Advantages and Disadvantages of Plane Table Survey

Plane table surveying is one of the easiest methods to measure a piece of land. In this method, the earth is considered plane, and its curvature are omitted. It is one of the fastest methods of surveying in which the surveyor plots the plan and monitors the field simultaneously. In this method, the geometrical condition of the site (whose map is being drawn) is created on the map sheet on a plane table, and then topographic details are plotted. Parallelism is the principle on which plane table surveying works.

Here is the list of all the objects or tools needed for conducting plane table surveying:

A plane table
A Tripod
Plumb bob and plumb fork
A simple and a telescopic alidade
Some paper clips
A compass
A Spirit level
A chain
Some screws
Drawing sheets and drawing tools

Plane table fixation

Set the table on the tripod stand and arrange the drawing sheets in the position from first to last on the table with the help of paper clips.

Table leveling: Spirit level is used to level the plane table.
Centering: A plumbing fork needs to be used for centering on arranging plotted points over the ground point.
Orientation: When more than one instrument station is used, orientation is required by back sighting and compass.

Uses of plane table surveying method

Urban Planning:-Urban Planning is an extremely useful method for architects and urban planners to draw accurate and detailed plans of an area. It locates the position of buildings, roads, and others in developmental projects.
Topographic Surveying:- This type of surveying involves mapping relief and elevations in an area. It is used to draw detailed maps of topography, often required in engineering and construction projects.
Archaeological Surveying:- This method is used by archaeologists to conduct their studies and record and plot their archaeological sites.
Site layout:- Plane table surveying is used to form a construction site layout, with a piece of detailed information on roads, buildings, and other monuments or structures.
Photogrammetry:- To create highly and properly detailed 3-D models of a particular area, plane table surveying is combined with photogrammetry.
Mapping small areas:- This plane table surveying is also beneficial for creating detailed maps of comparatively small areas like a town or a particular block of a city and proves helpful for archaeologists and urban planners.

Different methods of plane table surveying

There are four types of methods for this surveying: method of radiation, method of intersection, method of traversing, and method of resection.

1. Method of radiation

This method takes the whole to the part method into account. One station covers the entire area by making radial lines from it. This station is fixed, most probably at the center. A chain link measures the distance between the other and center points.

2. Method of Intersection

This method comes into consideration when the method of radiation fails or meets some obstacles in its path, such as mountains and rivers.

3. Method of Traversing

This method includes a sequence of connected lines. Here, a tape is used to measure the length and direction of the lines. In order to minimize errors, check lines are drawn to create an accurate land map. The length of these check lines can be determined by the triangle formula.

4. Method of Resection

In order to set up new stations, this method comes into account. In this method, the location of the plane table (when unknown) is calculated by taking the help of the known points in the plot. Three-point problem method and two-point problem method are used to get the station point.

Three-point problem method

In this method, already plotted three points on the map are considered to obtain the new station by bisecting these points. There are also four methods to use this method. These are the tracing, Lehmann, Analytical, and Graphical methods.

Tracing method- New station is determined by using the three points. In this method, the station or plane table is set up at a point in such a way that the other three points are visible. Plot the radiating lines on a trace paper by looking at the three points and follow it by placing the trace paper on the drawing and trying to match these lines passing through the corresponding points lastly plotted. This procedure will give you the position of the new station.
Lehmann Method: In this method, the plane table is set up at a point, stations A, B, and C are sighted, and rays are drawn. These rays lead to the formation of a little triangle known as the triangle of error. Now, another point is chosen, say it Z, and now points A, B and C are sighted from Z, giving another triangle of error. This procedure goes on like this until and unless the value of the triangle of error turns to zero.
Analytical method: In this method, points A, B and C are sighted from the station, and then values of angles and lengths are jotted down. Thus, from these values, the position or location of unknown points is gathered with the help of using the analytical formulae.
Graphical method: In this method, values of angles and lengths are obtained on a graph, and thus, the location of the plane table or station is identified.

Two-point problem method

Under this method, two points are located from the points given in the plane table sheet. In this method, two cases are encountered. One, when two points are possible to be drawn i.e, they intersect, and another, when two points are not possible to be drawn i.e, they do not intersect on the plane table.

Some points to keep in mind while conducting the plane table survey

Make sure that the plane table is leveled up properly and carefully plotted to get the more rightful outcomes and that it is easy to rotate during orientation.
The alidade should also be leveled properly and pivoted appropriately.
Try to avoid the local attraction while marking the magnetic north.
The whole area and all the points should get covered under a proper map scale.
Make sure a dashed line is used while connecting the objects and the survey stations.

Some common errors faced

Likewise other fields and subject matter, this field can also encounter some errors which can make your work or efforts appear and prove less efficient. The following are some common errors that one makes:

Plotting errors: Errors can be done while drawing the radiating lines and the known points.
Instrumental errors:Sometimes, instruments can also carry defects that may make your work ineffective. For instance, an uneven plane table.
Manipulation and sighting errors: Defects can also occur in the preparation of a plane table, like in leveling, setting, or during orientation.

Principles of plane table surveying

Represent points X, Y, Z, etc. (on the ground) as x, y, z, etc. on the paper so as to distinguish them properly.
While taking the sights, the alidade should be pointed in the center, and the plot table should be clamped in position on the drawing sheet.
The table should be turned at the time of orientation and again should be returned to its original position after the orientation.
The principle of plane table survey is based on parallelism. In order to measure the land, two or more stations are plotted, and the lines are drawn through the station point.
The line connecting two stations or points is a straight line, say point X and Y, the lines joining them XY, and the angle between these lines is considered the plane's angle.
This plane surveying method proves to be successful upto an area of 250 square km, for example, in constructing dams and bridges.

Advantages of plane table surveying

It is one of the fastest methods of surveying.
This method is very much useful in measuring small land areas.
It does not require any high costed or expensive equipment, machines, or tools.
This method minimizes errors or mistakes as all the work is carried out in the field at the time when plotting and surveying are being done and thus increases the correctness of the plotted work.
There is no need for skilled or professional personnel to plot the map. Not much skill is required to create or prepare the map.
It is one of the best methods to be used in magnetic fluctuation where a compass survey fails.
Here, office work is just limited to drawing.
There is no need to measure the lines and angles directly as it can be determined from the graph work.
In this method, the need or requirement for more time and labour is reduced.
This method bears less cost as compared to the odolite survey.
Since the work is carried out in the field at the same time when plotted, the error of omitting the important object or important measurement is reduced.
As work is done simultaneously in the field, objects like trees or ponds can be easily shown in the plotting and surveying, which helps in the appropriate drawn map.

Disadvantages of plane table surveying

One of its cons is that setting the table properly and its orientation consumes a lot of time.
This method is not considered as useful or right in bad weather conditions or rough weather conditions.
The plane table surveying method is considered less accurate when compared with other surveying methods.
All the instruments needed to conduct the plane table survey are heavy, big, and different or unusual to carry.
This plane table surveying method is not suitable to be carried out in wet climates or areas where high winds dominate.
Due to fluctuations in temperature, the plane table survey warps.
At the time of extreme sunlight, the strain may not allow the architect to plot accurately and properly.
As work is done simultaneously in the field on the plane table, it cannot take place at night, considering light and other uneven situations. The work is only done in the daytime.
This plane table surveying method is improper for big or large-scale areas compared to the theodolite or compass survey method.
Since this survey method does not maintain its field notes, it is easy to replot the survey or re-calculate the quantities to a new scale when needed.
This method is very disadvantageous in the thick wood lands.
The method is unsuccessful and fails in the areas or locations where highly dense trees and forests affect the viewpoints.

Objectives of plane table surveying

The plane table surveying method is intended towards some objectives, and these are mentioned as follows:

This method is focused on carrying out the small scale survey easily and fastly.
This method works to draw the lines between the stations which are previously fixed by the methods.
The plane table surveying method looks more adequately at the topographic details of the location.
It generally serves the area or locations that the compass survey can not serve for any reason.
This method is suitable for preparing small-scale maps.

Precautions to be taken while doing a plane table survey

While doing a plane table survey, some points should be kept in mind. These are mentioned as follows:

The table should be properly oriented at each station; otherwise, new lines drawn from the station will not align with the previous ones.
Rays should be drawn with the help of a ruler to assure a straight line.
In order to maintain accuracy, check lines should be protected.
The centering of data for alidade should be done with utmost care.

Preparation of plane table

In order to make the table ready for the work, some sets of operations need to be performed. These are as follows:

Centering

The process of plotting the point on the paper exactly as represented on the ground is referred to as centering. Centering is done with the help of plumbing folk. One end of the plumbing folk is put on the paper, and the other is attached to a plumbing bob. And, the board moves continuously until the bob hangs properly over the peg of the station.

Leveling

The process of keeping the drawing board properly parallel to the ground. The following methods are used to bring the plane table survey method into use:

Arrangement of ball and socket.
Tilting of the board.
Adjusting the tripod's legs.

Orientation

The process of placing the board parallel at every station. As this method requires continuous movement of the table from one station to another, the table should be oriented properly. In order to represent plotted lines exactly as represented on the field, the new station needs to be placed parallel to the precious one. There exist mainly two methods of orientation, orientation by back sighting and orientation by the magnetic needle.

Orientation by back sighting- Here, the survey lines drawn from the previous station are again redrawn from a new one to superpose the line drawn from the new station on the previous one. This method is accurate and is mostly preferred.
Orientation by magnetic needle- This method comes into action when the previous line drawn from the new station is not feasible. The method is useful when there are chances of a local attraction.

The procedure of setting up a plane table

Follow these steps to set up a plane table :

Fix the table on the tripod stand- Keep the tripod stand over the needed station with its legs apart, and fix the table by a nut at the bottom.
Level the table- Place the spirit level at various positions and in all the corners of the table.
Center the table- Fix a drawing sheet on the table and plot a point P at the suitable location in order to represent station p at the ground. And place the pin on the plotted point.
Mark the north line- Place the trough compass on the top corner at the right side with its north hand pointing towards the north. Now turn the compass clockwise or anticlockwise so as to align the needle with the 0-0 mark. Draw a line from the edge of the compass to represent the north line.
Orientation- Orientation is an essential and important step to be performed at every station by either of the processes (magnetic needle method and back sighting method). At the time of orientation needs of the centering and leveling should not be altered and must be satisfied.

Tools required for a plane table survey

Plane table: Plane table is a point made of teak, pine, and other material. It comes in the dimensions of 750 mm and 600 mm. The table is needed to fix the drawing sheet and locate the needed requirements of the maps. Its top surface is levelled properly, and the bottom has a circular plate to be fixed on the tripod.
An alidade: Alidade is a sighting device in the form of a straight-edge ruler whose one edge is bevelled, and the other edge is used to draw the line of sight. There are basically two types of alidade, a plane alidade and a telescopic alidade:
- Plane alidade- It has a wooden or metal ruler of about 50 cm. One edge is known as the fiducial edge as it is bevelled. It has vanes at its ends. One of them is called an object vane.
- Telescopic alidade-Here, a telescope is placed on a column which is placed on a ruler. Telescope is put parallel to the bevelled edge for the line of sight. Telescope has a vertical graduation arc and a level tube.
Spirit level: The table can be levelled by placing it at right angles towards each other in two positions when the spirit level is sensitive and can not fit the alidade.
Trough compass: It is mounted on a brass plate generally of a square shape with two bubble tubes placed at right angles towards each other. This is done to indicate the magnetic meridian direction.
Plumbing folk: It is used to centre the table at a specific station by attaching plumbing folk along with a plumbing bob at one end. It is useful in large-scale fieldwork.
Waterproof cover: It protects the drawing sheet from rain, which is placed on the table.
Drawing sheet: It is the paper which is used to plot the survey and is put on the table. The quality of the paper should be of top material, as plotting and changes are hard to draw. It should be well seasoned about a week before its usage by introducing it into the dry environmental conditions, which decreases its chances of being distorted. This sheet should not be folded. It is made up of zinc and celluloid. It is fixed by drawing pins on the drawing board for its usage.
Tripod: It is one of the main instruments which is needed for conducting this plane table survey. It is used for fixing the drawing board. It has a screw at its bottom so as to adjust the drawing board accordingly. It is almost hard or close to possible to do a plane table survey without it.