Class 11.pdf
- 1. Surveying (CE2005) Plane Table Surveying Dr. Bibekananda Mandal Assistant Professor Department of Civil Engineering
- 2. Outline • Introduction • Instruments • Working operations • Methods of Plane Tabling
- 3. • The first two methods are generally used for locating the details. • The third method is widely used to lay down survey lines between the instrument stations of a traverse. • Resection is used to plot position occupied by the plane table. Using a trough compass By back sighting Two-point problem Three-point problem Mechanical method /Tracing paper method Graphical method (e.g. Bessel’s method) Lehmann’s method /Trial and error method Radiation Intersection Traversing Resection Methods of Plane Tabling
- 4. i. Radiation A ray is drawn from the instrument station towards the point, the distance is measured using a chain/tape and the point is located by plotting to some scale. • The work is done from a single instrument station. • Orientation of the table not required (as all the rays are drawn from a single location of the plane table). • The method is suitable when the distances are small.
- 5. i. Radiation • Set the table at P, level it and transfer the point on the sheet using plumbing fork and mark it as point p. Also, mark the magnetic meridian. • Keep the alidade touching p and sight to A. Draw the ray along the edge of the alidade. • Similarly, sight different points B, C, D, E etc. and draw the corresponding rays. The table should be kept clamped throughout this process. • Measure the distances PA, PB, PC, PD, PE etc. in the field and plot their distances to a chosen scale along the corresponding rays and mark as a, b, c, d, e etc.
- 6. ii. Intersection • Intersection method is useful when the distance between the point and the instrument station is either too large or cannot be measured accurately due to some obstacles. • This method requires setting the table at minimum two stations. • The distance between the two instrument stations is required to be measured. • These two station are selected in such a way that the whole area to be surveyed can be viewed clearly.
- 7. ii. Intersection • Set the table at A, level it and transfer the point A on the sheet using plumbing fork as point a. Clamp the table. • With the help of the trough compass, mark the magnetic north direction on sheet. • Pivoting the alidade about a, sight it to B. Measure AB and plot it to get b. • Pivoting the alidade about a, sight C, D, E, F etc. and draw corresponding rays • Shift the table to B and set it. Orient the table by back sighting along ba to the signal at A. • Pivoting the alidade about b, sight C, D, E, F etc. and draw the corresponding rays along the edge of the alidade to intersect with the previously drawn rays in c, d, e, f etc.
- 8. iii. Traversing • A series of lines can be surveyed, which may be a closed traverse or, an open traverse. • At each successive station the table is set, a foresight is taken to the following station and its location is plotted by measuring the distance as in the radiation method. • The difference from the radiation method is that, the observations are made to those points which will subsequently be used as instrument stations and position of the table is needed to be shifted.
- 9. iii. Traversing • Set the table at A, level it and transfer the point A on the sheet using plumbing fork as point a. Clamp the table. • With the help of the trough compass, mark the magnetic north direction on sheet. • Pivoting the alidade about a, sight it to B. Measure AB and plot the distance as ab. Similarly, draw a ray towards E, measure AE and plot line ae. • Shift the table to B and set it. Orient the table by back sighting along ba to the signal at A. • Pivoting the alidade about b, sight to C. Measure BC and plot it to get bc. • Similarly, the table can be shifted to other stations and the traverse is completed.
- 10. iv. Resection Resection is the process of determining the position of the station occupied by the plane table, by the means of sights taken towards the known points, locations of which have already been plotted. • Objective is to plot the position occupied by the plane table and orient it properly. • Can be done by using the following methods: (a) Using a trough compass (b) By back sighting (c) Two-point problem (d) Three-point problem Mechanical method /Tracing paper method Graphical method (e.g. Bessel’s method) Lehmann’s method /Trial and error method
- 11. (a) Resection: Using a trough compass This method is rarely used as the errors due to local attraction is common. Say, station A and B on the ground are already plotted as a and b. The position of the table C is to be plotted. 1. Set the table at C and orient it using compass. 2. Pivoting the alidade about a, draw a ray. 3. Similarly, pivoting the alidade about b, draw a resector. The intersection of the two rays will give the required point c.
- 12. (b) Resection: By back sighting In this method, it is necessary to set the table on one of the known points and draw the ray towards the station to be located. Say, station A and B on the ground are already plotted as a and b. The position of the table C is to be plotted. 1. Set the table at A and orient it by back sighting B along ab. 2. Pivoting the alidade about a sight C and draw a ray as ac1. 3. Shift the table at C and centre it approximately with respect to c1. Keep the alidade on the line c1a and orient the table by backsighting A. Clamp the table. 4. Pivoting the alidade about b, sight B and draw a resector bB to intersect c1a at c (i.e. location of the instrument station).
- 13. Next class Will continue Plane Table Surveying… Two-point problem Three point problems Errors in plane tabling Advantages and disadvantages
- 14. Thank you…