Suitability for Scaling (Return to top)
Each projection has its own particular properties. These can be affected by scaling the X and Y axes on the map. For example, the Mercator projection is conformal and can be used for navigational purposes since angles on the map represent bearings on a compass (and so straight lines are loxodromes); any scaling needs to preserve this property - but since a complete Mercator projection is of infinite height, different rectangles can include different amounts of northern and southern extremities. Similarly the polar projections keep lines of lattitude as concentric circles, and this needs to be preserved in scaling; but for the infinite gnomonic and stereographic maps, different amounts of the sphere can be included at different scales. For the plain Longitude/Latitude, Equal Area Cylindrical and Mollweide projections, scaling can be done almost freely, enabling different lines of latitude to have distances consistent with those on the prime meridian near that line of latitude. A scaled Equal Area Cylindrical projection is known as a "secant" rather than the basic "tangent" projection, but the difference is in fact minor - for the azimuthal projections, the secant and tangent methods produce the same map, though at different scales. The triangular projection is simply there to show that virtually anything is possible (its properties are that it preserves areas, it has lines of latitude as horizontal lines, and it fits inside a triangle - it is similar to a sinusoidal projection at the top, and to the cylindrical projection at the bottom). A transverse projection is at right angles to a standard projection (e.g. with a pole at the centre), while a reverse projection is at two right angles so the penguins can feel superior to the polar bears and Pacific islanders can feel at the centre of the earth. An oblique projection is at any other angle, and is harder to calculate (Internet Explorer seems to handle it better than Netscape Navigator).
Particularly good sites for projections include: Hunter College which also has a big page of only slightly dated links, and the University of Texas. Eddie McCreary has a different Java applet, using vector rather than raster graphics. For links there is Oddens' bookmarks, aussi Liens Utiles en français, e Geogr@afia on line em português. An earlier version of this page seems to have been copied (without permission) at the Rijksuniversiteit Groningen.
Copyright 1999 Henry Bottomley. Any comments?
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