four distinct methods of plane tabling : 1. Method of radiation 2.Method of Intersection 3.Method of Traversing 4.Method of Resection

Method of Radiation

This is the simplest method and it is useful only when the whole traverse can be commanded from a single station . Set the table at ‘o’ and level it . Transfer point o , on to the sheet by means of plumbing fork. Keep the alidade touching O and sight A. draw the ray along the fiducial edge of the alidade .measure distance OA , on ground and put this length to a suitable scale on paper which will give point a on the paper. similarly, obtain point b , c , d and e on paper by drawing lines of sight for stations B,C,D and E and measuring the distances, OB,OC,OD and OE on ground respectively. Join a , b , c , d and e on paper. For checking the accuracy of work, measure the distance AB,BC,CD,DE and EA and compare them with the lengths ab , bc , cd , de and ea respectively on paper.

2. METHOD OF INTERSECTION

Set the plane table at A, level it and transfer the point A on to the sheet by way of plumbing fork . clamp the table. With the help of trough compass , mark the north direction on the sheet. Pivoting the alidade about a , sight it to B . Measure AB and plot it along the ray to get b. The base line ab is thus draw corresponding rays . Pivoting the alidade, about a , sight the details C,D ect . and draw corresponding rays . Shift the table at B and set it there orient the table by back sighting A . Pivot the alidade about b ,sight the detail C,D ect. And draw the corresponding ray to intersect the previously drawn ray at c,d ect. Thus, the plot ABCD can be plotted on paper as abcd .

METHOD OF TRAVERSING

This method resembles the work of a compass survey and it is useful for the survey work of roads, rivers ect. Set the plan table at A . use plumbing fork for transferring A on to the sheet . Draw the direction of magnetic with the help of trough compass. With the alidade pivoted about a , sight it to B and draw the ray measure AB and scale off ab to some scale . Similarly draw a ray towards E , measure AE and plot e. Shift the table to B and set it .orient the table accurately

TWO POINT PROBLEM This method of resection by two-point problem is not very accurate.

Select a suitable fourth point D as an auxiliary point near C such that the angles CAD & CBD are not small for good intersection at A & B . Set up the table at D & level it. Orient the table approximately using a compass or by placing ab parallel to AB by judgment and clamp it. With the alidade pivoted at a ray is drawn through a. similarly, with the alidade pivoted on b, B is sighted & another ray is drawn through b. the intersection d of these two rays is the approximate location of D as the orientation of the plane table at D is only approximate. Draw a ray to C by pivoting the alidade on d. by estimating the distance DC, cut off dc1 to scale and c1 so obtained is the approximate position of C. Now sift the place table to C, orient it by taking backsight to D & Centre it with reference to c1. the orientation is, thus, the same as it was at D.

Keep the alidade pivoted at a and sight it to A Keep the alidade pivoted at a and sight it to A. draw a ray through a intersecting the line dc1 in c. Pivoting the alidade about c, sight B. draw the ray to intersect with the ray drawn from D to B in b’. thus b’ is the approximate representation of B with respect to the orientation made at D. The angle between ab & ab’ is the error in orientation and must be corrected for. In order that ab & ab’ may coincide, keep a pole P in line with ab’ & at a great distance. Keeping the alidade along ab, rotate the table till P is bisected. Clamp the table. The table is thus correctly oriented. After having the table oriented, draw a resector (ray) from a to A & another from b to B, the intersection of which will give the position C occupied by the table.

POINTS TO BE KEPT IN MIND IN PLANE TABLING : The following points should be kept in mind while doing plane table survey. Ground points shall be marked as A, B, C … etc. and plan. points (on paper) shall be marked as a, b, c … etc. The rays from survey stations to the objects shall be drawn by dashed line. The alidade should be properly pivoted while sighting the objects. The first survey station and the scale of the map shall be so chosen that the entire area can be plotted on the paper.

5. While establishing magnetic north on the paper using trough compass, things causing local attraction shall be kept away of the table. 6. The plane table should be clamped after centring and levelling. The table should be rotated only at the time of orientation.

ERRORS IN PLANE TABLING : The various sources of errors may be classified as : Instrumental errors Errors in manipulation and sighting (personal errors) Errors in plotting.

1. Instrumental errors : The top surface of drawing board is not perfectly plane. The edge of alidade is not straight .(fiducial edge) The object vane and sight vane are not perpendicular to the alidade. The edge of alidade is not parallel to the line of sight. The fixing clamp is not proper. The level tube is defective.

2. Errors in manipulation and sighting : (personal errors) Defective levelling Defective sighting Defective orientation Defective centring Movement of board between sights. The alidade may not correctly pivoted to the station point.

3. Errors in plotting : Defective scale of map. Wrongly intersecting the rays drawn from two different stations. Incorrect measurements from the scale.

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