how map projection are classified
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The Classification of Map Projection
Next to their property (equivalence, equidistance, conformality), map projections can be discribed in terms of their class (azimuthal, cylindrical, conical) and aspect (normal, transverse, oblique).
The three classes of map projections are cylindrical, conical and azimuthal.The earth's surface projected on a map wrapped around the globe as a cylinder produces the cylindrical map projection. Projected on a map formed into a cone gives a conical map projection. When projected on a planar map it produces an azimuthal or zenithal map projections.
The three classes of map projections
The three classes of map projections
Projections can also be described in terms of their aspect: the direction of the projection plane's orientation (whether cylinder, plane or cone) with respect to the globe. The three possible apects of a map projection are normal, transverse and oblique. In a normal projection, the main orientation of the projection surface is parallel to the earth's axis (as in the second figure below). A transverse projection has its main orientation perpendicular to the earth's axis. Oblique projections are all other, non-parallel and non-perpendicular, cases. The figure below provides two examples.
tangent to the reference surface
A transverse cylindrical and an oblique conical map projection. Both are tangent to the reference surface
The terms polar, oblique and equatorial are also used. In a polar azimuthal projection the projection surface is tangent or secant at the pole. In a equatorial azimuthal or equatorial cylindrical projection, the projection surface is tangent or secant at the equator. In an oblique projection the projection surface is tangent or secant anywhere else.
A map projection can be tangent to the globe, meaning that it is positioned so that the projection surface just touches the globe. Alternatively, it can be secant to the globe, meaning that the projection surface intersects the globe. The figure below provides illustrations.
cylindrical, conical and azimuthal
Three normal secant projections: cylindrical, conical and azimuthal
A final descriptor may be the name of the inventor of the projection, such as Mercator, Lambert, Robinson, Cassini etc., but these names are not very helpful because sometimes one person invented several projections, or several people have invented the same projection. For example J.H.Lambert described half a dozen projections. Any of these might be called 'Lambert's projection', but each need additional description to be recognized.
It is now possible to describe a certain projection as, for example,
Polar stereographic azimuthal projection with secant projection plane
Lambert conformal conic projection with two standard parallels
Lambert cylindrical equal-area projection with equidistant equator
Transverse Mercator projection with secant projection plane.