Plane table surveying is a graphical method of surveying in which field observations and plotting are done simultaneously, it is simpler and cheaper than theodolite surveying mostly suitable for small-scale surveys or medium-scale maps for filling details between two stations by theodolite traversing on triangulation.
The plan is prepared by the surveyor in the field, while the area being surveyed is before his eyes, so it is not possible to omit the necessary measurements.
Principle of plane table surveying:
The principle of a plane table survey is that the rays drawn from different points should move through the position of a single level i.e. station point.
And the position of the table at any station should be the same as that of the earlier station i.e. the table should be accurately oriented at the next subsequent stations.
Methods of plane table surveying:
This is a method of orienting the table, the purpose to plot the station occupied by the table on the table rather than to obtain other stations or plotting details.
The two methods of orienting the table are method of back sighting & method using trough compass.
This method is used to connect two or more stations, however, it is similar to compass traversing but is used simultaneously.
Traversing is a method of surveying where surveying is done by multiple lines, while traverse can be open or closed.
This method requires setting the table at a minimum of two stations, while orientation is necessary and can be done with rear sight.
Two stations A and B have been selected so that they command a full view of the area to be surveyed.
This method requires a plane table to occupy a single station.
In this method, orientation is not required.
To survey an area, the table is placed at the convenient station P, the area to be surveyed.
Parts of plane table:
A drawing board or table mounted on a tripod:
A drawing board or table is made of well-seasoned wood i.e. teak or cedar, the board size varies from 40 cm x 30 cm to 75 cm x 60 cm or 50 cm x 60 cm square.
It is mounted on a tripod stand in such a way that it can be leveled and rotated about the vertical axis.
Also, it can be held in any position.
A straight edge called an alidade:
Alidade is made of metal, probably made of brass or gunmetal.
The straight edge or ruler is about 50 cm long, while ruling edge of the alidade is called a fiducial edge.
Alidade is used to view objects, they may be of two types.
Simple or plain Alidade:
Simple alidade is fitted with sight vanes at both ends.
One of the vision vanes is supplied with a narrow slit and the other with a central hair.
The telescopic alidade primarily consists of a small telescope with a level tube and no vane at both ends with a graduated degree mounted on the horizontal axis.
The horizontal axis rests on a frame fitted with verniers fixed, which is positioned in the identical manner in transit.
All parts are supported by a heavy rule, one side of which is used as the working edge along which the line can be drawn.
The tilt of the line of sight can be read on the vertical circle.
Adjustments of Plane table surveying instruments:
Fixing the table:
The plane table or board should be fixed to the tripod stand at a convenient height for work, about 1 m.
The legs of the tripod must be effectively separated and fixed firmly within the ground.
The table should be placed above the ground at the station so that it is plotted on a sheet corresponding to the station located at the point and should be exactly above the station on the ground.
This operation is known as the centering.
The process of centering:
A) Place the pointed end of the higher leg of the fork coinciding with the point of the paper and suspend a plumb bob on the middle of the station peg.
B) If the suspended plume bob station is not exactly above the center of the peg, adjust the table until the plumb bob station hangs above the center of the peg.
C) In the absence of a plumbing fork, the center is completed by dropping a small piece of stone from the point on the underside of the table.
D) The accuracy required at the center depends on the scale of the map.
E) Only in large-scale work is the precise center required.
A) The plane table or board is leveled through the bubble tube or spirit level by placing it on the table or board at two angles.
B) The board is adjusted by the tripod legs until the bubble comes to the center and stays in the center in both directions.
C) If the table is provided with leveling screws or ball and the socket arrangement is fastened.
The method of inserting a table at each of the successive stations, which is parallel to the position occupied on the first station is called orientation.
Necessity of orientation:
A) It is necessary when the equipment is to be installed at more than one station.
B) If the orientation is not done properly, different meridians will be used in each of the successive stations that are incorrect.
C) If the orientation is done properly, the lines on the paper are parallel to the corresponding lines on the ground.
Methods of orientation:
There are two methods of orientation i) magnetic meridian method ii) rear sight method.
After centralization, leveling and orientation is done, the point to be located is seen through the alidade.
The degree of accuracy to be achieved in plane tabling depends on the character of the survey, the quality of the instrument, the system adopted, and the degree to which accuracy is deliberately sacrificed for speed.
Errors in plane table surveying:
The various sources of errors may be classified as follows:
Non-horizontality of the board:
When the difference in height between observed points is greater, the effect of non-horizontality of the board is more severe.
- The accuracy of plane table mapping work depends largely on the accuracy with which the points are sighted.
- A telescopic alidade is better than a plain alidade because, in the case of the telescopic alidade, the line of sight is fixed.
- The orientation performed with the magnetic meridian method is unreliable due to the local attraction probability.
- When there is an erroneous orientation it contributes towards the deformation of the survey.
To avoid such errors, orientation needs to be checked so far as possible, by sighting distant prominent objects already plotted.
Board movement between sightseeing places:
The table may be disturbed between any two sites due to the observer’s negligence, resulting in disturbances of orientation.
To reduce the possibility of such movement, the clamp must be applied firmly.
To avoid error due to movement of the board, it is always appropriate to check the orientation at the end of the observation from a station.
It is very important to have a proper conception of the extent of error initiated by incorrect centering, as this avoids unnecessary time wastage in setting the table by repeated testing.
Advantages of Plane Table Surveying:
- Plane table surveying is fast and suitable for small-scale maps.
- Since the fieldwork and plotting work together, the surveyor can compare the plotted work with the actual characteristics of the area.
- It is rarely necessary to record readings hence these recordings are not prone to mistakes.
- Not much skill is required to produce a satisfactory map.
- It is best suited for areas where compass surveys cannot be done due to local attractions.
- Check lines detect measurement errors and plot on-field by themselves.
- It is less expensive.
- Office work consists only of drawing finishing.
Disadvantages of Plane Table Surveying:
- The plane table is unsuitable for working in a wet climate and high wind.
- It is inconvenient to re-plot the survey on different scales and to calculate quantities when proper field notes are not available.
- The plane table is suitable for open countries only.
- It is not intended for greater accuracy.
- The instrument is heavy.
- There are various items to be carried and they may be lost.
- In strong sunlight, due to stretch on the eyes, it becomes difficult to plotting.
Uses of Plane Table Surveying:
- For surveys that do not require high precision.
- For small-scale mapping.
- Where plotting is done concurrently with fieldwork.
- It is particularly advantageous in magnetic fields where compass survey is not reliable.
- No great skill is required to produce a satisfactory map.
Also read: Chain Surveying, Tacheometry & Contour Interval
The plane table is also suitable for filling in detail on the map already prepared and available on the drawing sheet.
It can also be used to produce a new map, in which linear measurements are taken with chains or tapes.