Contours

The above view is a perspective drawing of a coastal landscape, consisting of a shoreline, including a barrier spit (hooked sandbar), a river valley (with bluffs), and two hills. One hill has an inclined tabular slope, bounded by a steep cliff that grades into a terrace, and the other has a crest point, from which extend several ridges between stream cuts.

The above view above are a set of surveyed points with their elevations, starting from the coast at 0 ft sea level, up to two benchmarks (BM) atop the hills. This is typical raw data, from which we can draw contours. The contour-drawing procedure relies on interpolation of values set by the chosen contour interval of 20 ft between pairs of survey points.

Drawing contour lines

Note three other characteristics (besides non-crossing) of contour lines:

1) the gentler the slope, the wider the spacing between lines (very closely spaced lines denote a cliff),

2) where lines cross a stream, they must bend upstream (remember, a stream moves directly downslope and therefore we must walk horizontally to cross it and get to the same elevation point on the other side), and

3) by convention, we draw every fifth line in a heavier weight-for the contour interval in this case that is at 100 ft and 200 ft.

Rule of V's

The rule of V's: sharp-pointed vees usually are in stream valleys, with the drainage channel passing through the point of the vee, with the vee pointing upstream. This is a consequence of erosion.

Hachured contour

The rule of O's: closed loops are normally uphill on the inside and downhill on the outside, and the innermost loop is the highest area. If a loop instead represents a depression, some maps note this by short lines radiating from the inside of the loop, called "hachures".

Label the map below:

Test yourself

Topographic Profile - A topographic profile is a cross sectional view along a line drawn through a portion of a topographic map. In other words, if you could slice through a portion of the earth, pull away one half, and look at it from the side, the surface would be a topographic profile.

Gradient - Gradient tells you how steep your route is. Gradient can be expressed numerically in many ways. One common way to describe the average steepness or gradient between two points is to state the difference in elevation (ED) divided by the horizontal distance (HD):

GRADIENT = ED/HD

To get the distance along a curvy line such as a river, first one could lay out a piece of string along the line. Then, straighten out the string and hold it up to a ruler or the scale bar.

What is the gradient between points G and H? and which way is the river flowing?