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Part
1: Mapping
A. Introduction to Mapping
The following content information can
be found at the United States Geological Survey’s (U.S.G.S) website: http://mac.usgs.gov/mac/isb/pubs/booklets/symbols/index.html.
“What is a Topographic Map?
In this activity, students are introduced to topographic maps with a discussion and examination of actual topographic maps. Students will notice that the distinctive characteristic of a topographic map is that the shape of the Earth's surface is shown by contour lines. Contours are imaginary lines that join points of equal elevation on the surface of the land above or below a reference surface such as mean sea level. Contours make it possible to measure the height of mountains, depths of the ocean bottom, and steepness of slopes.
A topographic map shows more than contours. The map
includes symbols that represent such features as streets, buildings, streams,
and woods. These symbols are constantly refined to better relate to the
features they represent, improve the appearance or readability of the map, or
to reduce production cost. Consequently,
within the same series, maps may have slightly different symbols for the same
feature. Examples of symbols that have changed include built-up areas, roads,
intermittent drainage, and some type styles. On one type of large-scale
topographic map, called provisional, some symbols and lettering are hand drawn.
Interpreting the colored lines, areas, and other symbols is the first step in using topographic maps. Features are shown as points, lines, or areas, depending on their size and extent. For example, individual houses may be shown as small black squares. For larger buildings, the actual shapes are mapped. In densely built-up areas, most individual buildings are omitted and an area tint is shown. On some maps, post offices, churches, city halls and other landmark buildings are shown within the tinted area.
Students are asked to notice features such as vegetation (green), water (blue), and densely built-up areas (gray or red). Students may also discover that many features are shown by lines that may be straight, curved, solid, dashed, dotted, or in any combination. The colors of the lines usually indicate similar kinds or classes of information: brown for topographic contours; blue for lakes, streams, irrigation ditches, etc.; red for land grids and important roads; black for other roads and trails, railroads, boundaries, etc.; and purple for features that have been updated using aerial photography, but not field verified.
Names of places and features also are shown in a color corresponding to the type of feature. Many features are identified by labels, such as "Substation" or "Golf Course."
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Ground Configuration shown by contours |
Introduce topographic contours to students by observing the brown lines of different widths. Each contour is a line of equal elevation; therefore, contours never cross. They show the general shape of the terrain. To help the user determine elevations, index contours (usually every fourth or fifth contour) are wider. The narrower intermediate and supplementary contours found between the index contours help to show more details of the land surface shape. Contours that are very close together represent steep slopes. Widely spaced contours, or an absence of contours, means that the ground slope is relatively level. The elevation difference between adjacent contour lines, called the contour interval, is selected to best show the general shape of the terrain. A map of a relatively flat area may have a contour interval of 10 feet or less. Maps in mountainous areas may have contour intervals of 100 feet or more. Elevation values are shown at frequent intervals on the index contour lines to facilitate their identification, as well as to enable the user to interpolate the values of adjacent contours.
(For further content information, check the USGS website at: http://mac.usgs.gov/mac/isb/pubs/booklets/symbols/index.html)
Once students have a general
understanding of topographical maps and contours, discuss how to construct a
two-dimensional map from a three-dimensional object, which is described in the
following section.
Constructing
a two-dimensional map of a three-dimensional object
When viewed in Profile, from a side, any object fills what is commonly known as the “x” and “y” axis. They have length, expressed on the “x” axis and height, expressed on the “y”axis.
The challenge is to be able to convert the profile or side view to a birds-eye view or top view. It is imagined that the top view is from a spot directly over, at right angles to the “x” axis, the area in question. When looking at a Contour map, of this shape it is viewed from directly above the “y” axis (see arrow above).
Because the “x’ axis represents height we can draw in planes at constant intervals and stack to resulting outlines atop each other like a layer cake. If we draw lines parallel to the “y” axis extending above or below the profile, the lines of contour begin to take form. A top view can be drawn from a front view and a side view, figure 2.
Figure 2.
Figure 3 depicts an imaginary mountain giving a profile.
Figure 3.
If lines of elevation are inserted into this Figure, it would look like Figure 4.
Figure 4
Using the same type of graphical transformation illustrated in Figures 1 and 2 above, we can draw a contour map of the mountain depicted above in Figure 4.
Please note that if we refer to the illustration above as the side view, no front view is given.
Figure 5 illustrates the result of conversion from the side view to a top view with heights or elevations shown as contour lines.
Figure 5.
As stated in the Introduction to Mapping, “…contours are imaginary lines that join points of equal elevation on the surface of the land above or below a reference surface such as mean sea level. Contours make it possible to measure the height of mountains, depths of the ocean bottom, and steepness of slopes”. Note that when a steep slope is shown the lines extending from the side view, top, to the top view, bottom, become more closely packed, conversely, when areas are of little incline contour lines are spread apart. Students can explore these concepts further by completing the following 5 sets of worksheets.