Contour Lines
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Maps & Map Reading |
Maps are two dimensional representations of three dimensional features. They are drawn to a scale, which is printed on the map. A scale of 1:50,000 means that an object measuring 1 cm on the map will be 50,000 times that in real life, or 0.5km. However, since maps are diagrammatic they also use symbols to represent items. These symbols (shown later on) are not to scale. Thus the symbol for a church is a + . The size of the cross does not represent how big the church is, only that it is there.
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To understand the shape of the mountain it is helpful to use the contour lines to build up an image of the feature, either in your mind or to draft out a profile on paper.
Here is a profile of Mt.Wrongagain. A contour
line is a continuous line of the same elevation (or height) around the
edge of a feature. Think of it as the edging trim along each layer of a
wedding cake.
Each line
gives an outline of what a feature looks like at regular intervals of
elevation. The closer together the lines are, the steeper the slope. For
example the close gathering of contour lines on the north side of Mt.
Wrongagain represents a steep slope. The more spread out contour lines to
the east indicate a gentler slope. Depending on which map you are using
the contour lines are at different intervals.
For example, on OS 1:50,000 maps the contour lines are at 10 metre intervals (i.e. each line represents an outline of the mountain 10 metres higher than the line below it). On OS 1:25,000 maps, the contour lines are at 5 metre intervals. As contour intervals will vary with different maps, it is important to check the interval specified on the map you are using. Contour lines are usually printed in brown, with every 5th line thicker. This thicker line will carry a number somewhere along its length telling you the height. The intermediate lines you will have to work out for yourself.
The
mountain shown has two peaks, with the higher summit on the left. The
lower slopes are moderate, becoming very steep towards the summits.
Because the contour lines are at 50 metre intervals in this example, we
can only estimate what the terrain between each contour looks like. A
sizable cliff could easily hide between contours and not be recorded on
the map. With this in mind it is good to remember that while these maps
are generally very good, there is still room for the odd surprise. One of
the most difficult things to interpret on a contour map is whether the
contours indicate ridges or valleys. A useful tip is to look for streams
and rivers, which will always lie in valleys.
When contours form a bulge that points from a lower elevation to a higher elevation, it is valley or bowl. When contours form a bulge from higher elevations to lower elevations it is a ridge.
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On British Ordinance Survey maps there are two sets of parallel grid lines. These lines are in black and run vertically and horizontally on the map. They represent 1km squares and enable people to accurately locate a place.
On
the side of the map are a set of numbers (
On the top and bottom of the map are another set of numbers (known as Eastings), which number each of the vertical grid lines.
To identify a grid reference, follow the vertical line to the left of the chosen location down to the foot of the map to read its Easting - for example 04. Then estimate the number of tenths from the grid line to the location - in this case 5. Therefore, the first part of the grid reference is 045.
Repeat this with the horizontal grid line just below the location (410). The full six figure grid reference is, therefore, 045410.
You will also find two letters in
front of the six-figure grid reference. These letters relate to the
Ordnance Survey’s grid
system shown in the figure to the right. If these letters are left out of
the grid reference, the six-figure number would be repeated across the
country at intervals of 100 kilometres. You will find the relevant letter
combination in the key of the OS map you are using. Putting the letter
code and grid reference together you get, for example, SK045410.
TIP
for how to remember which set of numbers within the OS grid reference go first - There are loads of saying and methods, but here are two. To learn that the Eastings go first followed by the Northings:"along the corridor and up the stairs"
"walk along the flat before you fly upwards"
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There are many symbols used on Ordnance Survey maps and these are shown in a key on each map usually at one side. Spend some time studying these as it is common to confuse a boundary with a path or track. There are some differences between the 1:50,000 series and the 'Explorer' map series, which many find easier to use.
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•
The word MAP comes from the Latin word, Mappa and means napkin, cloth or sheet.•
The first map to represent the known world was created by Anaximander, a Greek philosopher in the 6th century BC.•
A cartographer is a person who creates maps.•
A map on a spherical surface is called a globe.•
Orienteers call a small valley a “re-entrant”. They also don’t have numbers on their contour lines.