Contour lines on a topographical map represent imaginary lines connecting points of equal elevation above or below a datum. The vertical distance between contour lines is called the contour interval. Index contours are drawn with a heavier line every fifth contour to aid identification of elevations. Intermediate contours fall between index contours. Contour lines can be marked in the field using a homemade A-frame leveling device to identify points of equal height and indicate slope. The spacing of contour lines depends on the steepness of the slope, with closer lines used for steeper slopes to prevent soil erosion.

3.  A CONTOUR LINE is a line
representing an imaginary
line on the ground along
which all points are at the
same elevation.
 Contour lines indicate a
vertical distance above or
below a datum plane.
Starting at sea level, normally
the zero contour, each
contour line represents an
elevation above sea level

4.  CONTOUR INTERVAL:-
It is the vertical distance
between adjacent contour
lines.
 INDEX CONTOURS :-
 The amount of the contour
interval is given in the marginal
information. On most maps, the
contour lines are printed in
brown. Starting at zero elevation,
every fifth contour line is drawn
with a heavier line. These are
known as INDEX CONTOURS.

5.  INTERMEDIATE
CONTOURS:-
Someplace along each index
contour, the line is broken and
its elevation is given. The
contour lines falling between
index contours are called
INTERMEDIATE CONTOURS

6.  In Topographical map
 In Hilly Region for Farming Purpose
 To Avoid Soil Erosion on the slope
 To Stop the flow of Water from the slope
so that it can be infiltrate

7. You can mark contour lines out with a home made
A-frame.
1. MAKING AN A-FRAME
 2 pieces of wood, approximately 5cm x 2.5cm and
2m long
 1 piece of wood, 1m long
 3 nails or screws, 5cm long (if not available, you
can use string instead)
 a short length of string
 a weight (for example a stone)

8. 1. Lay the two longer pieces of
wood on the ground, with
their ends touching and the
other ends 2m away from
each other. Fasten the 2
pieces together at one end
with a nail or screw. If you
have not got nails or screws,
cut notches and use string to
bind the two ends together.
Fix them loosely at first.

9. 2. Fix the third piece of wood
(1m) at exactly the middle
of each of the long pieces.
When the3 pieces are in
position
3. Hang a string down from
the tip of the A frame, and
attach a weight to it, so
that it swings below the
cross-bar

10. 4. Calibrate the A-frame by placing
it upright on ground you know
is level. When the stone stops
swinging, mark the position
where the string crosses the
crossbar. Turn the A frame
around, putting the legs in
exactly the opposite position,
and measure the centre-point
again. If it is in the same place,
then it is the level ground marker

11. Start near the top of the
slope. Stand on the
hillside with the
A-frame, adjusting it
until you find level
ground as indicated
when the weighted
string hangs at the level
ground marker.

12. The distance between
contour lines depends on
the steepness of the slope.
It could be as little as 8
meters or as much as 30
meters. The steeper the
slope, the closer the
contour barriers have to be
to prevent erosion. It also
depends upon the amount
of rain that falls, and on
what you are going to do
with the land.

13. Lines are
with respect
to the Slope

14.  The Major advantage of
Contour lines is in the field
Agriculture..
 It is very necessary to stop
the flow of water on the
slope so that water can be
infiltrate
 By this we can stop the soil
erosion

15.  Provide water to crops during drought years
 Trap nutrients washed away from the field
 Can be converted to compost pits
 Reduce siltation to river systems down slope
 Can be used to grow crops
 Can be used for fish-farming