and Projections – EB/ES/GE 351
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).
|Latitude-longitude grid centered on the|
equator (W-E) and prime meridian (N-S).
Image obtained from NASA/JPL.
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°.
|Mercator projection – cylindrical and conformal. Most famous of all map projections, first developed in 1569 by Gerardus Mercator of Flanders. Meridians are equally spaced and parallel; latitudes are unequally spaced and parallel. All rhumb lines (lines of constant angle or bearing) are straight lines. Image obtained from NASA/JPL.|
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.
|Plate Carrée projection – cylindrical, equidistant projection that is neither conformal nor equal area. The simplist of all projections to construct: all meridians and latitudes are equally spaced, straight lines that intersect at right angles. Meridians are half the length of the equator; scale is true along the equator and meridians. Image obtained from NASA/JPL.|
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.|
|Albers equal-area conic projection – Compare spacing of latitude lines between this projection and the next one. Both are widely utilized for mapping (national atlases) in Canada and the United States. Image obtained from NASA/JPL.|
|Lambert conformal conic projection – Image obtained from NASA/JPL.|
|Bonne projection – a heart-shaped pseudoconic projection. The central portion of this projection was utilized by Ptolemy for his "world map" during the Roman Era. The projection was expanded for the whole Earth and became quite popular for global maps during the 16-17th centuries. A novelty map today–see stamp below. 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).
|Orthographic projection – azimuthal and perspective, but neither conformal nor equal-area. Produces a map that resembles a globe. Excellent means to display a single continent, ocean, or hemisphere; also used to simulate the appearance of the Earth from space. Image obtained from NASA/JPL.|
|Stereographic projection – azimuthal and conformal. In polar aspect, all meridians are straight, radial lines (like spokes of a wheel) and latitudes are concentric circles–see title stamp at top. The basis for the Universal Polar Stereographic (UPS) grid for use in high-latitude, polar areas of the world. Image obtained from NASA/JPL.|
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.
Topographic Maps for the Nation—US Topo
USGS topographic map symbols
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EB/ES/GE 351 © J.S. Aber (2014).