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The U.S. Geological Survey (USGS) creates topographical maps for use by government agencies as well as geologists and private individuals such as hikers and campers. These maps function as the basis for measuring elevation around the country with a geographic grid, allow for precise positioning and planning. In addition, topo maps are often used as the base for any number of other maps. These extremely precise terrain maps show natural and man-made features at a large scale to include landscape details.
Creating these maps involves thousands of man-hours of planning, application and update. Topo maps were initially hand-constructed. The mathematics of the geographical grids were hand computed and drawn in the past, as were all of the topography and features. These original base topographical maps that cover the United States are the basis for future digital cartographic updates. -
The construction of topographical maps involves placing features based on exact geographical position. As a result, collecting accurate data is the first step in topographical map making. Control points depicting accurate elevation and position form the basis for the investigation and placement of every item on a topo map.
Most topographic map features are added based on aerial photography. Planes fly during clear weather, often during the winter when trees are free of leaves to track changes in the surface features. Planes take two sets of photos from different flight paths at exact altitudes. Using control points for positioning accuracy, mapmakers view these photos with a stereoscope to see the images in three-dimensions.
Topo mapmakers always have an eye out for changes in charted details. Map makers verify positions of new features such as boundaries, buildings as well as any significant changes to the maps. The verification process must meet high standards for accuracy of details as well as geographic positioning.
Once details are confirmed, map makers hand scribe charting changes onto base maps. These base maps include as many as 10 layers in different colors based on the feature type. Cartographers etch changes into individual layers which later become the print negatives used in the printing process. From start to finish, it takes 5 years to construct a topographic map. - Each color on the topographic map represents a different type of map feature. Contour lines and terrain are brown. Topo mapmakers depict water in blue and forests in green. Minor roads and highways are shown in black and major roads are shown in red. Any changes between editions are depicted in purple to stand out clearly. There is a standardized "legend" for all features depicted on topographic maps.
- Topographical map making is based on the measurement of the relationship between the earth and the surface of the map. This measurement, called the map scale, represents a ratio referring to one inch versus a comparative distance on the earth's surface. The most detailed topographic maps are 1:24,000-scale charts. In essence, one inch on the map equals 2,000 feet on the earth. Scale is standardized on topographic maps with the most detailed charts labeled as "large-scale" maps.
- The USGS has a continuous series of charts for the lower United States and Hawaii at the 1:24,000 scale. These charts are called quadrangles because each represents a four-sided depiction of 7.5 minutes of latitude and 7.5 minutes of longitude. These topographic quads continue with seamless coverage when two connecting maps are placed side by side.
