Saturday, January 11, 2014

Features on a topo map - Contour lines

Contour lines, projection and satellite image
Contour lines, projection and satellite image

Contour lines are one of the most useful features on a topo map. They are also the feature that can be most intimidating to the beginner. Contour lines are essentially the 2D representation of the 3D landscape.

The above image shows three different representations of Mount Gordon (in Mount Carleton). The top right is the topo map, the top right is a projected view of the contours of a topo map and the bottom is the satellite image. Being good at reading contour lines is being able to read the lines and interpret them into a mental image of the actual terrain.

Contour lines can be thought of as slices through the terrain at specific intervals. Think of the stacked layers of a cake and, with each layer being the same height but different shapes.  Each contour line is essentially a layer stacked on the layer below.

The following rules will help you interpret these lines:

  • Contour lines never intersect or cross, they are always nested. 
  • All topo lines on a map are equal height difference. This height is called the "interval". In Canada the interval is often 10m. The map will usually describe the interval in the title block. 
  • Contour lines that are closer together mean steeper terrain than those that are further away. 
  • Some features that happen in between the interval height might not show up on the map. (ex. small cliffs or gulleys less than 10m in height)
  • If you follow the wide part of a V shape from a lower contour level to a higher contour level, the feature is probably a valley
  • If you follow the small end of a V shape from a lower contour level to a higher contour level, the feature is probably a ridge
  • A singe contour loop with no other contours inside is probably the top of a hill or mountain. (there are exceptions)

Contour map features
Contour map features
The image to the right shows some basic features to help with this process. Here is a description of these features:

A) This large area with few contours would be a very flat section of land

B) Closely stacked contours means that this is a steep slope rising approximately 80m over a short distance.

C) The looped contour to the left of the label (at 140m) is the top of this hill.

D) The V shaped contours shows a valley with a stream running through it. We can see 70m of rise on the map, starting at the river below.

Mount Sagamook - Great contours!
Mount Sagamook - Great contours!
 Using the contour lines you can keep track of where you are on the map. Are you rising, falling, at the top of a hill, at the bottom of a valley, etc...

Often, control points will be located at interesting geographic features that are highlighted by contours. The tops of hills are a common place for a CP.

A good understanding of contours will also help you travel the most efficient route. Often there will be options available with varying terrain difficulty. It might be better to go around a mountain than across one.

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