Over time many features on maps change. Trees can
be felled, tracks become over grown, new houses built .... but
generally the shape of the land does not alter greatly. It takes a
major event to make significant changes to the bumps and hollows of
the land. Given this situation you would think that using land forms
would be orienteers' first choice for aiding navigation. So why is this
not so? Perhaps because contour lines tend to fade into the back
ground when compared to roads, rivers and other clearly defined
features. Also it takes time and practice to be confident in reading
contour lines and let's face it, it is not easy reading a map while
running.
So how good are you at interpreting the contour lines on an
Orienteering map? Do all those brown squiggly lines just look as if
some one has been careless with a bowl of spaghetti? If you can
quickly and confidently distinguish high land from low and spurs from
re-entrants just by looking at a map you probably have a reasonably
good grasp of what is involved. However making full use of this
information in the heat of the race is much more difficult and many
orienteers aren't completely successful. You will know if you are in
this category if you:-
1) habitually navigate using line features even when this can involve
a long detour.
2) always get a sinking feeling when you see the map you are about to
run on is lacking in line features but has a plethora of brown lines.
3) routinely have problems when orienteering in unfamiliar forests
where the map is old and there are many discrepancies between the map
and the land.
The benefits of being able to read contour lines are many. At the
most basic level they show the shape, height and steepness of the
land. Here is some basic information about them:-
1) If you look closely at an orienteering map you will discover that
every 5th contour is marked with a slightly thicker line. These are
called index contours. They are there to help calculate heights -
it is all too easy to lose track of which line has been counted when
all lines are the same thickness.
2) You will find that most orienteering maps have a contour interval
of 5 meters. (This information is usually found close to the scale of
the map.) This means that the space between one contour line and
another represents 5 meters in height when translated onto the ground.
Consequently if the lines are densely packed we can surmise that the
ground rises steeply and conversely if there are large gaps between
the lines then the ground is much flatter. Thus some impression of
height and steepness of the terrain is imparted even to the casual map
reader.
To use contour lines you need to be able to distinguish high land
from low land. This can be a useful skill when making route choices.
If faced with two technically similar route choices of comparable
length, one with little climb and a second with substantial climb, it
is sensible to opt for the less physically demanding option. Look at
the following sections of map to see the differences between high and
low land. In the first you will notice that the contour lines form
closed loops. This represents a rise of 15 meters.
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In the second example the initial impression is that this is also a
hill but if you look closely you will notice that there are small
lines pointing into the centre of the area enclosed by the contour
lines. These are called tag lines and are used to show low land (the
tags always point to the low land and will appear on any contour line
which is below the level of the surrounding land ). This example
shows a depression 15 meters deep and 70 meters long.
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Building on this information the next step is to work on identifying
spurs and re-entrants - another area which frequently causes confusion.
This example shows a re-entrant. You can see that the contour lines
form a "u" shape. Note that the bottom of the "u" points towards the
high land. In some cases identification of re-entrants is made easy by
the presence of a stream running through it - streams never run along
the ridge of a spur.
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The next section of map shows a spur. This is also a "u" shape but
this time the base of the "u" points away from the high land.
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Registration No. SO-01-369
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Once you become confident in reading contour lines it becomes easier
to visualise what the terrain will look like and awareness of
deviations from your chosen route occurs more quickly. From this it
is a short step to using land forms to aid navigation by using them as
· "hand rails" to lead you to your control,
· collecting features behind the control,
· an attack point to assist in locating the control.
This is a complicated topic and this article is really just a starter:
more details and specific examples will appear in future articles.
To whet your appetite here are two more sections of map - see if you can work
out what features the controls are on. The answers are at the very
bottom of this article.
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Registration No. 0101SO
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Registration No. 0172SO
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Answers
3 Depression.
11 Spur.
12 Re-entrant.