How Topo Maps are Made:
Field personnel use aerial photographs to mark and verify map features. A field check is necessary because information on an aerial photograph often can be ambiguous. For example, a worker in the field can indicate the difference between a perennial stream and one that dries up at certain times of the year. This is necessary because a perennial stream would be marked with a solid line on a map while an intermittent stream is designated by either a dash-dot or lighter weight solid line on a map. People who know the local area well, such as fishermen or farmers, are excellent sources of such information.
Another important job in the field is the verification of place names and political boundaries. This work often requires looking at courthouse records and talking to local residents. It can even include a visit to the local cemetery to check the spelling of a feature that has been named after a person buried there.
Compiling the Map
Upon completion of the field survey, the map manuscript is compiled using stereoscopic plotting instruments. Overlapping aerial photographs are placed in a special projector connected to a separate tracing table. The projected photographs are viewed through an optical system that causes the left eye to see one photograph and the right eye to see another. The result is a three-dimensional impression of the terrain.
Map features and contour lines are traced as they appear in the stereomodel. As the operator moves a reference mark, the tracing is transmitted to the tracing table, producing the map manuscript.
These illustrations show a portion of 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.
Map Scribing, Editing, and Printing
After the map manuscript is compiled, several steps remain before a map is completed. First, a map-size film negative of the compiled manuscript is made. This negative is then photochemically reproduced on several thin plastic sheets to which a soft translucent coating (called scribecoat) has been applied. These serve as guides for scribing.
Working over a light table, the scriber then uses engraving instruments to etch the map's lines and symbols. This is done by removing the soft coating from the hard plastic guide sheet. All features to be printed in the same color on the map--such as blue for water features--are etched onto separate sheets. A map is edited several times before final scribed sheets are completed.
Type for the words on the map is selected according to standards that will ensure consistency of type sizes and styles from map to map. Type placement is important for map legibility, so type must be carefully positioned on clear plastic sheets that are overlaid on the scribed separations. Photographic negatives are made of the type for printing.
The Digital Mapping Revolution
Computer-assisted map production is making it easier to produce new paper maps and to revise existing ones. The USGS is responding with innovative ways of compiling map data and using them for map production. Many of the mapmaking processes described above are being changed or eliminated. Improved efficiencies in most facets of production will shorten the 4 to 5 years it takes to produce a map by traditional methods.
Map digitization resembles the original map scribing process in that it requires that each feature on each map separate be located, classified, and traced. A map can have 10 or more different layers--roads, contours, boundaries, surface cover, and manmade features, for example--that require digitization. Maps can be digitized by hand, tracing each map's lines with a cursor, or automatically with scanners.
After digitizing, several editing operations remain. For example, attribute codes must be added to identify what each digitized line or symbol represents. A variety of other tasks must be performed to ensure that information is complete and correct, including matching features with adjoining files, matching features relative to each other within the file, and controlling the accuracy of attribute coding and positions.
Digital techniques will continue to influence mapmaking, enabling more rapid production of accurate, current maps. Computers can also help us manipulate data derived from traditional maps in increasingly sophisticated ways.