Incorporated in a map is the understanding that it is a “snapshot” of an idea, a single picture, a selection of concepts from a constantly changing database of geographic information (Merriam 1996). GIS represents a major shift in the cartographic paradigm. In traditional (paper) cartography, the map was both the database and the display of geographic information. For GIS, however, data storage, analysis, and display are physically and conceptually separate aspects of handling geographic information. Maps are usually made flat according to a projection of the curved surface onto the map plane. Globes are maps displayed on a sphere, which approximates the true shape of the Earth. Georeferencing refers to the manner by which locations in raster and vector GIS files are related to actual earth-surface positions (IDRISI manual). Minutes and seconds are awkward units for GIS applications, so decimal degrees are employed instead. One decimal minute is 1/60, or 0.016667°, one decimal second is 1/3600, or 0.00027778°. North latitudes and east longitudes are given positive values, whereas south latitudes and west longitudes are negative. For example, the boundaries of the Topeka quadrangle become, in decimal degrees: 39.0°, 39.125°, -95.625°, and -95.75°. This map is used primarily for navigation and for display of relatively small regions of the globe. High latitudes are greatly distorted in area, poles cannot be shown. For these reasons the Mercator projection is not suitable for showing the entire world. Prior to global positioning system (GPS), navigation on the high seas as well as aeronautical navigation were difficult tasks accomplished with Mercator maps, magnetic compass, star sightings, and landmarks. Often 'dead reckoning' was involved based on a best guess of travel direction and distance. This projection is used for simple outline maps of regions or the world or for index maps. This projection may have been devised by Eratosthenes (275?-195? B.C.), Marinus of Tyre is also credited with its invention around A.D. 100. It was widely used during the 15th and 16th centuries. This projection is ideally suited for display of GIS data in a latitude-longitude raster grid, although it has major distortions at high-latitudes. Mollweide projection – equal area, pseudocylindrical. A good map for depicting the whole world with moderate distortion. Image obtained from NASA/JPL. Polyconic projection – neither conformal nor equal-area. Central meridian is straight line, all others are complex curves, equally spaced along the equator. Equator is a straight line, other latitudes are nonconcentric circular arcs spaced at true distance along the central meridian. Scale is true along the central meridian and along each latitude. This projection was widely used for topographic and coastal mapping of the United States prior to the 1950s. It is still utilized for many large-scale topographic maps in the U.S.–see Topeka quadrangle. It was devised about 1820 by F.R. Hassler, first Director of U.S. Survey of the Coast (later U.S. Coast and Geodetic Survey). Having emphasized the advantage of maps and geospatial analysis, it is wise to note some cautions. All maps are estimations, generalizations, and interpretations of true geographic conditions, no map can depict all physical, biological, and cultural features for even the smallest area. A map may display only a few selected features, which are portrayed usually in highly symbolic styles according to some kind of classification scheme. Furthermore, all maps and GIS datasets are products of human endeavor, which may lead to unwitting errors, misrepresentation, bias, or outright fraud. In spite of these limitations, maps have proven to be remarkably adaptable, and geospatial analysis is an essential part of modern society. U.S. National Atlas—map projections Topographic Maps for the Nation—US Topo USGS topographic map symbols Source.


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