Topographic mapTopographic maps
, also called topo maps or topo quads (for quadrangles), are maps that show topography, or land contours, by means of contour intervals
. Contour intervals are lines that connect contiguous points of the same altitude
. In other words, every point on the marked line of 620 feet elevation is 620 feet above mean sea level.
In the United States, the United States Geological Survey produces several national series of topographic maps. The oldest series is the fifteen-minute series, meaning that each map portrays an area fifteen minutes of longitude wide by fifteen minutes of latitude high. The newer standard series is the 7.5-minute series, 7 1/2 minutes high by 7 1/2 minutes wide. Naturally, there are four of these per fifteen-minute quad, and they show far more detail. There are also other series, including some county maps and maps of special interest areas, such as the Grand Canyon, and there are also smaller-scale maps showing much larger areas.
There are several rules to note when viewing topo maps:
- The Rule of Vees: Sharp-pointed vees usually are in stream valleys, with the drainage channel passing through the point of the vee, with the vee pointing upstream.
- The Rule of Ohs: Closed circles are normally uphill on the inside and downhill on the outside, and the innermost circle is the highest area. If a circle instead represents a depression, this is noted by short lines radiating from the inside of the circle, called "hachures".
- Spacing of Contours: When contour lines are close together, it denotes a steep slope, and when they are far apart, it denotes a flatter area. If two or more contour lines merge together for a distance, it denotes a cliff or bluff.
Of course, to determine differences in elevation between two points, the contour interval, or distance in altitude between two adjacent contour lines, must be known, and this is notated at the bottom of the map. In most cases, contour intervals are consistent throughout a map. Sometimes dashed contour lines are present; these represent half the noted contour interval.
These maps show not only the contours (in light brown), but also any significant streams or other bodies of water (in light blue), forest cover (in light green), human-made development (in black), and other features and points of interest. Between full-scale topographic surveys, photographic revisions are performed, and the changes that result from these are shown in light purple.
See also geography.
Sources of topographic maps