Detail plotting by resection method.

Introduction


Position resection is a method for determining an unknown geographic position (position finding) by measuring angles with respect to known positions. In resection, the one point with unknown coordinates is occupied and sightings are taken to the known points.
This experiment is useful to study detail plotting by resection method.

Principle


Position of the ground station occupied by the Plane Table is determined graphically with the help of three knowm control points, so that the plane table is oriented for plotting the details. Its principle is opposite to the intersection method. Orientation of plane table may be done by -
1) Two point problem.
2) Three point problem.

Equipments Used


1) Compass
2) Plane table with stand
3) Alidade
4) Ranging rods
5) Tape
6) Plump bob
7) Level bubble tube
8) Theodolite
9) Staff

Dos and Don'ts


Dos


1) Center and level the base of plane table accurately.
2) Don't forget to orient the plane table with respect to magnetic north.
3) Always use good quality drawing sheet for mapping purpose.
4) Drawing sheet must be firmly pasted on plane table to avoid any wrinkles.
5) Make a diagonal scale and use it for plotting the distances so as to compensate expansion or shrinkage of srawing sheet.
6) Bisect the ground object accurately using alidade.
7) Use this method when accurate positions of two or three control points are already known and plotted on plane table.
8) Three point problem is always preferred over two point problem to get accurate position of ground point occupied by plane table.
9) Solve the position of ground point inside the great triangle which is always preferred than outside great triangle.
10) Remember lehmann's rules to solve three point problem quickly.

Don'ts


1) Don't measure the long distances with measuring tape. Instead use tacheometric method to get long distances.
2) Don't waste too much of time to carryout exact centering of plane table, particularly in hilly area. It should be set close by within the plotting accuracy limit.
3) Ignore plotting the fields having dimensions less than the plotting accuracy.
4) Don't use this method in plain and small areas.



Observations


1) Don't waste too much of time to carryout exact centering of Plane Table, particularly in hilly area. It should be set close by within the plotting accuracy limit.
2) Ignore plotting the fields having dimensions less than the plotting accuracy.
3) Don't select three control points which make an acute or obtuse angle.
4) Don't set plane table close to one side of great triangle.
5) Do not disturb the centering or leveling of plane table during solving three point problem using trial and error method. If disturbed, repeat the process from beginning.