Plane table principles; advantages, disadvantages

Principles of plane table surveying

Plane tabling is a graphical surveying method in which fieldwork and plotting are done together on the spot, using a drawing sheet fixed on a board (plane table), an alidade for sighting and a tripod.

Principle: the table is set up so that the rays drawn on the sheet from a station point towards the objects are parallel to the corresponding lines on the ground. The plotted figure is then similar to the ground figure. This needs three things at every station:

1. Levelling - the board is kept horizontal (spirit level on the board).
2. Centring - the point on the sheet is kept vertically above the ground station (plumbing fork and plumb bob).
3. Orientation - the board is turned so that all lines on the sheet stay parallel to their ground lines (by magnetic needle or by back-sighting a previously plotted line).

Advantages and disadvantages

Advantages

• Fieldwork and plotting are done together, so omissions and mistakes are detected on the spot.
• Contours and details can be drawn directly by looking at the ground.
• Less field book work; no angular booking is needed.
• Not affected by local attraction when oriented by back-sighting.
• Quick and economical for small and medium-scale maps.
• Inaccessible points can be plotted by intersection and resection.
• Needs less skill than theodolite surveying and the instruments are simple.

Disadvantages

• Not suitable in rainy, windy, foggy weather or in thick forest and built-up areas where intervisibility is poor.
• Instruments are heavy and many accessories must be carried; difficult to transport.
• Less accurate than a theodolite survey for large areas; accuracy depends on drawing quality.
• The whole work depends on the plan only; a plan cannot be re-drawn to a different scale from field data as there are no field notes.
• Paper may shrink or expand with weather.
• Plotting is done in the field, so work stops in bad weather, and an unskilled person may make a poor drawing.