Understanding Map Projections

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planar projection A form of projection where the earth is projected onto a conceptual tangent or secant plane. Usually, a planar projection is the same as an azimuthal projection (Snyder and Voxland, 1989). polar aspect A planar projection with its central point located at either the North or South Pole. prime meridian A meridian of reference that defines the origin of the longitude values, 0° east or west. projected coordinate system A reference system that defines the locations of points on a planar surface. radius The distance from the center to the outer edge of a circle. reference ellipsoid See ellipsoid. rhumb line A complex curve on the earth’s surface that crosses every meridian at the same oblique angle; a straight line on the Mercator projection. Also called a loxodrome (Snyder and Voxland, 1989). scale factor A value (usually less than one) that converts a tangent projection to a secant projection. Represented by ‘k 0 ’ or ‘k’. If a projected coordinate system doesn’t have a scale factor, the standard point or lines of the projection have a scale of 1.0. Other points on the map have scales greater or lesser than 1.0. If a projected coordinate system has a scale factor, the standard point or lines no longer have a scale of 1.0. secant projection A form of map projection where the conceptual surface of the projection (cone, cylinder, or plane) cuts through the earth’s surface. semimajor axis The equatorial radius of a spheroid. Often known as ‘a’. semiminor axis The polar radius of a spheroid. Often known as ‘b’. sphere A three-dimensional shape obtained by revolving a circle around its diameter. spherical coordinate system A system using positions of longitude and latitude to define the locations of points on the surface of a sphere or spheroid. spheroid When representing the earth, the three-dimensional shape obtained by rotating an ellipse about its minor axis. This is an oblate ellipsoid of revolution, also called an ellipsoid. standard line A line on a sphere or spheroid that has no length compression or expansion after being projected. Commonly, a standard parallel or central meridian. standard parallel The line of latitude where the projection surface touches the surface. A tangent conic or cylindrical projection has one standard parallel, while a secant conic or cylindrical projection has two. A standard parallel has no distortion. State Plane Coordinate System A projected coordinate system used in the United States that divides each state into one or more zones to minimize distortion caused by the map projection. Also known as SPCS and SPC. tangent projection A form of map projection where the conceptual surface of the projection (cone, cylinder, or plane) just touches the earth’s surface. true-direction projection A form of projection that shows lines with correct azimuths from one or two points. unit of measure See angular units or linear units. Universal Transverse Mercator A projected coordinate system that divides the world into 60 north and south zones, six degrees wide. 104 • Understanding Map Projections
UTM See Universal Transverse Mercator. WGS 1984 World Geodetic System of 1984. A geocentric datum and geographic coordinate system created by the United States military. Also known as WGS84. Glossary • 105
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Understanding Map Projections GIS b
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Contents CHAPTER 1: GEOGRAPHIC COOR
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State Plane Coordinate System .....
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GEOGRAPHIC COORDINATE SYSTEMS A geo
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SPHEROIDS AND SPHERES The shape and
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DATUMS While a spheroid approximate
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2 Projected coordinate systems Proj
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WHAT IS A MAP PROJECTION? Whether y
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PROJECTION TYPES Because maps are f
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maintained for both small-scale and
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Planar projections Planar projectio
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OTHER PROJECTIONS The projections d
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The four parameters above are used
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GEOGRAPHIC TRANSFORMATION METHODS M
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Unless explicitly stated, it’s im
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National Transformation version 2 L
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LIST OF SUPPORTED MAP PROJECTIONS A
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Loximuthal McBryde-Thomas Flat-Pola
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AITOFF USES AND APPLICATIONS Develo
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ALASKA SERIES E LIMITATIONS This pr
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AZIMUTHAL EQUIDISTANT Area Distorti
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BIPOLAR OBLIQUE CONFORMAL CONIC DES
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CASSINI-SOLDNER Area No distortion
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CRASTER PARABOLIC Distance Scale is
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DOUBLE STEREOGRAPHIC Oblique aspect
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ECKERT II The central meridian is 1
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ECKERT IV parallels. Nearer the pol
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ECKERT VI central meridian. Scale i
Page 60 and 61: EQUIDISTANT CYLINDRICAL LINES OF CO
Page 62 and 63: GALL’S STEREOGRAPHIC Area Area is
Page 64 and 65: GEOCENTRIC COORDINATE SYSTEM Geogra
Page 66 and 67: GNOMONIC PROPERTIES Shape Increasin
Page 68 and 69: HAMMER-AITOFF LIMITATIONS Useful on
Page 70 and 71: KROVAK Direction Local angles are a
Page 72 and 73: LAMBERT CONFORMAL CONIC Area Minima
Page 74 and 75: LOXIMUTHAL Distance Scale is true a
Page 76 and 77: MERCATOR Greenland is only one-eigh
Page 78 and 79: MOLLWEIDE Distance Scale is true al
Page 80 and 81: ORTHOGRAPHIC PROPERTIES Shape Minim
Page 82 and 83: PLATE CARRÉE Direction North, sout
Page 84 and 85: POLYCONIC Direction Local angles ar
Page 86 and 87: RECTIFIED SKEWED ORTHOMORPHIC DESCR
Page 88 and 89: SIMPLE CONIC Equirectangular projec
Page 90 and 91: SPACE OBLIQUE MERCATOR DESCRIPTION
Page 92 and 93: Montana—The three zones for Monta
Page 94 and 95: TIMES LIMITATIONS Useful only for w
Page 96 and 97: 10,000,000 meters to ensure that al
Page 98 and 99: UNIVERSAL POLAR STEREOGRAPHIC Area
Page 100 and 101: VAN DER GRINTEN I Distance Scale al
Page 102 and 103: WINKEL I Distance Generally, scale
Page 104 and 105: WINKEL T RIPEL Distance Generally,
Page 107 and 108: Glossary angular units The unit of
Page 109: High Accuracy Reference Network A r
Page 114 and 115: Eckert VI 52 ED 1950 6 Ellipse 101
Page 116: Projections (continued) planar 17,
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Understanding Map Projections

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