Cartography

Data for Cartography

Maps are created from information collected about the locations and attributes of features in the environment. Locations are positions in two- or three-dimensional space, and sometimes the time of data collection is treated as a fourth dimension. Attributes are qualitative characteristics of features (type of forest) or quantitative values or rankings (heights of trees). Cartographers are experts at working with a wide variety of location and attribute data collected by different organizations using a range of data collection techniques and technologies.

Consider the types of data cartographers use to create topographic maps showing ground features such as rivers, roads, buildings, public land surveys, forest areas, and topography. After the extent to be mapped, the map scale, and the map projection for the area have been selected by the cartographer, the mapping process begins with plotting the locations of geodetic control points. These points give the precise latitude, longitude, and often elevation of ground positions relative to a three-dimensional ellipsoid that very closely matches the true size and shape of the earth. Cartographers do not measure these points, but rather they obtain them from professionals in the fields of geodetic surveying and photogrammetry. In the United States, these control points are determined by our National Geodetic Survey (NGS), and cartographers can download points from Web sites maintained by the NGS or private companies. The measurements needed to define locations to the highest surveying accuracy levels were traditionally made using electronic surveying instruments, but now survey-grade global positioning system (GPS) receivers are used that independently acquire points of the same accuracy.

The next step in creating a topographic map is to collect the ground locations for each type of feature to be mapped, using the geodetic control points as a geometrical “skeleton” for the map. Remote sensor imagery, particularly aerial photography, is the primary data source for finding the positions of roads, buildings, forested area boundaries, and other features visible on the photographs. Professional photogrammetrists collect this information by first marking the locations of geodetic control points on each aerial photograph covering the area to be mapped, then placing adjacent overlapping photographs in an expensive stereoplotting instrument. Each photograph is then geometrically rectified to match the latitude, longitude control point coordinates as placed on the map projection at the selected map scale. The photogrammetrist then views the overlapping portions of the photographs stereoscopically, seeing a planimetrically correct three-dimensional (3D) image of the area, from which contours can be traced by placing a cursor on the surface and creating a line of constant elevation. Positions of roads, houses, and other features can also be captured. The attributes for each feature (e.g., elevations for contours) are also entered into the data-base for the map. Modern stereoplotting instruments are completely digital, performing the geometrical rectifications analytically and storing the points, lines, polygons, and attribute information in digital files that can be directly read into computer mapping systems.

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