Plane Table Surveying with Methods and Examples | Advantages & Disadvantages
What is Plane Surveying?
Plane surveying is a type of surveying in which distances are measured between two points on a flat surface, such as a plane or a sheet of paper, by means of a surveyor’s chain or tape
This method is used to establish horizontal control networks and to determine the shapes of objects on the ground.
Plane tabling is typically used for surveys where great accuracy is not required. It is mostly used for small-scale or medium-scale mapping.
Plane table surveying is a quick way of surveying. Plotting of the plan and field observations can be done concurrently in this sort of surveying.
Geometrical conditions of the site are Manu scripted on the map sheet using a plane table and alidade, and then topographic features are put on the map in the case of plane table surveying.
Equipment Used in Plane Table Survey
The plane table is a survey apparatus that resembles a drafting table. It is used in plane table surveying.
The table has two legs with cross-pieces at the top, which hold a wooden tabletop horizontal when set on a level surface.
There are means for attaching the alidade and tripod at the center of this tabletop. The instrument whose use obviates such a high degree of accuracy as must otherwise be attained is known as the plane-table or camping sextant.
Alidade for sighting (telescopic or simple)
A device for sighting along the ruler of the transit. It is an instrument constructed so that it can be attached to a transit to enable its use in finding distance to objects at unknown elevations.
It is used in plane table surveying. The alidade enables the longitudinal lines of sight to be accurately measured by means of its vertical rod, which is slid over the horizontal section of the ruler.
It is a metal device used for ascertaining whether an object is horizontal. It is common in plane table surveying.
Plumb bob and plumb fork
A device used to indicate vertical direction. It is fixed to a point on the plate and is swung up and down when the geometric conditions of the site change, so that it remains upright. The plumb bob is a cylindrical device made of wood.
It is a heavy chain made of metal links. It is used to measure the distance between two points.
It is a magnetic instrument used for determining direction. It can be used both in-plane table surveying and in transit surveying as well.
The compass needle aligns itself with the earth’s magnetic field, which thus determines the bearing of an object, point or line of sight relative to the earth’s north-south axis. Compass is used in plane table surveying and transit surveying.
Alidade and chain are fixed to the compass by a chain, which enables the compass needle to be moved around its circle.
Thus, it is possible to make measurements over a very large area. The compass has to be used in plane table surveying as well as in transit surveying.
Drawing sheet and drawing tools
Every field work has to be done on a drawing sheet. It is a large rectangular paper of tracing paper. The drawing tools are all the instruments used to extract precise measurements, as well as to prepare plans, sections and sectional elevations, and finished drawings.
The drawing tool consists of several instruments that help in the accurate depiction of objects on the paper. They are used in plane table surveying just as they are used in transit surveying.
The ranging rods are instruments consisting of a solid rod and a plumb bob that is attached at one end. When the rod is held upright, the plumb bob falls down vertically. The rods are used to measure distances between points.
A metal tripod used for setting up the alidade and other instruments in order to survey the ground more accurately. It is used in plane table surveying.
Paper clips or screws
They are small fasteners that are used for attaching the alidade to the plane table.
Trek-Sight (a telescope attachment for the plane table)
It is a portable instrument consisting of telescopic sight and a sight scale.
It is used to locate points on the ground at known elevations.
In plane table surveying, it is used to survey areas with low relief where the topography is flat or only slightly ascending, i.e., in glaciated regions, both in winter and summer.
The equipment setting for plane table survey:
The table’s setup consists of three operations:
(1) leveling the table;
The plane table should be set up at a comfortable height (almost a meter) by spreading the legs to maintain the table roughly leveled above the specific station.
If the instrument has a ball and socket configuration, the leveling is performed by using leveling screws (if provided) or by tilting the board by hand.
On the table, a level tube or circular level is put parallel to two screws and then over the third screw.
If no leveling or ball-and-socket system is supplied, the leveling can be done by adjusting the legs.
(2) leveling the instrument;
Orientation is defined as the process of keeping the table parallel to the position held by the table at the first station at each consecutive station.
If the table is aligned, all of the lines on the paper will be parallel to the corresponding lines on the ground. When there are several instrument stations, this process becomes essential.
If the board is not oriented, it will not be parallel to itself at various instrument stations, and the plan will be derived by using a different meridian at each station, resulting in the relative projected locations of various points in the region being significantly different from the real ones.
The table can be oriented in two ways:
- By rear sighting and
- Using a magnetic needle
- Leveling the tripod.
In a basic table survey, centering is a necessary step. The term “centering” refers to positioning the table vertically on the ground.
To conduct this operation, align the pointed end of the fork’s upper leg with the point on the paper, suspend a plumb-bob from the lower leg, then adjust the table body until the fork’s plumb-bob hangs precisely over the center of the station peg.
All operations are performed simultaneously, and any two operations can be done after completing the third one. Before starting plane table surveying, care should be taken to ensure that the instrument is mounted on a level surface, otherwise data reading may be inaccurate.
Methods of plane table surveying
Re- section method
Resection is a plane table surveying approach in which the plane table’s position is unknown and is found by sighting it to known or charted points. It is also known as the orientation method, and it may be carried out under two different field circumstances, as shown below.
- The three-point issue
- The two-point issue
- The Three-Point Issue
Three points and their locations in the field are known under this circumstance. The plane table is positioned in such a way that all three points are visible. So, by seeing those three spots, we may find the location of the equipment.
This can be accomplished in a variety of ways, including the ones listed below.
- Method of tracing
In the tracing method, the plane table is located at a point from where three points are visible. The table is oriented with respect to the plotted lines of those three points.
Place the tracing paper on the drawing sheet and again sight the three points and plot the radiating lines.
The tracing paper is then moved above the drawing sheet until the three radiating lines pass through corresponding points previously plotted on the map. Finally, the position of the plane table is marked.
- The Lehmann technique
In this procedure, a plane table is placed at a point P, and the stations A, B, and C are seen, and the rays Aa, Bb, and Cc are plotted.
The rays create a tiny triangle known as the triangle of mistake. Another point P1 is picked to lessen the mistake and sight the point A from P1 similarly to B and C, resulting in another error triangle. Rep this method till the error is zero.
- Methods of analysis
Many analytical approaches have been developed in the context of a three-point issue. This approach involves seeing A, B, and C from station P and noting the values of angles and lengths. Analytical equations are used to calculate the location of unknown points based on these variables.
- Method using graphs
Angles and lengths are also obtained and shown on a graph in the graphical approach, which determines the placement of the plane table.
- The two-point issue
Under this circumstance, only two points are visible from the plane table. Therefore, three limitations will be reflected in locating the plane table:
- The error triangle
- The circle of error
- The point of intersection
When two points are visible, the triangle of error is expressed in the form of three angles.
The circle of error is a circle centered on one station and passing through the other two. The location of the plane table is found to be at where the circle intersects with the triangle. In addition, a third point should be visible to further determine the position of plane table.
This is similar to the compass survey. It is used to find topographic features by drawing survey lines between stations that have already been set by other methods of survey.
The plane table is fastened in place (say A)
From there, a line of sight is drawn toward B, and the distance AB is measured.
The plane table is moved to station B and aimed at A. (this is called back sighting). BA’s distance was measured.
On the drawing paper, the average distance between AB and BA is plotted to an appropriate scale.
The distance was estimated after observing point C from point B. This procedure is done for each station.
Perform some checks at regular intervals. Finally, on the drawing page, draw the traverse lines. Back sighting was only performed on the first two stations.
Radiation method of plane table survey
The plane table is set up at only one station.
In this type of plain table survey, and numerous points are identified by radiating (drawing) a ray from the instrument station to each of the locations, and plotting to scale along the ray the distance measured from the station to the point is sighted.
The radiation approach is appropriate for surveys of limited regions that are likely to be controlled by a single station.
It is beneficial in large-scale projects when combined with other strategies.
If the distances are acquired tacheometrically with the use of telescopic alidade (equipped with stadia-hairs), the task may be completed very quickly, and therefore the approach has a broader reach.
Intersection or triangulation method of plane table surveying
This approach is commonly used for plotting information on maps. It may also be used to plot the location of points that will be utilized at later stations.
The intersection of rays taken from two separate stations (A and B) establishing a baseline may be used to find the various spots.
The only linear measurement necessary is that of the ground baseline AB.
The approach may also be used to locate far and inaccessible items, rivers, in a survey of mountainous terrain (where distances are difficult to measure), and to inspect the remote things.
Errors in Plane Tabling Surveying
- Instrumental flaws
(i) The board’s surface is not perfectly flat.
(ii) The alidade’s fiduciary edge is not a straight line.
(ii) The alidade’s fiduciary edge is not a straight line.
- Manipulation and sighting mistakes.
(i) The alidade’s fiduciary edge is not a straight line.
(ii) The table is not properly centered.
(iii) The table is not properly arranged.
- Plotting mistakes.
(I)By utilizing high-quality paper and appropriately stretching it on the board.
(ii) By using vigilant caution when sketching and using scales.
Advantages of Plane Table Surveying
- Accuracy in the short run.
- Low instrument cost due to easy use and working of alidade.
- Multiple surveys may be done in a short period of time.
- It is precise in the determination of distances due to the nature of optical process involved.
- Suitable for surveying of small areas.
- Shortens the time by eliminating correction process.
Disadvantages of Plane Table Surveying
- The machine has limited field of view; therefore, it cannot survey large areas.
- Long periods need to be spent in reading and studying the desired items of information for being able to extrapolate it accurately on a map after collecting all necessary data from the field (i.e., plane table surveying is time-consuming and laborious).
- The plane table must be transported to areas where surveys are conducted.
- It is weak in the long-range survey, because of optical factors such as atmospheric refraction and distortion.
- It requires the surveyor to be exceptionally well-trained and skilled.
- It is difficult to use in mountainous terrain.
- Difficulties are experienced while surveying under very poor weather conditions such as rainstorms, snowstorms etc., where seeing the ground through a partly cloudy sky becomes extremely difficult and limited by the errors of error triangles and circles of error; thus, not giving us much confidence in estimating accurate survey points.
What is Plane Table Survey?
Plane table survey is a method of surveying small areas by using a plane table.
How is Plane Table Survey Done?
The plane table is used to intersect the plane of the horizon. The plane table uses two stations.
For performing plane table survey, the two stations are required for which two sets of distances are measured and plotted on the map, one between these two sets of distances.
If a line can be drawn from one station to another; this procedure is called back sighting.
From these two sets of distances, the sum of their lengths will give the total distance between them. This method is used for plotting the distance from one station to another.
Why is Plane Table Survey Used in Surveying?
This method is used to survey large areas and also to mark points that may be later used as a starting point for further surveys. Plane table survey allows surveying small areas quickly and accurately. It gives results of up to about 90 percent accuracy in surveying a relatively small area.
Which instrument is not used in plane table survey?
A compass is used to determine magnetic north. Theodolite is not employed in this situation since the entire operation is performed manually without the use of any instruments.
How many types of plane table surveying are there?
Plane table surveying can be done in four ways: radiation, intersection or triangulation, traversing, and resection.
What is the difference between plane table surveying and the use of a total station?
The main differences between plane table surveying and the use of a total station are as follow:
The plane table is an optical instrument, while a total station is an electronic-optical instrument that can determine its position by means of satellites/GPS.
Can two or three points be visible simultaneously?
Yes. The point may be visible at different times, say A, B and C.
The surveyor can look at A and B first and determine the position of C with the help of a plane table.
Can a plane table survey be done horizontally?
Yes, it can be done horizontally too (using an angle plate).
How is the accuracy of plane table surveying?
As mentioned before, this type of surveying is not accurate in measuring large distances.
A plane table survey has an accuracy of 2-3%.
The accuracy may be enhanced if a marker is used.
The accuracy of plane table surveying is 2-3% in long distance and 2-3% in short distances.
What is the speed of plane table surveying?
It varies from one field to another, but in general it can be said that about 150 feet per minute under normal weather conditions.
How to store plane table data?
The place of intersection of the lines is generally known to be accurate within 1-2%.
The distance from A to B is generally method and sometimes made by direct reading with the aid of a horizontal scale, so it is not more than 3-4% error.
If a traverse is made with an alidade, and cross-hair sights are used, the error does not exceed 0.1%.
The time required to make a survey is 15 minutes for 150 feet and 40 minutes for 30 feet.
The vertical distance measurement requires about 30 minutes for 150 feet and about 60 minutes for 30 feet.
Are there any problems in using plane table surveying?
Yes, there are several problems that can occur in surveys by plane table surveying. The main problems are as follow:
- Non-homogeneity of the ground over a large area.
- Poor or no light; or clouds may affect the sighting accuracy.
- Dangers due to excessive wind, rain or snow fall, etc.
Where are plane table surveys used?
The plane table is used mainly in areas where there is little level ground, or where it is too difficult to stake out, although it can be used in large areas as well.
A plane table survey may be carried out using a total station in extremely mountainous terrain, so that its survey can be made much more accurate.
The machine has greater accuracy than the human eye and an alidade may be used with it to obtain a better survey of small areas.
What is the simplest way of using plane table?
The simplest way of using plane table is to make a sketch on the board, then by noting down the dimensions and angles found in it, map out the area concerned.
What are some limitations of plane table?
- The use of a plane table is not suitable for surveys in mountainous areas.
- The surveyor must be very skillful and trained to use it properly.
- It gives only relative measurements; thus, this method is not sufficient for determining accurate distances.