Site Selection, Survey and Treatment Plan - Afforestation, Reforestation and Forest Restoration in Arid and Semi-arid Tropics - page 54

Particulars such as the name of the site, area and perimeter estimated, date of
survey, name of surveyor, instrument used, and other relevant data should be recorded
on the map. It is also important to plot the permanent reference mark on the plan. If
the reference mark is too far away to fall on the sheet, the direction and bearings can
be marked, and distance can be shown using ellipsis and a figure. An unambiguous
verbal description of the reference point should also be recorded on the plan.
The site plan should be traced on butter paper and multiple copies should be
made for use in marking the treatment plan. An accurate and correct site plan is of
critical importance in arriving at a correct treatment plan which is the blueprint of
the plantation work.

3.3 Plane Table Survey

For greater accuracy and a more complex site involving many field features to be
marked on map, plane table survey is more convenient. The equipment used in
plane table surveying consists of a flat-top table mounted on a swivel base so that
the table can be turned about both the horizontal and the vertical axes. A drawing
sheet is fixed on this table with the help of thumb tacks. A sighting device (allied
alidade), consisting of a metal ruler with bevelled edge and two metal frames hinged
at ends, is used for sighting and marking field features and other points. Plane-table
surveying applies the principle of geometrical similarity to locate points on sheet,
so that no distance measurements need be made except the distances between the
traverse stations.
The plane table is set up at each of the traverse stations. To set up the table at a
traverse station it is necessary to centre it, that is, to bring the ground point exactly
under the point representing the station on the sheet of paper. A U-fork with a plumb
line is used for transferring the ground point (the instrument station) to the sheet. The
table is then made horizontal with the help of a spirit level, and clamped securely
thereafter.
A pin is stuck into the board at the point representing the instrument station. To
locate a ground object the alidade is kept touching this pin and swung to obtain the
object exactly in alignment with the sighting vane and hairline (the hairline is then
said to bisect the object). After bisecting the object, a ray is drawn along the edge of
the alidade. This process is repeated for all the objects to be surveyed, including the
next traverse station.
The instrument is then shifted to the next traverse station. The distance between
the two traverse points is measured with a tape and scaled off to mark the next
traverse station on the sheet. The table is centred on this station and then oriented
by back-sighting to the last traverse station along the ray already drawn. The pin is
shifted to the new station point on the sheet and keeping the alidade touching the
pin, the same objects are bisected again. The intersection of the bisection rays to an
object gives the location of the object on the sheet. Since any number of rays can be
drawn from a given station without distance or angle measurement (also, no scaling