METHOD OF PLANE TABLING

METHOD OF TABLING

  The following are the four methods of the plane tabling.

[1]. Radiation

[2]. Intersection

[3]. Traversing and

[4]. Resection.

Radiation: The method is suitable for locating the objects from a single section. In this method, rays are drawn from the station to the objects, and the distance from the station to the objects are measured and plotted to any suitable scale along the respective rays.

Procedure [a] Suppose P is a station on ground from where the objects A, B, C, and D are visible.

[b]. The plane table is set up over station P. a drawing sheet fixed on the table which is then levelled and centred. A point p selected on the sheet to represent the station P.

[c]. The north line is marked on right-hand top corner of the sheet with tough compass or circular box compass.

[d]. With the alidade touching p, the ranging rod at A, B, C, and D are bisected and rays drawn.

[e]. The distance PA, PB, PC, and PD are measured and plotted to any suitable scale to obtain the point a, b, c, and d, representing the objects A, B, C, and D (Fig. 10), on paper.

The Intersection method: The method is suitable for locating inaccessible points by intersection of the rays drawn from two instrument stations.

Procedure [a] Suppose A and B are two station and P is an object on the far bank of river. Now it is required to fix the position of P on the sheet by intersection of rays, drawn from A and B.

[b]. The plane table is set up at A. It is levelled and centred so that a point a on the sheet is just over station A. The north line is marked on the right-hand top corner. The table is then clamped.

[c]. With the alidade touching a, the object P and the ranging rod at B are bisected and rays are drawn through the fiducial edge of the alidade.

[d]. The distance AB is measured and plotted to any suitable scale to obtain the point b.

[e]. The table is shifted and centred over B and levelled properly. Now the alidade is placed along the line ba and orientation is done by back-sighting. At this time, it should be remembered that the centring, levelling and orientation must be perfect simultaneously. 

[f]. With the alidade touching b, the object P is bisected, and ray is drawn. Suppose the ray intersect the previous ray at p. This point p is the required plotted position of P (Fig. 11).

The Traversing method: The method is suitable for connecting the traverse stations. This is similar to compass traversing or theodolite traversing. But here fielding and plotting are done simultaneously with the help of radiation and intersection methods.

Procedure [a] Suppose A, B, C and D are the traverse stations.

[b]. The plane table is set up at station A. A suitable point a selected on the sheet in such a way that the whole area may be plotted in the sheet. The table is centred, levelled and clamped. The north line is marked on the right-hand top corner of the sheet.

[c]. With the alidade touching a, the ranging rod at B is bisected and a ray is drawn. The distance AB is measured and plotted to any suitable scale.

[d]. The table is shifted and centred over B. It is then levelled, oriented by back-sighting and clamped.

[e]. With the alidade touching b, the ranging rod at C is bisected and a ray is drawn. The distance BC is measured and plotted to the same scale.

[f]. The table is shifted and centred over C. It is then levelled, oriented by back-sighting and clamped.

[g]. In this manner, all stations of the traverse are connected.

[h]. At the end, the finishing point may not coincide with the starting point and may be some closing error. This error is adjusted graphically by ‘Bowditch’s rule’.

The resection method: The method is suitable for the establishing new stations at a place in order to locate missing details.

Procedure [a] Suppose it is required to establish a station at position P. Let us select two points A and B on the ground. The distance AB measured and plotted to any suitable scale. The line AB is known as the ‘base line’.

[b]. The plane table is set up at A. It is levelled and centred and oriented by bisecting the ranging rod at B. The table is then clamped.

[c]. With the alidade touching a, the ranging rod at P is bisected and a ray is drawn. Then a point P₁ is marked on this ray by estimating with the eye.

[d]. The table is shifted and centred in such a way P₁ is just over P. It is then oriented by back-sighting the ranging rod at A.

[e]. With the alidade touching b, the ranging rod at B. Suppose this ray intersects the previous ray at a point P. This point represents the position of the station P on the sheet. Then actual position of the station P is marked on the ground by U-fork and plumb bob (Fig. 13).

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