Site Selection, Survey and Treatment Plan - Afforestation, Reforestation and Forest Restoration in Arid and Semi-arid Tropics - page 55

and plotting is required), this method is therefore ideal for locating a large number
of details. The problem of ground distance being different from the true horizontal
distance does not crop up here because what one gets is the true horizontal distance
(assuming, of course, that the table has been levelled properly).
The principle involved in plane table surveying assumes that all the positions of
the table are parallel to each other, and the accuracy of results will depend upon
the truth of this assumption. It is therefore of utmost importance that the table is
oriented carefully by back sighting to the previous station accurately. To avoid errors
in bisecting objects, proper field notes are maintained to describe what each ray of
the previous station points to. For this, the rays are marked with small alphabets to
identify them, while the sighted objects may be marked with capital letters.
At the end of the traverse the sheet may look like a criss-cross web of rays. The
rays are usually rubbed off since these are merely means of arriving at the positions
of the points of interest. The remaining details are traced off the sheet on a clean
paper, and the trace may be photocopied to get plain paper copies of the site plan.

3.4 Tachymetric Survey

Traverse survey using a theodolite fitted with a stadia diaphragm can be much faster
method of survey. No distance needs to be measured since it can be read off a staff
kept at the point of sighting. All angular measurements are read on the horizontal
scales of the theodolite, giving much more accurate results. However, this method
requires more costly equipment and trained and skilled manpower which may not be
available or even desirable to hire.
Except for reading of angles and distances this method is identical with that of
chain-and-compass survey. The traverse is identified, stations are marked, instrument
is set up at each of these stations, inner details if desired can also be sighted and
included in the main traverse, and horizontal angles and stadia readings are noted in
a prescribed tabular format.
Tachymetric survey is also useful when it comes to surveying cross sections of a
stream or the longitudinal ground profile of a stream bed for locating and designing
earthen dams, stopdams, and other engineering structures. A dumpy level could also
be used for this purpose.

3.5 Advanced Automatic Surveying Equipment

With the advent of advanced electronics and microprocessor technology it is possible
to dispense with the need of any kind of direct measurement. It is enough to choose an
object to be surveyed, point at it, and click it. The software and the technology does
the rest. Printed maps with any degree of complexity (and with both horizontal and
vertical control) can be generated within minutes. Such equipment, however, is not