Plotting means to represent on paper, to a suitable scale, the previously surveyed objects in accordance with their shape and size. Plotting is commenced after the field-work is over.
The plotting of a chain survey is done in the following steps:
(i) A suitable scale is chosen before starting the plotting work. The scale depends upon the importance of the work and extent of survey.
(ii) Leave a suitable margin (2 cm to 4 cm) all round the paper.
(iii) Select a suitable position of the base line so that the map or plan is shown to the best advantages. The base line should be plotted as accurately as possible because the entire accuracy of the frame-work depends upon it.
(iv) Mark the intermediate stations on the base line and complete the frame-work of triangles.
(v) Check the accuracy of the plotted frame work by means of check and tie lines. If the error is within the permissible value, then adjust the lengths of the sides of the wrong triangles. But if the error exceeds the permissible limits, then resurvey the wrong lines.
(vi) For plotting the offsets, mark the changes of the points along the chain line from where offsets were measured and then draw the perpendicular lines with set squares and scale off lengths of the offsets.
The method of plotting the offset is much simplified if offset-scale (Fig. 3.19) is used for plotting them. In this method, the long scale is placed along the chain line with its zero exactly at start of the line. The offset scale is then placed at right angles to the long scale and is then moved along it to the required changes and the offset lengths are marked with a pricked.
(vii) While plotting keep the field-book side by side in the same direction as the work proceeded in the field parallel to the chain line to be plotted and then plot the various offsets. After plotting one line completely, transfer the offset scale along the second line and open the filed-book page for that line, keep it in the same direction and plot the off-sets. Similarly plot all the lines and details and complete the plan.
(viii) After completing the plan in pencil and checking it, ink the lines and objects and then colour them in according with the conventional signs. The inking and colouring should be commenced from the top left hand corner of the sheet working from left to right and downwards.
(ix) Print the title of the survey in right hand corner at the bottom or at the top of the drawing and then draw the scale of the plan below it.
(x) Mark the north direction in any convenient blank space near the top.
Equipment required for Plotting:
(i) A drawing-board.
(ii) A tee-square.
(iii) Set squares.
(iv) A protractor.
(v) A drawing instrument box.
(vi) A rolling parallel ruler for drawing parallel lines.
(vii) A set of French curves for drawing irregular and curved figures.
(viii) A set of metric scales.
(ix) A set of offset scales which are 5 cm long and are divided in exactly the same way as the long scales. They are much useful for plotting the offsets.
(x) Drawing-paper of good-quality of required size.
(xi) Pencils of grade 2H, 3H or 4H.
(xii) Ink and colours of the required shades.
(xiii) Sundries such as rubber, brushes, drawing-pins, weights, sandpaper, knife etc.