Topographic Map
Interpretation

TECHNIQUE NUMBER 7 

PROFILE CONSTRUCTION

(X and/or Y), Z axes

Remember from the beginning of this chapter that a topographic map is a two-dimensional representation of our three-dimensional world. On a map, the visual impact of elevation is sacrificed so that horizontal relationships can be immediately seen. A topographic profile is a restatement of spatial relationships which emphasizes the vertical axis and deemphasizes the horizontal axes. A topographic profile has a vertical axis (elevation) and a horizontal axis (any line on the topographic map). Profiles are used to emphasize slopes and changes in slope angles across a map. They are usually aligned perpendicular to major structural/topographic trends. They may be constructed with a horizontal scale in common with the base map, larger, or smaller, and with no or significant vertical exaggeration.

Vertical Exaggeration. The vertical exaggeration is the ratio between the vertical scale and the horizontal scale. If the two are the same the ratio is 1 and there is no vertical exaggeration. If the horizontal scale is 1:24000 and the vertical scale is 1:2400, the vertical exaggeration is:

(1/2400)/(1/24000) = 10

NOTE: Vertical exaggeration is NEVER used unless there is a definite need for it. If there is no vertical exaggeration, slopes will be shown to scale and approximate slope angles can be scaled from the profile. Use vertical exaggeration only if there are subtle slope features which must be shown, and even then use the smallest exaggeration which has the desired results. Large vertical exaggerations are the prime cause of difficulty in the interpretation of profiles.

Common scale profiles. Topographic profiles generated at a common scale with the base map are the most frequently used. They have the advantage of being able to be placed on the map along the line of profile and compared to the base map. This is particularly important in the preparation of geological cross-sections (below).

To generate a common-scale profile: 

First draw a light, thin line on the base map between the two end points of the profile.
Place a piece of precise graph paper (such as 20 x 20 to the inch) such that its edge lies along the line, with one end of the line even with a (heavy) starting line on the graph paper.  Draw the vertical axis along that line, perpendicular to the edge of the paper. The units on the vertical axis should bracket the highest and lowest elevations to be shown on the profile, and should be at the same scale as the horizontal scale, which is that of the map. (This is the hardest part - if your map scale is 1:24,000 your profile scale should be the same. This translates to 1 inch = 2000 feet, so if you are working in English units you can use 20 x 20 to the inch graph paper and each division will represent 100 vertical feet.) Mark the starting elevation on the vertical axis and proceed along the profile line, marking an elevation whenever you cross a significant contour line. If only a generalized profile is required you can use only the index contours - this results in a smoothing or averaging of the topography. If a detailed profile is required, use each contour line.
Connect the marks to yield a profile which is at a common scale to the base map. Note that interpretation is required here.  At the bottom of a narrow valley the profile might make a sharp "V", whereas equally spaced contours at the top of a hill might imply a broad crest.

Enlarged Profiles. A profile at a scale smaller (thus a view larger) than that of the base map, most useful where direct comparisons will not be made and great detail must be shown, can be constructed almost as easily as a common-scale profile. First, you must decide what scale you wish to use. Usually a scale which is easy to graph, such as 1:10,000, is chosen. Then the graph paper, with vertical and horizontal scales and axes already on the paper, is placed on the map with one end of the profile touching the graph paper and the paper angled away from the profile line. The angle should be such that a perpendicular from the other end of the profile line intersects the graph paper at the other end of the profile. A triangle is the most convenient tool to use to define the perpendiculars. The graph paper should be taped (with masking tape) to the map so that it will not shift as perpendiculars are drawn from the contours on the profile line on the map to the graph paper. The profile is then constructed as above.

Reduced Profiles. Reduced profiles are drawn where direct comparisons will not be made with the base map and there is a need to generalize or fit a long profile on a short piece of paper. Like enlarged profiles, the scale should be chosen for convenience. Also like an enlarged profile, a reduced profile is constructed by placing the graph paper at an angle to the profile line on the base map. The only difference in the construction method is that in order to reduce the size of the profile the perpendiculars are run from the graph paper to the profile line, whereas in order to enlarge the profile they are run from the profile line to the graph paper.