Consider this diagram (adapted from one used by the U.S. Geological Survey, so that here and in subsequent maps we will use English units exclusively):

The upper 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 with an inclined tabular slope, bounded by a steep cliff that grades into a terrace, the other topped at a crest point from which extend several ridges between stream cuts. In the lower view are a set of surveyed points with their elevations, starting from the coast at 0 ft sealevel, up to two benchmarks (BM) atop the hills. This is typical "raw data" from which contours can then be drawn. The procedure relies on interpolation of values set by the chosen contour interval of 20 ft between pairs of survey points. To see how, print out the above diagram. Take the two points at 52 and 90 ft (left of panel center): Assuming equal spacing, a short mark (say, a dashed line) valued at 60 ft can be placed just left of 52, then another at 80 feet - these are located by "eyeball" such that they are at proportional positions where one might anticipate them to be located as horizontal lines (thus, contours) if on a planar surface inclined between 90 and 52 feet elevation. That slope plane is divided into 38 1-ft units and the two short lines lie at 8/38ths (60)and 28/38ths (80) upwards from the 52 ft point. Using the same equal spacing approach, along a path directed to the coast between 52 and 0 short lines can be drawn at 12/52ths (40 ft) and 32/52ths (20 ft). The same interpolation can be done between the 100 and 56 ft points (near top center), and likewise for any other pair of points. This leads to a large number of interpolation lines of different contour (elevation) values. Now, much as is done in the parlour game of "connecting the dots", lines connecting the various short contour line marks can be drawn with the proviso that they cannot cross one another. Your end result should look like this:

Note three other characteristics (besides non-crossing) of contour lines: 1) the gentler the slope, the wider the spacing between lines (very close-spaced lines denote a cliff); 2) where lines cross a stream, they must bend upstream (remember, a stream moves directly downslope and therefore you must walk horizontally upstream to cross it and get to the same elevation point on the other side); and 3) by convention, every fifth line is drawn in a heavier weight) - for the contour interval in this case that is at 100 and 200 ft. Note, too, that on most maps, contours are drawn in brown. This is illustrated on this part of a larger topographic map:

This map covers the region just south and east of Mammoth Caverns in Kentucky (below the surface in the upper left). The upper third of the map, judging from the 20-ft contour spacing and the shapes outlined by the contours, is an area of rolling hills with several hundred feet of moderate relief (relief refers to the amount of difference in elevation between two points; low relief implies relatively flat terrain, while high suggests steep slopes) with narrow stream valleys. The hilly appearance can be enhanced by a technique sometimes used by map makers of shading (here in uniform brown) steep slopes on one (and the same) side of the hills. In the map center are large numbers of roughly circular contours, each grouping quite small. These mark the sites of sinkholes - this is limestone country in which the so-called karst topography develops by solution and downward draining of the bedrock. The lower third of the map resembles the upper but with notably less relief.

Now, as an exercise for you to test your ability to associate depiction of surface shapes by contours, look at this next map.

If you are curious to try, determine which scene in Section 6, the cross-country flight, contains landforms similar to the ones you should be able to visualize from the map. The map area is not in that scene but lies just to its east (next Landsat orbit to the right).

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