Saturday, 1 June 2013

Topographic Map - Contour Lines

Contour maps make a valuable tool for understanding the elevation of your land and surrounding terrain.  Contour lines are lines drawn on a map connecting points of equal elevation, meaning if you physically walked along the contour line, your height above sea level would remain constant.

In order to keep things simple, topographic maps show lines for certain elevations only. These lines are evenly spaced apart. We call this spacing the contour interval. For example, the map below uses a 10m contour interval, you will see contour lines for every 10 meters of elevation. Different maps use different intervals, depending on the topography and how precise the mapping needs to be.

How a topographic map represents the terrain.   

To make topographic maps easier to read, every fifth contour line is an index contour. Because it's impractical to mark the elevation of every contour line on the map, the index contour lines are the only ones labeled. The index contours are a darker or wider brown line in comparison to the regular contour lines. You'll see the elevations marked on the index contour lines only.
 To determine elevations, pay attention to the amount of space in between lines. If the contours are close together, you're looking at a steep slope. If the contours have wide spaces in between -- or aren't there at all - the terrain is relatively flat.
OpenStreetMap provides contour mapping for the world with 10m contour intervals i.e each contour line is 10m apart from each other. It's a great way to read the slope of the land, can help tremendously when planning rainwater harvesting land works, and allows a view of your surrounding watershed.   

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