The term topo map is short for the lengthier topographic map. Topo maps, also known as contour maps, are maps that show the Earth’s surface in a series of contour lines. Contour lines give reference to the elevation of any given point, and the lines are drawn in a series of intervals that show mountain heights, ocean depths, and the steepness of slopes. Topo maps usually give a reference surface to any given map, and the contour lines are drawn from that point. For instance, in Hawaii, the reference surface would be sea level, while in Colorado, it might be a low river valley or the lowest point in the state, depending on the size of the map.
Contour lines on topo maps always have a line of equal elevation, meaning that if a topo map is drawn for a particular hill or mountain, any given elevation will have a contour line to match that height, and the line will never cross itself. Elevations are usually printed on topo maps just along the edge of the contour line, or sometimes superimposing it. Contour lines drawn very close together indicate steep slopes, while those that are few and far between indicate a generally level ground surface.
Contour lines on topo maps vary significantly in contour interval. Contour interval indicates the steepness of an incline. On a topo map representing a sharp mountain grade, for instance, the contour interval may be 100 feet (30.48 meters) or more, meaning that there is a 100 foot (30.48 meter) elevation change between each contour line. Topo maps depicting level ground might have a contour interval of only 10 feet (3.48 meters).
Topo maps are not limited to contour lines for mountains, slopes, and plains. Other geographical data is also typically printed on topo maps, such as rivers, lakes, and canals, as well as significant roadways, railroads, property lines, and hiking trails. Topo maps even have symbols for campgrounds and buildings, though often, a highly populated area is simply depicted with a darker colored tint across the map; this way, elevations can still be shown under the city streets and structures.
Contour lines on topo maps are generally drawn in brown, important roads in red, other roads and trails in black, and waterways in blue. Topo maps designed to show the slopes and contours of the ocean floor have contour lines known as bathymetric contours. Not surprisingly, these contour lines are drawn in either blue or black.
By Paulla Estes