PLANE TABLE - Civil Engineering psc short notes

Plane tabling is a graphical method of surveying in which the field work and plotting are done simultaneously 
• It is most suitable for filling-in of the details in a plot 
• Used in small scale work with low accuracy 

INSTRUMENTS USED 

PLANE TABLE 
• The board IS made of well-seasoned wood such as pine or teak 
• Size : 75 x 60, 60 x 45 cm 
i. Simple plane table : tripod 
ii. Johnson's plane table : ball and socket
iii. Coast survey plane table : foot screws 

ALIDADE 
• The alidade consists ofa metal (brass or gunmetal) or boxwood straight edge 
• 50 cm length 
• The bevelled (ruling or working) edge of the alidade is called the fiducial edge 

i. Plane alidade : has only eye vane , object vane . Only plane 
ii. Telescopic alidade : has a telescope. Can rotate about the horizontal axis 

 PLUM BOB, PLUMBING FORK 
• U shaped metal frame with upper horizontal arm lower inclined arm 
• Plum bob hangs at hook from lower arm 

SPIRIT LEVEL : to check the horizontality of plane table 

TROUGH COMPASS : to mark n-s direction 

DRAWING SHEET 
• Well-seasoned paper is used 
• For better result 2 sheets overlapped with grains perpendicular 
• For important work paper backed with thin aluminum sheets are used 

WORKING OPERATIONS 

1. FIXING : fixing the board to the tripod and clamping 

2. SETTING UP 

i. Levelling : adjusting legs or ball and socket or foot screws 
ii. Centering : setting up the plane table exactly over the point on the ground 
iii. Orientation 
• Process of setting up plane table on a station such that all the line drawn is parallel to that on the ground 
• Accuracy of the project depends on accuracy of orientation 
a. Orientation by trough compass : less accurate , fast , used when no points for backlighting 
b. Orientation by back sighting : most accurate , used when more than one station available 

3. SIGHTING : sighting is done through alidade and recectors are drawn 

METHODS OF PLANE TABLING 

1. RADIATION 
• Used for small works or 1 instrument station 
• Rays are drawn and distance is taken with tape or chain 

2. INTERSECTION METHOD 
• Used when point is inaccessible 
• Point is sighted from 2 instrument stations 
• Aka graphical triangulation method 
• Line joining instrument station is called baseline 

3. TRAVERSING : plane table carried from one point to other and plotted simultaneously 

4. RESECTION 
• This method is used for establishing the instrument stations only 
• Rays drawn from unplotted location to known location - resectors 
i. Two point problem : consists in locating the position on the plan of the station occupied by the plane table, by means of observations to 2 well-defined points. 
ii. The three point problem : consists in locating on the plan the position of the instrument station on the ground by means of observations to three well defined points whose positions haw been already planned on plan 

Types of resection method 

1. MECHANICAL OR TRACING PAPER METHOD : tracing paper is used 

2. GRAPHICAL / BESSEL'S GRAPHICAL METHOD 
• Most easiest & commonly used method 

3. LEHMAN'S / TRIAL AND ERROR / TRIANGLE OF ERROR METHOD 
• If orientation is accurate the resecors pass through a single point 
• If orientation is wrong a triangle is formed called triangle of error 
• Its solved using Lehman's method 
• Circle passing through 3 well defined points - great circle 
• Triangle formed by the points - great triangle 
 
Strength of fix : accuracy with which point can be plotted 
• SOF is good if instrument located within the great triangle 
• SOF is bad if it's in the great circle


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