How Topo Maps are Made:
A look into how topo maps are made reveals a long and complex process. It can take 5 years from the identification of a mapping requirement to the printing of a large-scale map like one of the USGS 7.5-minute, 1:24,000-scale quadrangle maps. This process requires a team of professionals and a series of closely coordinated steps.
A closer look at the procedures traditionally involved in topographic mapmaking demonstrates the combination of science, technology, and artistry required to produce a USGS map.
The first step in producing a topographic map is acquiring aerial photographs of the area being mapped. A pair of aerial photographs--each showing the same ground area taken from a different position along the flight line--are viewed through an instrument called a stereoscope, producing a three-dimensional view of the terrain from which a cartographer can draw a topographic map.
Most photographs used for the USGS's topographic mapping program are now obtained through the National Aerial Photography Program (NAPP). NAPP flights are flown in a north-south direction along carefully determined flight lines. It takes 10 precisely positioned NAPP aerial photographs to provide the stereoscopic coverage needed for each 7.5-minute quandrangle map.
Every aspect of the aerial photography process requires precision and meticulous planning.
- Specialized cameras are used to meet the exacting geometry needed to faithfully reproduce the stereoscopic model. Such a camera can cost more than $250,000.
- To ensure that all NAPP photographs are at a scale of 1:40,000, NAPP flights are flown at a consistent altitude above the terrain.
- Photographs must be taken when the sky is clear and with the sun at the proper angle for the type of ground being photographed.
- Even seasonal factors must be considered. In an area of hardwood forest, for example, it is usally best to take the photographs when leaves are off the trees so that terrain features are more clearly visible.
A pair of stereoscopic aerial photographs taken over Villanueva, New Mexico, in 1984. The originals were at a scale of 1:24,000, which are reduced here. Overlapping photographs such as these can be viewed through a stereoscope, resulting in a three dimensional view of the terrain to be mapped.
Another important element in how topo maps are made is that technology has reduced the requirement for mapping work in the field. Gone are the days of planetable surveying when the topographer sketched the map by hand. Nevertheless, the field survey still plays an important role in making and revising topographic maps. After aerial photographs are obtained, field survey work may be required to establish and measure the map's basic control points and to identify objects that need visual verification.
Survey measurements are taken carefully to establish the control points that become the framework on which map detail is compiled. Two types of control points are needed to position map features accurately. Horizontal control points identify the latitude and longitude of selected features within the area being mapped. They establish correct scale and map orientation and allow accurate positioning of the map's features. Vertical control points determine the elevation of selected points for the correct placement of a topographic map's contours. These control points are often in the form of a bench mark, seen below.
Bench marks such as this are placed
in the field by USGS survey
teams to establish control
points for maps.
What's in a Name?
The fact that almost 2 million natural and manmade features are identified in the USGS topographic map series affects how topo maps are made. These geographic names form a primary reference system essential for the communication of cartographic information. Beyond map labeling, geographic names are part of the Nation's living heritage. The origins and meanings of geographic names, derived from many languages, show national, personal, and social ingredients of life, past and present.
Some of the oldest geographic names found on U.S. maps are from Native American languages. Names like Adirondack, Chippewa, Chesapeake, Shenandoah, Choctaw, Yukon, and the names of 28 States are derived from various Native American languages. Other names reflect the European naming traditions of the early settlers. New London, Yorktown, Grover Hill, and Lancaster are derived from English; Fond du Lac, Baton Rouge, Marietta, La Salle, and St. Louis are French; El Mirage, Guadalupe, Rio Grande, San Francisco, and De Soto are Spanish names.
U. S. Geographic names are often rich in description, local color, and national history. Names like Stone Mountain, Ragged Ridge, Big Muddy River, Carmel-by-the-Sea, Grandview, and Long Island paint descriptive pictures of the places, features, and areas they represent. Last Chance, Hells Canyon, Liberty, Thief Lake, Enterprise, Rattlesnake Creek, Dread and Terror Ridge, Hell-for-Certain Creek, and Paradise Flats evoke the dreams, fears, and color of the frontier.
The standardization of geographic names in the United States began late in the 19th century. The surge in mapping and scientific activities after the Civil War left the accuracy and spelling of a large number of names in doubt. This posed a serious problem to mapmakers and scientists who require nonconflicting nomenclature. The U.S. Board on Geographic Names was established in 1890 as the central authority to deal with naming conflicts. This interagency body, chaired by the U.S. Department of the Interior, helps standardize the spelling and application of geographic names on maps and documents published by the U.S. Government.
Knowing how topo maps are made makes us appreciate the complex methods used and can even help us use topo maps more effectively. Continue to How Topo Maps are Made: Part Two