How do we use topographic maps to find Gradient and Steep Slopes?

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Because topographic maps show us relief and elevation, we can use them to identify areas with steep slopes.  These are areas of interest for architects  engineers, and developers because they present challenges ands risks to building homes, roads, and other infrastructure.   

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Finding Steep Slopes

Contour lines show us the change in elevation between points.  If we find an area on the map where the elevation changes a lot in a very small space, that indicates a steep slope.  To find steep slopes we look for areas on the map where contour lines are close together.

Steep Slopes = Contour Lines Close Together

In contrast, areas where the contour lines are very spread out represent very flat, level surface with low slopes.

Measuring Steepness

Now that we can identify areas with steep slopes, the next thing we want to be able to do is measure exactly HOW steep it is. 

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This is done by measuring the GRADIENT  between two points.  To find the gradient we need to know several things:

  • The elevation of the two points
  • The distance between the two points

Looking at the map to the right... 

Elevation A = 400ft

Elevation B = 300ft

Distance =  5mi

 

Now we use this information to calculate the gradient of the slope between A and B.  the equation for gradient can be found on the cover of the ESRT. 

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In this case, the field value is elevation.  So gradient is equal to the change in elevation from A to B, divided by the distance 

Gradient AB = (400-300) / 5 = 20 ft/mi

The higher the gradient value is, the steeper the slope is.