TOPOGRAPHIC POSITIONS

Someone learning to read a language that has an unfamiliar alphabet (e.g. a child with any written language, or an adult American learning Russian) has to look carefully at each individual letter at first. In time, however, the unit of perception becomes the syllable, then the whole word, and perhaps even a group of words. The speed of reading increases dramatically as the mind learns what information is essential and what can be ignored.

Likewise, a beginner is often overwhelmed by the amount of detail on a standard topographic map, whereas a skilled map reader seldom bothers to trace individual contours. Taken as a group, contour lines often form patterns that indicate familiar terrain features. Over the years, people have developed a number of ways to classify terrain elements. This section introduces a simple but useful classification of basic landforms. The small X on each idealized contour map shows the point from which four people m might try to walk in different directions, as described in the text paragraphs. The four paths are always at right angles to each other, but the set is oriented so that one individual goes directly up or down the steepest slope.

1) Plain (flat) - a generally level area, with few contour lines and no significant change in elevation in any direction. If four people walked away in all directions from a point on a plain, all four would remain at about the same elevation.
2) Slope -- a flat area that is tilted, not level. Contour lines on a slope are generally straight and parallel to each other. If four people walked away from each other on a slope, one person would go downhil, one would climb, and two would stay at the same elevation.
3) Summit (peak) - the highest point in a given area. At least one contour line goes all the way around a summit. If four people walked in different directions from a point on a summit, they would all go dowriill. If they descend at the same rate, the summit is symmetrical.
4) Ridge (crest) - a long and narrow high area. Contour lines are generally straight and parallel to each other. If four people walked away from a point on a ridge, two would go downhill and two would stay at the same elevation.
5) Spur (nose) a tilted ridge, where the contours form a roughly parallel set of rounded 'V' shapes. If four people walked away from a point on a spur, one would climb, one go gently downhill and two would have to descend more steeply.
6) Cliff -- an abrupt and very steep slope, with crowded parallel contours. If four people walked away from a point near the top of a cliff, two would stay about the same level, one could stay level or climb, and one may not survive the fall.
7) Bench (terrace) - a level area on a slope, with parallel straight contours. If four people walked away from a point on a bench, all would start out level, but one would soon climb and another would go downhill, while the other two stayed generally at the same level.
8) Valley - a long and narrow low area, lie an upside-down ridge. The contour lines are often straight and parallel to each other, although a valley, like a edge, can curve. If four people walked away from a point in a valley, two would stay at about the same elevation and two would go uphill.
9) Draw -- a tilted low area, like an inverted spur. Contour lines form a parallel set of rounded "I' shapes. If four people walked away from a point in a draw, two would climb steeply, one would go more gently uphill, and one would descend.
103 Depression (hole or pit) - the lowest point in a local area (an upside-down summit). At least one contour line completely encircles a depression; tiny hachures indicate the downhil direction. Four people wanting away from a point at the bottom of a dep depression would all have to go uphill.
11) Butte -- hill with a flat top and steep sides, marked by closely spaced concentric contours that encircle a level area with few lines. If four people tried to walk away from each other on a butte, no one could walk too far! (A big butte is called a mesa).
12) Pass (saddle) -- a low point in a high area, with contours that look like a combination of two spurs and two draws. If four people walked away from a point in a pass, two would go downhill and two would have to climb.
13) Fan -- a wide spur, where the contour lines form a set of nested arcs or concentric half-circles. If four people walked away from each other on a fan, one would climb gently uphill and the other three would descend slowly.
14) Scarp (cuesta or hogback) - an asymmetrical ridge, with one slope significantly steeper than the other. If four people walked in opposite directions from a point on a scarp, two would stay nearly level, one would go gently downhill, and one would go s steeply downhill.

These topographic positions (terrain elements, or topographic features) do not exist in isolation. On the contrary, they tend to occur in more-or-less predictable groups (associations) of related landforms. Two summits, for example, usually are separated by a pass, which in turn serves as the top of two draws. Similarly, a long draw leading out onto a plain in a dry region will usually have a fan where it leaves the hillier area In many cases, these associations of topographic features can provide some valuable clues about the geologic structure that underlies the landscape. At the same time, however, the close association of different topographic features can make it difficult to separate one feature from another (or even to decide whether a particular f feature actually exists in an area). In short, the identification of topographic features is at least partly an arbitrary process, just like all other aspects of the language of maps.

To complicate things further, the scale of individual terrain elements can vary widely in different regions. For example, a pile of dirt exposed to the rain in North Carolina will develop an intricate set of draws and tiny fans, often no more than a few f feet across. The same process working a long time in the radically different climate and rock material of Nevada can build fans that are many miles wide and thousands of feet thick. With practice, a map reader learns what shapes and saplings of contour lines are normal within a given region. Perhaps even more important, a skilled reader can "read between the contour lines" and see what the slope actually is lie. For example, a wiggly contour line may indicate a rough and rocky slope with many irregularities too small to show with individual contour lines.


This Material was adapted from: The Language of Maps by Philip J. Gersmehl, N.C.G.E. Pathways in Geography Series, No. 1, 1991. pp.60-62.