Plane Table

plane table photo

Measured survey using plane table and alidade

This is a method of survey that allows a surveyor to plot and create an accurate scale drawing on site. The idea is to scale down the site to a size that fits onto the drawing board. A plane table is a drawing board which is fixed to a tripod, levelled, and orientated. An alidade is a sighting device used to observe the salient points of the site.

Have a good look at the site before you start the survey. Time should be taken to walk round and have a good look at the site or building that you are going to record. On a building, the obvious features that you would want to record are the corners, door and window openings, fireplaces. More subtle features that you would look out for are changes in wall direction, straight joints, blocked doors or windows, cruck slots, changes in the stonework, changes in floor covering, byre drains.

Select a survey station

Once you have had a good look at the site, you will select a position for your survey station from which you have a line of site to as many of the main points as possible. It is particularly important to be able to see corners and places where a feature has a change in direction. Set up the tripod legs over your selected survey station. Prepare the drawing board A piece of polyester drafting film is taped onto the drawing board with masking tape. The corners are taped first, with the film pulled tight, and then the edges are taped. The board is fixed onto the tripod.

Orient the drawing board

When drawing a building you would orientate the long side of the board so that it is parallel to the long axis of the building. If possible, for a site plan, you would oriented the board so that north is up the page. For all plans it is important to check that the whole site is going to fit on the board at your chosen scale. A pin is positioned into the board to give a fixed point from which observations and measurements can betaken. The pin marks the position of your survey station on the board. Once the survey has commenced the plane table should not be moved.

Carry out the survey

The alidade is placed against the pin and used to observe in turn, selected points. Rays are drawn on the film and measurements between the pin and the points taken with the 30m tape. The measurement is written onto the drawn ray as a record. Subsequently, each point is plotted along the ray at the selected scale, ' with a scale rule. Observations and measurements are taken to as many points as required to allow the shape to be recorded. These plotted points create an accurate framework around which the rest of the plan can be constructed. This method is most useful for plans of single buildings or small sites within a radius of 30m from the plane table.

Larger sites

For larger sites multiple stations will be required. The second station must be plotted accurately onto the plan from the first station and marked on the ground. It must lie within 30m of the first station.

Equipment required for a plane table and alidade survey:

Plane table, tripod legs, alidade, spirit level, 30m tape, polyester drafting film, hi-polymer 6H pencil, masking tape, pin, scale ruler, compass, ranging rod for sighting, plumb bob.

photo of plane table

diagram

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diagram

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