# Toposheet

To study tools of toposheet

Aim: To study the tools of toposheet
Requirements: Toposheet maps of different states.
Procedure: Look at the toposheet map & study the different aspects related to the different symbols indicating different landmarks. For
e.g. Roads, bridges, streams, dams, river banks, wells, embankments, railways, contours sand features, towns or villages, huts, temples,
palms, heights( triangulated), bench marks, post office, telegraph office, combined office, police station etc.
Introduction
Maps are representation of features on the surface of the earth drawn to scale. It is a tool of communication & in use right from the days
of primitive man who was required to move about constantly in search of food & shelter. He resorted to some method of recording his
surroundings & used symbology to depict various features relevant to him. This resulted in the idea of map & the symbols as means of
expression.

The tools of toposheet is shown below

Index to sheet

The grilled portion shown in the center of the adjoinin g sheet is the toposheet map which is in current
use by the observer. If all the sheets are arranged in the order according to the index shown in the
adjoining sheets, then the area can be studied on a larger scale.

Map Scales:
Maps are prepared correct to the scale. Scale is expressed as Representative
Fraction, abbreviated as R.F. written as 1/ 25,000 or in the form of ratio 1:
25,000.It means that 1metre on paper is equal to 25,000 metres on the ground.

Mean Sea Level:

One important information which we can get from a map is height of any point on
the ground. For small areas it may be enough to assume an arbitrary reference
surface form which these heights can be measured. But for large areas, the
reference surface has to be unique, should be natural & can always be re-
established so that heights of points measured with respect to it are compatible to
each other. Mean Sea level is the datum surface form which all heights are
measured. It is the mean level of the sea obtained by taking the average of all the
heights of the tides measured at pre- decided intervals over a long period covering
whole number of complete tides.
For India, 9 Sea Ports of Karachi, Bombay, Karwar, Beypore, Cochin,
Nagappattinam, Madras, and Visakhapatnam & False Point were selected along its
sea coast where tidal gauges were installed for taking measurements of the sea water level. Water level at these nine ports were
continuously recorded for  19 year & an
average level was  calculated.
This average level has  been accepted
as Mean Sea Level or  Zero Level for
India & all heights given  in the map are
with respect to this Zero  Level.
However at present there  are 23 tidal
observatories in India.

salem toposheet