Agriculture Reference
In-Depth Information
ground surface, if the relief is not too marked. In a hilly terrain the ground distance will
have too wide a variation with the actual horizontal distance, and the traverse will not
close (except by mistake or by chance). In such cases, plumb lines should be dropped
and long distances should be broken into step-like pattern to determine the correct
horizontal distance. Distances should be noted in the field topic immediately. The sur-
veyor should not rely upon memory and think of noting a couple of distances together.
Back-bearings should not be considered extra or redundant work. They ensure
that the surveyor does not have to go back to the point where he began and redo the
whole work. It is useful to include as many checks and redundancies as possible in
order to eliminate chances of mistakes, gremlins, and blunders entering in the survey
work. However, back-bearings need not be measured twice where these agree with
the fore-bearing.
It is very important to mark every station with some permanent or semi-permanent
artefact such as a piece of stone slab, monolith, wooden peg, or masonry pillar. Even
if excellent survey work has been done, and the area has been estimated accurately,
it serves little purpose if the survey stations cannot be located when one comes to
the site 3 months later to start fencing work. In such a case, the entire exercise of
surveying will have to be re-done and the treatment plan will have to be revised to
make it agree with the new survey.
The position of at least two of the survey stations should be determined by record-
ing their bearings and distances with respect to a permanent reference mark having
unique identity so that it would be possible to reach and spot it in field just on basis of
a verbal description. A permanent reference mark could, for instance, be the corner
of a school compound wall, a large and isolated tree, or a water well. If it is possible
to have a reference point that is available on the general topographical (GT) sheet of
the area it will be even better. Alternatively, the latitude-longitude of the point can
be recorded with the help of a global positioning system (GPS) handset.
In case of a hilly terrain, it is desirable to survey the hillocks falling within the
plantation area using sub-traverses. The foothill demarcation line for the different
hillocks has to be somewhat arbitrary, though criteria such as soil depth and slope may
be used to demarcate the hilly from the flat terrain. Such an exercise does entail a little
extra work, but it enables more accuracy in preparation of the treatment plan later.
Map of the area should be plotted on a standard scale. A scale of 1 cm to 50 m is
appropriate for most occasions. If any sub-traverses have been surveyed these will
also be plotted. While plotting distances and bearings the traverse will seldom, if
ever, close without error. A closing error of less than 1% of the perimeter of the
area can be considered insignificant and the traverse can be closed by choosing the
mid-point of the error-line as the first and last station. In this process two distances
and two angles, as observed, will have to be modified. However, for more accurate
work proper error-closure algorithm such as the Bowditch rule should be followed.
The area of the traverse can be determined using a graph sheet or by dividing
the figure into triangles and measuring the bases and heights of these triangles. The
area can also be calculated using the formula for generic polygon after the Cartesian
co-ordinates of the stations have been read off a graph. It is always better to calculate
the area by two independent methods, so as to eliminate chances of a major error.
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