It is unsuitable for work in a wet climate, and is difficult in high wind.Advantages & Disadvantages of Plane Table Survey
1. Meaning of Plane Table Surveying:
Plane table Surveying also called plane tabling, is a method of surveying in which field-work and office work are done simultaneously on a plane table. The field observations are taken and recorded side by side on the sheet fixed upon the plane table and a map of the area is obtained. The use of field-book is altogether eliminated.
It is most suitable for the survey of details between stations which have already been fixed by theodolite or any other accurate method of surveying. It is commonly employed for small and medium scale mapping of comparatively large areas where great accuracy is not the main consideration such as for topographical surveys.
The equipment essentially needed for plane tabling is a plane table or drawing board which carries a drawing sheet and is mounted on a tripod stand and an alidade which provides line of sight and a straight graduated edge.
2. Advantages of Plane Table Surveying:
Some of the advantages of plane table surveying are:
(i) It is one of the most rapid method of surveying.
(ii) Field-notes are not required, and thus the possibility of mistakes in booking is eliminated.
(iii) Measuring of lines and angles is mostly dispensed with since they are obtained graphically.
(iv) Since the map is plotted in the field, there is no chance of omitting necessary measurement.
(v) The surveyor is fully confident about the true representation of the area since he can always compare his work with the actual features on the ground and cannot, therefore, over-look any essential detail.
(vi)The surveyor can check the accuracy of his work more frequently and from any position he may desire, thus eliminating all error at the spot.
(vii) It is particularly suitable for filling in details in hilly areas and in magnetic areas where chain and compass surveys are not suitable.
(viii) Contours and other irregular objects may be accurately represented on the map since the tract is in view.
(ix) It is less costly than theodolite survey.
(x) No great skill is required in making a satisfactory map and the work can be entrusted even to a subordinate.
3. Disadvantages of Plane Table Surveying:
Some of the disadvantages of plane table surveying are:
It is unsuitable for work in a wet climate, and is difficult in high wind.
It is not useful for large scale surveys and accurate work.
It is not suitable for surveying a densely wooded area.
The instrument is heavy and cumbersome and the various accessories, being loose, are likely to be lost.
The absence of field-notes is sometimes inconvenient if the survey has to be replotted to a different scale.
Only day time can be availed of for the field and plotting work whereas in other methods of surveying, day time can be used for field- work and night or even hot and rainy days can be utilised for plotting.
4. Equipment Needed in Plane Table Surveying:
The equipment needed in plane table surveying is dealt with under different heads as follows (Fig. 6.1):
1. Drawing board and tripod.
2. Alidade.
3. Accessories.
1. The Drawing Board and Tripod:
The drawing board on which the field measurements are plotted is fixed on the plane table or drawing board [Fig. 6.1 (1)].
The board is made of well-seasoned good quality wood such as teak or pine and is available in sizes:
(i) Small: 50 cm 40 cm x 1.5 cm.
(ii) Medium : 50 cm x 50 cm, and
(iii) Large: 70 cm x 60 cm x 1.5 cm.
The upper surface of the board must be perfectly plane. It is mounted on a tripod in such a way that it can be levelled, rotated about a vertical axis and clamped in any position. The tripod is generally of the open frame type [Fig. 6.1(2)] and can be folded for convenience of transportation.
The board is sometimes fitted with a levelling head or a ball and socket arrangement for the purpose of levelling the table:
2. The Alidade:
The alidade is a straight rule fitted with such an arrangement as to provide a line of sight. It is used for sighting objects and drawing rays along one of its bevelled edges known as ruling or working or fiducially edge. The fiducially edge is graduated so as to serve as a scale for plotting distances.
It is available in two forms:
(i) Plain alidade, and
(ii) Telescopic alidade.
(i) Plain Alidade (Fig. 6.2):
It consists of a wooden or brass ruler 50 to 75cm long, or of a length equal to the smaller dimension of the board and is provided with a pair of vertical sight-vanes at its ends. One of the vanes is provided with a narrow slit, while the other is open and carries a horse-hair or fine-wire.
A small circular bubble is sometimes attached to the alidade for levelling the table. The alidade with plain vanes provides a sufficiently definite line of sight for many purposes, but is somewhat inconvenient for work in hilly country owing to the limited range of inclination possible in the line of sight.
The highly inclined sights can be taken by stretching a thread tightly between the centres of the tops of the vanes and sighting the object through the eye-slit and this thread.
(ii) Telescopic Alidade (Fig. 6.3):
In this type of alidade, a telescope is fitted on the straight rule which increases considerably the accuracy and the range of sighting. The telescope is provided with a vertical circle to measure the angles in the vertical plane and stadia wires to measure horizontal distances. Thus the telescopic alidade serves the purpose of a small theodolite. It is commonly used when the sights are long and much inclined.
3. Accessories:
The accessories to the plane table are:
(i) A trough compass. Fig. 6.1. (4). It is used for marking the direction of the magnetic meridian on the sheet of the plane table.
(ii) A Plumbing fork or U-frame Fig 6.1. (5) with a plumb-bob Fig. 6 1, (7) is used for centering the table.