Unit 4Unit 4

Plane Table SurveyPlane Table Survey

Introduction Introduction

A plane table surveying is a graphical method A plane table surveying is a graphical method of surveying. In this method of surveying, field of surveying. In this method of surveying, field observation and plotting are done observation and plotting are done simultaneously helping the surveyor to simultaneously helping the surveyor to compare the plotted details with actual features compare the plotted details with actual features of the ground. of the ground.

Advantages Advantages

Suitable for location of details as well as contouring for large scale Suitable for location of details as well as contouring for large scale maps directly in the field. maps directly in the field.

Surveying and plotting are done simultaneously in the field, Surveying and plotting are done simultaneously in the field, chances of getting omission of any detail get less. chances of getting omission of any detail get less.

The plotting details can immediately get compared with the actual The plotting details can immediately get compared with the actual objects present in the field. objects present in the field.

Contours and specific features can be represented and checked Contours and specific features can be represented and checked conveniently as the whole area is in view at the time of plotting. conveniently as the whole area is in view at the time of plotting.

Only relevant details are located because the map is drawn as the Only relevant details are located because the map is drawn as the survey progresses. survey progresses.

The plane table survey is generally more rapid and less costly The plane table survey is generally more rapid and less costly than most other types of survey. than most other types of survey.

The instruments used are simple. This method of survey requires The instruments used are simple. This method of survey requires no field book. no field book.

DisadvantagesDisadvantages

The plane table survey is not possible in unfavorable The plane table survey is not possible in unfavorable climates such as rain, fog etc. climates such as rain, fog etc.

This method of survey is not very accurate and thus This method of survey is not very accurate and thus unsuitable for large scale or precise work. unsuitable for large scale or precise work.

As no field book is maintained, plotting at different scale As no field book is maintained, plotting at different scale require full exercise. require full exercise.

The method requires large amount of time to be spent in The method requires large amount of time to be spent in the field. the field.

Quality of the final map depends largely on the drafting Quality of the final map depends largely on the drafting capability of the surveyor. capability of the surveyor.

This method is effective in relatively open country where This method is effective in relatively open country where stations can be sighted easily .stations can be sighted easily .

Plane Table Plane Table

A plane table mounted on A plane table mounted on a tripod stand and a a tripod stand and a number of accessories number of accessories are used during plane are used during plane table survey. The table survey. The accessories consist of accessories consist of alidade, spirit level, trough alidade, spirit level, trough compass, plumbing fork, compass, plumbing fork, plumb bob, drawing plumb bob, drawing sheet. sheet.

AlidadeAlidade

It consists of a straight-edge ruler, It consists of a straight-edge ruler, made of a metal or wood, with one of made of a metal or wood, with one of the edges is graduated known as the edges is graduated known as fiducial edge.fiducial edge.

It consists of two vanes which are It consists of two vanes which are perpendicular to its ends, fitted with perpendicular to its ends, fitted with hinges at their bases, known as sight hinges at their bases, known as sight vanes. vanes.

These are kept folded down on the These are kept folded down on the ruler when not in use. ruler when not in use.

Thus, the line passing through the Thus, the line passing through the slit of the eye vane joining the thin slit of the eye vane joining the thin wire of the object vane and passing wire of the object vane and passing beyond is known as the line of sight beyond is known as the line of sight of a plane alidade. of a plane alidade.

Telescopic alidadeTelescopic alidade

The telescopic The telescopic alidade is designed alidade is designed for greater precision for greater precision and longer range of and longer range of sights. It can be sights. It can be used with used with advantages for advantages for contouring and contouring and plotting of details plotting of details during topographic during topographic surveying surveying

Plumb forkPlumb fork

Plumbing fork Plumbing fork with a plumb bob with a plumb bob is used in large is used in large scale surveying scale surveying for Centering of for Centering of plane table and plane table and for transferring of for transferring of ground points.ground points.

Spirit level Spirit level

It is used to check the It is used to check the level of plane table by level of plane table by placing it on the board in placing it on the board in two positions at right two positions at right angles to each other.angles to each other.

When the bubble tube When the bubble tube remains in the centre at remains in the centre at any point on the the any point on the the table is considered to be table is considered to be properly leveled. properly leveled.

Trough compass Trough compass

The trough compass is used The trough compass is used for marking the magnetic for marking the magnetic north line on the drawing north line on the drawing sheet of the plane table. In sheet of the plane table. In this case, the magnetic needle this case, the magnetic needle point to 0° - 0° of the point to 0° - 0° of the graduated scale and a line graduated scale and a line drawn parallel to the edge of drawn parallel to the edge of the trough compass is along the trough compass is along the magnetic meridian. the magnetic meridian.

A trough compass is also A trough compass is also used to orient the plane table used to orient the plane table with respect to the magnetic with respect to the magnetic meridian. meridian.

Working operation Working operation

FixingFixing Setting: (i) Levelling the tableSetting: (i) Levelling the table

(ii) Centering(ii) Centering

(iii) Orientation(iii) Orientation Sighting the points Sighting the points

Fixing Fixing

In this operation, first the top of the tripod In this operation, first the top of the tripod stand is fixed in level by eye estimation at stand is fixed in level by eye estimation at convenient height with its legs uniformly convenient height with its legs uniformly spread and shoes fixed firmly into the spread and shoes fixed firmly into the ground. The board is fixed to the tripod ground. The board is fixed to the tripod head by tightening the clamping screw. head by tightening the clamping screw.

Leveling Leveling

The top of the table is leveled by moving The top of the table is leveled by moving the legs of the tripod. The level of plane the legs of the tripod. The level of plane table is first judged by eye estimation. table is first judged by eye estimation. Further, it is checked by keeping spirit Further, it is checked by keeping spirit level at different positions on the table level at different positions on the table and if required, legs are further adjusted. and if required, legs are further adjusted.

Centering Centering

The centering with reference to already The centering with reference to already plotted position of the station on the plotted position of the station on the sheet.sheet.

The centering with reference to ground The centering with reference to ground location of the station. location of the station.

Orientation Orientation

To maintain the orientation of the table constant at all To maintain the orientation of the table constant at all the stations in any particular plane table surveying i.e., the stations in any particular plane table surveying i.e., the four edges of the plane table will always be in the the four edges of the plane table will always be in the same direction at all the stations same direction at all the stations

During orientation, the leveling of the plane table During orientation, the leveling of the plane table generally gets disturbed so it is usually carried out with generally gets disturbed so it is usually carried out with leveling simultaneously iteratively. The orientation of leveling simultaneously iteratively. The orientation of plane table can be carried out by:plane table can be carried out by:

Using a trough compass;Using a trough compass; Backsighting;Backsighting; Resection. Resection.

Methods of plane tableMethods of plane table

RadiationRadiation IntersectionIntersection TraversingTraversing Resection Resection

Radiation Radiation

The direction of the objects or points to The direction of the objects or points to be located are obtained by drawing radial be located are obtained by drawing radial lines along fiducial edge of alidade after lines along fiducial edge of alidade after getting the objects or points bisected getting the objects or points bisected along the line of sight of the alidade.along the line of sight of the alidade.

The horizontal distances are then The horizontal distances are then measured and scaled off on the measured and scaled off on the corresponding radial lines to mark their corresponding radial lines to mark their positions on the drawing. positions on the drawing.

Radiation

Intersection Intersection

In intersection method of plane table In intersection method of plane table surveying, the objects or points to be surveying, the objects or points to be located are obtained at the point of located are obtained at the point of intersection of radial lines drawn from two intersection of radial lines drawn from two different stations. different stations.

Intersection Intersection

Z Station

Y Station

Traversing Traversing

It is not much different from radiation as It is not much different from radiation as far as working principles are concerned.far as working principles are concerned.

Used to lay down survey lines between Used to lay down survey lines between the instruments station of a closed or the instruments station of a closed or unclosed traverse.unclosed traverse.

Traversing Traversing

Traversing Traversing

Resection Resection

This method of plane table surveying is This method of plane table surveying is employed to locate and plot the position employed to locate and plot the position of the plane table during surveying.of the plane table during surveying.

This also results in the orientation of the This also results in the orientation of the plane table. plane table.

Three point problem Three point problem

Three point problemThree point problem

if X, Y and Z are well defined objects present in if X, Y and Z are well defined objects present in the field whose plotted positions are x, y and z. the field whose plotted positions are x, y and z. Now, if the plane table is oriented correctlyNow, if the plane table is oriented correctly

The three resectors Xx, Yy and Zz get The three resectors Xx, Yy and Zz get intersected at p which represents the location intersected at p which represents the location of the plane table station, P on the drawing of the plane table station, P on the drawing sheet. sheet.

Thus, through solution of three-point problem, Thus, through solution of three-point problem, both orientation and resection of plane table both orientation and resection of plane table gets accomplished simultaneously. gets accomplished simultaneously.

Methods of resection Methods of resection

Trial and Error method, Trial and Error method, Mechanical method, Mechanical method, Graphical method, Graphical method, Analytical method andAnalytical method and Geometrical construction method. Geometrical construction method. Of these, the trial and error method is easy, Of these, the trial and error method is easy,

quick and accurate. It is commonly used in quick and accurate. It is commonly used in practice and hence, has been discussed in practice and hence, has been discussed in detail. detail.

Triangle of errorTriangle of error In three point problem, if the In three point problem, if the

orientation of the plane table orientation of the plane table is not proper, the intersection is not proper, the intersection of the resectors through the of the resectors through the three points will not meet at a three points will not meet at a point but will form a triangle, point but will form a triangle, known as triangle of error.known as triangle of error.

The size of the triangle of The size of the triangle of error depends upon the error depends upon the amount of angular error in amount of angular error in the orientation. the orientation.