First-time users of topographic maps may be overwhelmed by all the colored lines, areas, and other symbols it contains. Looks can be deceiving, however, so don't let all the graphics discourage you from using these maps. With the help of the map legend, you'll be reading them like an expert (well...almost) after a few tries.

Features on topographic maps are shown as points, lines, or areas, depending on their size and extent:

• Lines may be straight, curved, solid, dashed, dotted, or any combination thereof. Line color indicates similar kinds of information. Colors used include brown for topographic contours; blue for lakes and streams; red for land grids and important roads; black for other roads, railroads, etc.; and purple for updated features that have not been verified.
• Area features have color designations also: vegetation - green; water - blue; information added during update - purple; and densely built-up areas - gray or red.
• Various point symbols are used to depict features such as buildings, campgrounds, springs, water tanks, mines, survey control points, and wells.
• Names of places and features are shown in color corresponding to the type of feature. Others are identified by labels

Pictured below is a portion of a topo map legend.

As I pointed out in my previous article, the distinguishing feature of the topographic map is its' use of contour lines to show the shape and elevation of the land.

• Topographic contours are shown in brown by lines of different widths. Each contour is of equal elevation, therefore they never cross. They show the general shape of the terrain. Coutours very close together represent steep slopes; those farther apart or absent represent relatively level ground. The elevation difference between adjacent contour lines is called the contour interval. A 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.
• Bathymetric contours are generally offshore since they show the shape and slope of the ocean bottom. They are shown in blue or black.
• Depth curves are shown along coastlines and on inland bodies of water where the data are available from hydrographic charts or other reliable sources.

Topographic map of Charlestown, Indiana, showing
many of the features listed above.

Making Topographic Maps

Mapmaking has come a long way from the days of planetable surveying. Today, aerial photography and photogrammetry (the science of obtaining reliable information by measuring and interpreting photographs) are used to produce accurate topographic maps - a process that can take 5 years from the identification of a mapping requirement to the printing of a large-scale map.

Step 1:	Aerial photographs of the area being mapped are acquired. A pair of aerial photos - each showing the same ground area taken from a different position along the flight line - are viewed through an instrument called a stereoscope. This produces a three-dimensional view of the terrain from which a cartographer can draw a topographic map.
Step 2:	Field survey work may be required to establish and measure the map's basic control points and to identify objects that need to be verified visually. There are two types of control points which lay the framework for map detail: (1) horizontal control points identify the latitude and longitude of selected features; and (2) vertical control points determine the elevation of selected points for the correct placement of a topographic map's contours.
Step 3:	Map features must be verified by field personnel. Field checks are an important step as information on aerial photos can often be ambiguous. For instance, a worker in the field can indicate the difference between a perennial stream and one that dries up at certain times of the year. Since a perennial stream is marked with a solid line on a map and an intermittent stream with a dash-dot line, this is crucial to the accuracy of the map. Field workers also verify place names and political boundaries.
Step 4:	The map manuscript is compiled, after the field survey is finished, using stereoscopic plotting instruments. The result is a three-dimensional impression of the terrain.
Step 5:	A map-size film negative of the compiled manuscript is made and then photochemically reporduced on several thin plastic sheets to which a soft translucent coating (scribecoat) has been applied. These serve as a guide for scribing.
Step 6:	The scriber uses engraving instruments to etch the map's lines and symbols. Features to be printed in the same color on the map, such as blue for water, are etched onto separate sheets. A USGS topographic map (top left) and three of the six colors used to print separate features. The green layer shows areas of woodland, and the brown layer shows topographic features, including contour lines. The purple layer shows features that are added from aerial photographs and other sources, but are not field checked.
Step 7:	Type for the words on the map is selected and carefully positioned on clear plastic sheets that are overlaid on the scribed separations. Photographic negatives are made of the type for printing.
Step 8:	A color proof is prepared. The proof looks much like a finished map, however, it is edited once again for content, legibility, accuracy, and spelling. When the final proof is approved, the map is ready for printing.
Step 9:	The map is printed by repeatedly running the map paper through a lithographic printing press (once for each color), or running the paper through a press capable of printing several colors in sequence.

The Future of Mapmaking

The widespread acceptance of computers and the possibilities they present, has accelerated the demand for mapping information in computer-compatible form. Government agencies and private businesses now require digital mapping information for their computer-based systems. Many of the mapmaking processes described above are being changed or eliminated. Digital techniques will continue to influence mapmaking, enabling more rapid production of accurate, current maps.