METHODS OF SURVEY:
USING PLANE TABLE AND ALIDADE SURVEY
1. 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.
2. A plane table is a
drawing board which is fixed to a tripod, leveled, and orientated. An
alidade is a sighting device used to observe the salient points of the
site.
3. 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, crack slots, changes in the stonework, changes in floor
covering, byre drains.
STEP 1:-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 all the 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.
STEP
2:- 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 orient 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 be taken. The pin marks the position of your survey
station on the board. Once the survey has commenced the plane table should not
be moved.
STEP
3:- 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 30mtape. 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 siteswithin a radius of 30m from the plane table.
