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topographical features

"Topographic Map. A map that presents the horizontal and vertical positions of the features represented; distinguished from a planimetric map by the addition of relief in measurable form."

An A-Z street map is not, therefore, a topographical map; it does not provide information about the shape of the land. This sort of information may be vital to navigation in otherwise featureless terrain.

contours

Contour lines are imaginary lines that join points that are equally high above sea-level (i.e. points with equal elevation or altitude). Occasionally there is a depression that is below sea-level.
Since maps are flat they cannot show the "shape" of the land they depict. Contours provide that information.

This presentation covers the main features to understand about contours and ends with a self-test.


Contours
Reading and understanding contour maps
Contour interpretation (MCofS)

things to remember about contours

In 1:50000 OS maps the contour lines are 10m apart (the vertical interval).
In 1:25000 OS maps the contour lines are either 5m apart (lowland areas) or 10m apart (high ground).
In Harvey maps the contour lines are 15m apart.
In all three of these types every 5th contour line is thicker - the index contour.
Contour lines are hard to read if you are using a red light at night to preserve night vision.

In OS maps & Harvey maps the top of the number in a contour line will be uphill. This helps to distinguish valleys from spurs.
contour_image_with_arrow.png

slopes

The closer the contour lines the steeper the ground.
Steeper ground:
steep_ground.png







Less steep ground:
less_steep_ground.png

A slope that is steeper at the top than at the bottom is called a concave slope. Contours are closer at the top than the bottom of the slope.

A slope that is steeper at the bottom than at the top is called a convex slope. These can be dangerous to descend as you may not be able to tell - without the map - that the slope wil become too steep to walk down. Contours are more widely spaced at the top than at the bottom of the slope.

A slope steeper than 30 degrees will usually require hands to climb and care to descent.
A 30 degree slope has 6 index contours per 500m (1 cm on 1:50000 and 2cm on 1:25000).

Spreadsheet for calculating slope from contours:

video introduction to contour lines (using US topo map)


video introduction using OSGB map

terrain features

Apart from slopes, contours can demonstrate a number of features:
Summit: usually a ring surrounded by larger rings (not necessarily circular)
Ridge: a narrow elevated area, with U or V shaped contour lines in which the apex of the V is pointing to lower groound.
Valley/re-entrant: has U or V shaped contour lines, in which the apex of the V points to higher ground. May have a stream in the bottom.
Saddle: an area of low ground between two summits. The contour lines often have an hourglass shape.

Heights/altitudes are shown with spot heights as well as by contours.

to determine the height of a feature on the map or of your position:

Make sure you have identified the right feature/position on the map
Look at the contour lines either side of the feature
  1. If one is an index line (ie thicker) follow that line until the altitude of that line is found,
  2. If neither is an index lines count the number of thinner lines crossed to reach the index line then follow the index line to find its height. The add/subtract the number of thin lines x the contour interval to/from the height of the index line (you’re probably less likely to make an error with the maths if you find the index line downhill from the point and then add to the height of that).

examples:
on the map below, what is the height of the bothy at 674 347 & the start of the stream labelled Kirkdale at 673 344?
kirkdale.JPG


Bothy at 674 347
Bothy is next to index line with height 700m
It is uphill from that index line
No thin contour lines lie between the index line and the bothy
The contour interval is 10 m
The height is between 700m and 710m
Start of stream at 673 344
This lies between the 650m and 700m index line
There are two thin contour lines between the 650m index line and the start of the stream
The contour interval is 10m
The height is between 650m + 2 x 10m and 650m + 3 x 10
The height is between 670m and 680m

To calculate the height difference between 2 points:

work out the height of each then subtract the lower from the higher.
This will NOT necessarily represent the ascent you will make from one to the other – the ground between the two points may undulate, making the ascent greater.
In the first diagram the blue line on the right indicates the vertical height difference between point A and point B (wavy blue line shows outline of land and horizontal lines show where contour lines would cross the landscape)

height_difference_1.JPG
In the second diagram the blue stars indicate where a contour line is crossed going uphill. In this example the actual ascent is more than twice the height difference.
To calculate the ascent you will make going along a route count the number of contour lines crossed going up hill then multiply this number by the contour interval

height_difference_2.JPG

aspect of slope

This is the direction a ball would follow if you let it roll down a hill.
It will be perpendicular to contour lines: e.g. if contour lines were running west to east and south was higher than north then a ball would roll north.

video
The ability to use this can help to stay on course and to relocate when off course:
• Face downhill and aim direction of travel arrow downhill
• Turn housing until orienting arrows line with north arrow then deduct 3 degress (mag to grid get rid)
• Place compass on map with orienting lines parallel to NS grid lines then move until edge of compass is perpendicular to contour lines (ensuring height lost in direction of compass); this is a possible location

Alternatively if you know from you map which slope you want to travel on and want to use aspect ot help confirm you are there:
• Place compass edge on slope with edge perpendicular to the contour lines and the direction of travel arrow pointing the right way (downhill if the that is the way you will be travelling, uphill if you will be ascending the slope)
• Turn housing until orienting lines parallel with NS grid lines (check north at north of map) then add 3 degress (grid to mag, add)
• When you reach the slope, when the compass is orientated so north arrow pointing to N on the houing, the direection of travel arrow will point direclt up or down the slope (depending on how set)

aspect of slope article

Terrain association is matching land features to contours marked on a map. It is covered on the page relating map and ground.