After reading this article you will learn about: 1. Meaning of Plane Table Surveying 2. Advantages of Plane Table Surveying 3. Disadvantages 4. Equipment Needed 5. General Instructions 6. Errors 7. Testing and Adjusting.
Contents:
- Meaning of Plane Table Surveying
- Advantages of Plane Table Surveying
- Disadvantages of Plane Table Surveying
- Equipment Needed in Plane Table Surveying
- General Instructions while Plane Table Surveying
- Errors in Plane Table Surveying
- Testing and Adjusting the Plane Table Surveying
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.
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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:
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(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.
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(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.
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(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:
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(i) It is unsuitable for work in a wet climate, and is difficult in high wind.
(ii) It is not useful for large scale surveys and accurate work.
(iii) It is not suitable for surveying a densely wooded area.
(iv) The instrument is heavy and cumbersome and the various accessories, being loose, are likely to be lost.
(v) The absence of field-notes is sometimes inconvenient if the survey has to be replotted to a different scale.
(vi) 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.
(iii) A spirit level (if not fitted to the alidade) Fig, 6.1 (6) for levelling the plane table.
Besides these the other accessories are drawing sheet, a water proof cover to protect the drawing sheet and drawing instruments like pencils, rubber, scales etc.
Drawing Sheet or Paper:
For other than rough work, the paper used for plane tabling should be of the best quality, paper is very sensitive and liable to expand or contract due to changes in the humidity of the atmosphere. To lessen this effect, it should be well seasoned for about a week by exposing it alternately to a damp and dry atmosphere.
This effect is practically negligible if double mounted sheets (two sheets of paper pasted to a sheet of muslin in between) are used for the plotting work. For in damp climates, sheet of zinc and of celluloid may be used, and to reduce strain on the eyes, the paper many be tinted faint green or grey.
The paper may be fixed on the drawing board by means of drawing pins, spring-clips or screw-clamps. But the better way is to paste down the edges of the paper. The sheet should never be folded or rolled but should be carried fiat in a suitable cover.
5. General Instructions while Plane Table Surveying:
The following points should be kept in view while surveying with a plane table:
(i) The points A, B, C etc. on the ground should be denoted by the corresponding small letters a, b, c, etc. when plotted on the sheet.
(ii) The table should be turned only when it is to be oriented. After performing the orientation, it must be kept clamped in position.
(iii) The table should be kept clamped in position while the objects are sighted. Only the alidade should be moved on the table to bisect the objects.
(iv) While the sights are being taken, the fiducially edge of the alidade must be set touching the plotted station-point on the sheet and not the other edge.
(v) The lines should be drawn as fine as possible and with a good quality hard pencil.
(vi) The lines should be drawn close to the edge of the alidade.
(vii) Unnecessary complication of rays should be avoided. The rays and reference marks as soon as they can be dispensed with, should be rubbed off.
(viii) The drawing should be kept as clean as possible.
6. Errors in Plane Tabling Surveying:
The common sources of error may be classified as:
1. Instrumental errors.
2. Errors of manipulation and sighting.
3. Errors of Plotting.
1. Instrumental Errors:
(i) The surface of the board not being a perfect plane.
(ii) The fiducially edge of the alidade not being a straight line.
(iii) The sights of the alidade not being perpendicular to its base.
(iv) The fittings of table and tripod being loose.
(v) The defective trough compass.
All the above errors are adjustable and the effect of the residual errors of adjustment is quite negligible.
2. Errors of Manipulation and Sighting:
(i) The boar not being horizontal.
(ii) The table not being accurately centered.
(iii) The table not being correctly oriented.
(iv) The table not being properly clamped.
(v) The objects not being correctly sighted.
(vi) The alidade not being correctly centered on the station-pointing the paper.
(vii) The rays not being accurately drawn through the station point.
3. Errors of Plotting:
Errors of plotting are a common source of inaccuracy and can only be minimised;
(i) By using a good quality paper and stretching it properly on the board.
(ii) By constant care in drawing and in the use of scales.
7. Testing and Adjusting the Plane Table Surveying:
1. The Board:
(i) The upper surface of the board should be a perfect plane:
(a) Test:
Apply a straight edge in all directions.
(b) Adjustment:
If the surface is not perfectly plane, remove high parts by planning or sand papering.
(ii) The upper surface of the board should be perpendicular to the vertical axis of the instrument:
(a) Test set up and level the table:
Place a spirit level on the table and bring the bubble in the central position. Turn the table through 180 and see if the bubble remains central or not. Then place the level at 90 to its previous position, and repeat, if the bubble remains central on reversal in both the above cases, the adjustment is correct.
(b) Adjustment:
If not correct half the apparent error by inserting a packing or washer between the underside of the board and its support. Repeat the whole process until the bubble remains central after reversal in each case.
2. The Alidade:
(i) The ruling edge of the alidade should be a straight line:
(a) Test:
Select two points on the sheet at a distance approximately equal in length to the length of the alidade. Join these points by drawing a fine line along the edge. Reverse the alidade end for end and place it against the end points, and again draw a line. If the two lines coincide, the alidade is correct.
(b) Adjustment:
If not, correct the edge by repeated filing and testing.
(ii) The alidade spirit levels should have their axes parallel to the base of the alidade:
(a) Test:
Place the alidade on the table and bring the bubble of one of the levels of the alidade central by the foot-screws of the table. Mark the position of the alidade, lift and reverse it through 180 and replace it within the marks. If the bubble still remains in the centre of its run, the adjustment is correct.
(b) Adjustment:
If not, bring the bubble half way back by means of level tube adjusting screws and the remaining half by the foot-screws and repeat until the test is satisfied. Test and adjust the second level tube in the same way.
(iii) The sight vanes of the plain alidade should be perpendicular to the base of the alidade:
(a) Test:
Suspend a plumb line at a short distance from the instrument. Place the alidade on the levelled table and observe whether the sighting slit and vertical hair of the object vane appear parallel to the plumb line.
(b) Adjustment:
If not, adjust by filing or packing the base of the sights.
In the case of telescopic alidade, the conditions of adjustments are:
(i) The line of collimation should be perpendicular to the horizontal axis of the telescope.
(ii) The horizontal axis should be parallel to the base of the alidade.
(iii) The telescope level should be parallel to the line of sight.
(iv)The vertical circle should read zero when the line of sight is horizontal.