Plane Table Survey (lecture Part 1)

 

Plane Table Survey

Plane table survey is a survey in which drawings are prepared during the survey work. A plane table is a drawing board-like instrument that is surveyed by placing it on top of a stand. Also the plumbing fork is used to bring the ground point and its corresponding point on top of each other on the drawing. This tool is also called U- frame.

Purpose Of The Plane Table Survey

The purpose of the Plane Table Survey is to produce on-the-spot data about small areas on the drawing. This is usually done to display on a drawing the information left over from previous surveys with the help of theodolite. Sometimes traversing is done with the help of a plane table, but this is done only when the survey does not require much accuracy because more precision is not done with the help of a plane table.

Plane Table Survey

Principle Of Plane Table Survey

The principle of plane table surveying made in the plane table survey is to place the two points in the field on the driving seat parallel to the line joining them.  This is done by applying rays. Plane table survey work is graphical. According to which feed work and drawing work are done simultaneously and no field book is required for this purpose.

Instrument Used In The Plane Table Survey

The following instrument is used in the Plane Table Survey.

·         Plane Table Board With Tripod

·         Alidade

·         Plumbing Fork

·         Trough Compass

·         Ranging Rod

·         Pegs

·         Chain

·         Tape

Instrument Used In The Plane Table Survey

Setting The Plane Table

Plane Table before the survey work begins it is prepared to work This preparation is called plane table setting. The following are the three steps:

·         Leveling

·         Centering

·         Orientation

Leveling

This means bringing the plane table to a horizontal position. The legs of the tripod above ground stations are kept properly open. Its height is kept appropriate so that it is easy to work on the table.  This height is kept about one meter or three feet. The distance between the three legs of the stand is kept equal to level the table on the flat ground. Spirit level is used to check the leveling.  If it is not available, the round pencil is placed in a different place on the plane table and left free. If the pencil stops in one place, the level is correct and if it moves in any way, it will be on the down side and the leg on this side is raised slightly to correct it.  If the table has a ball and socket, it can be leveled very easily.

Centering

Centering refers to the stations shown on the drawing sheet above the plane table in a vertical position above its original ground station. The principle of the survey is that the ground station is overlaid with capital letters such as A,B,C etc. while on the drawing sheet they are represented with lower case letters like a,b,c etc. To center, the top edge of the U-frame is placed on top of the drawing sheet station and the top edge is tied to the top of the station  And if there is any difference between them, the tripod is corrected by moving, but keep in mind that the leveling of the table is not bad during this time.

Orientation

The process of placing the plane table in the correct direction according to the shape of the field while conducting the Plane Table survey is called orientation. If you have to work on more than one station in the field, then each station must have the same direction of the plane table. In this case, the process of correcting the table is also called orientation. This process is done in two ways.

By Trough Compass

In this method the plane table is set at the first station and the direction is corrected and the north direction is indicated with the help of a compass. The compass at each station is then set to the plane table in the first direction with its help. This method is not considered more accurate because the magnetic north direction does not reflect correctly due to local attraction.

By Back Sighting

This method is considered better in plane table. Because in this way the error is not affected by the local attraction.  In this method, after finishing work on one side of the line, the plane table is set on the other side and the back edge is looked at.  This is called the backside method. In this way, to orient the plane table, the rulling edge of the ledge is mixed with the line and the plane table is moved in the right direction by looking at the ranging rod on the rear edge.

Advantages Of Plane Table Survey

The following are the advantages of a Plane Table Survey.

·         The drawing is prepared during the survey which saves time .Only finishing is required in the office.

·         Field book does not require measurement writing and notice.

·         Measuring angles is not required.

·         There is no possibility of error due to magnetic attraction.

·         Field details are unlikely to be left out.

·         Field details are obtained in less time with more accuracy.

·         Suitable for small area surveys.

·         Surveying in uneven area is done this way.

·         It doesn't take much skill to create a drawing

Disadvantages Of A Plane Table Survey

The three tables have the following disadvantages of the survey.

·         This method is not used in bad weather.

·         This method is not suitable in forested areas.

·         Drawing in the hot sun is quite difficult.

·         There is a risk of the board becoming crooked due to changes in temperature.

·         Since field notices are not prepared in this type of survey, it becomes quite difficult to get the drawing accurate.

·         This method is not suitable for large area surveys as it involves changing the drawing sheet.

·         The drawing sheet gets damaged while drawing in the field.

·         The instrument used in plane table surveys are heavy and difficult to change over and over again