| Course on Maps & Graphs | Cartography Working Group | The Virtual Geography Department |
Aspect: Each projection surface (family) can be positioned over the globe from one of four aspects (also referred to as perspectives or viewpoints): Polar - over the North or South Pole, Equatorial - over the equator, Transverse - 90 degrees from the normal position, or Oblique - over any mid-latitude location. The appearance of the graticule will vary depending on the projection aspect. Aspect should be selected so that the area of greatest interest takes central focus on the projected map.
Azimuthal Projection: With Azimuthal projections, the spherical (globe) grid is projected onto a flat plane, thus it is also called a plane projection. When projected from the center of the globe with the normal (polar) aspect, the typical grid appearance for Azimuthal projections shows parallels forming concentric circles, while meridians radiate out from the center.
Case: Tangency or Case refers to the location or locations that a projection surface touches or cuts through the globe. There are two types of tangency: the Tangent Case and the Secant Case.With the tangent (simple) case, the projection surface (azimuthal plane, cylindrical or conic surface) touches the globe at one point or along one line.In secant case, the projection surface cuts through the globe to touch the surface at two lines.
Central Meridian: The Central Meridian is the meridian that passes through the center of a projection. Distortion is minimized along the central meridian, thus, it is important to select a central meridian that runs through the center of the area of interest on a map.
Conformal: On a conformal projection scale is the same in every direction from any point on the map, thus deformation of scale increases regularly in all directions. Parallels and meridians intersect at right angles and the shapes of very small areas ("orthomorphic"), and angles with very short sides are preserved. As there is no angular deformation, and true angles are maintained, angular measurements can be made from conformal projections.
Conic: With Conic projections, points from the globe graticule are transferred to a cone which has been enveloped around the sphere. The cone is then unrolled into a flat plane. The normal aspect for conic projections is the north or south pole where the axis of the cone (the point) coincides with the pole. When projected from the center of the globe, the typical grid appearance for Conic projections shows parallels forming arcs of circles facing up in the Northern Hemisphere and down in the Southern Hemisphere; and meridians are either straight or curved and radiate outwards from the direction of the point of the cone.
Cylindrical: Cylindrical projections are formed by wrapping a large, flat plane (e.g., a large sheet of paper) around the globe to form a cylinder. The points on the spherical grid are transferred to the cylinder which is then unfolded into a flat plane. The equator is the "normal aspect" or viewpoint for these projections. When projected from the center of the globe with the normal aspect, the typical grid appearance for cylindrical projections shows parallels and meridians forming straight perpendicular lines. The spacing varies depending on the type of cylindrical projection.
Distortion in Map Projections: The projection process involves the transformation of the spherical surface onto a flat plane through tearing, shearing and compression of the sphere to make it lie flat. All map projections, thus, produce some forms of distortion of shape, area, distance, direction, or scale. Distortions vary per map projection.
Ellipsoid: A geometric solid that simplifies the geoid's bulges and depressions to a smoother surface that is slightly wider at the equator than at the poles, and much more adaptable to mapping and survey measurements (see also Geodesy). Various reference ellipsoids have been developed and used in different parts of the world for large scale mapping purposes. the United states uses the Geodetic Reference System (GRS80) as the reference ellipsoid for surveying and topographic mapping.
Equal Area: Equal area or equivalent maps maintain true relationships of areas. That is, at a given scale, for every part, as well as the whole, map area is proportional to the corresponding area on the Earth. Deformation occurs in elliptical fashion away from tangency thus shapes are distorted.
Equidistant: In the equidistant projections scale is preserved (not distorted) in the direction perpendicular to the line of zero distortion or radially outwards from a point of zero distortion. The name arises from the fact that in the normal aspect of Cylindrical, Conic and Azimuthal projections the principal scale is preserved along the meridians and therefore all parallels on the map are equidistantly spaced.
Geodesy: The science of earth measurement. Geodetic surveyors collect precise measurements of latitude, longitude and elevation for selected points that serve as ground control for other land surveys, air photo/image rectification, and GIS and other geocoding and computer mapping needs. The geodetic control points are often regularly spaced across a given area to form a series of connected triangles. The process of measuring points in such a network is called triangulation.
Geoid: The earth is not actually round, but rather somewhat wider at the equator than at the poles, and with many surface irregularities including bulges and depressions. This irregular shape is referred to as a geoid and is used to describe the actual shape of the earth.
Globe: The reduced model of the spherical earth surface which must be distorted in various ways in order to transform earth features onto a flat planar surface; also called the generating globe or reference globe.
Gnomonic: The light source for the Gnomonic perspective is from the center of the earth through to the spherical surface where it is projected onto a plane.
Graticule: A map projection is the systematic arrangement of the earth’s (or generating globe’s) parallels and meridians onto a plane surface. These meridians and parallels become the projection graticule.
Oblique: The Oblique Aspect of a projection surface is placed above or on any position between, but not including, the equator and the poles. It may be centered on a parallel or on a meridian.
Orthographic: In the Orthographic perspective, the spherical surface is transformed to a projection plane from infinity, that is, as if a light source were shown from an infinite distance through the globe and onto a planar surface.
Perspective: Azimuthal projections are constructed from one of three perspectives where for each it is as if a light source were shown upon the globe and the arcs of the parallels and meridians were projected onto the flat, tangent, straight line surface. The three projection perspectives are Gnomonic, Orthographic, and Stereographic.
Projection: The systematic arrangement of the earth's meridians and parallels (graticule) onto a flat planar surface.
Pseudocylindrical: Within the cylindrical family are pseudocylindricalprojections. These "cylinders" are not rectangular, but rather, curve inwards at the poles. The resulting grid thus shows straight line parallels and central meridian (the meridian in the center of the projected map), and all other meridians form arcs which are concave from the perspective of the central meridian.
Sphere: The ellipsoid shape is smoothened to form a spherical surface from which map projections can be more easily created. The globe is spherical in shape.
Stereographic: In the Stereographic projection the perspective is a point at the opposite end of the globe. In other words, the light is a point source shown from a point on the globe through to the other end of the globe.
Tangency: See Case.
Transverse: The Transverse Aspect places the projection surface 90 degrees from the normal position, e.g., for a Polar Azimuthal projection the equator would be the Transverse Aspect, while for an Equatorial Cylindrical projection the poles would be the Transverse Aspect.
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Created 6/5/97 by Laurie A. B. Garo. Last updated 9/20/00 by lg. The URL for this page is http://www.uncc.edu/lagaro/cwg/mapproj/glossary.html