Hi Martin, As Adrian refered, I know Mercator cann't act when the point close to 90 N or 90 S.
Here is a screenshot in MapInfo, in this software, when the region close to the polar, it extends image , never abort work. I had implement a WebGIS application with MapXtreme like Google Map, now my boss want change to OpenSource, I found geotools. Or can you tell me how to refer a clip region to render map, such as MapPainter mapPainter = new MapPainter(shpMap); mapPainter.render(85, -180, 80, -85, "test.png"); A greed idea. Regards. 2007/5/15, Martin Desruisseaux <[EMAIL PROTECTED]>:
新买 a écrit : > Yes, with Mercator Projection, only the part where latitude between > -85 to 85 can be rendered. I trace the code of Mercator Projection, > there is some code as "throw Exception...". > I try to comments these word, then it dont work. > May you solve this problem?\ As explained in my previous email, you can NOT display a map up to 90°N or 90°S using Mercator projection, no matter which software (mapXtreme or whatever) you use. This is mathematically *impossible*. The formula for the spherical case is: log(tan(PI/4 + 0.5*latitude)) With latitude = PI/2, we get log(tan(PI/4 + PI/4)) = log(tan(PI/2)) = log(sin(PI/2) / cos(PI/2)) = log(1 / 0) = log(infinity) = infinity See also Wikipedia: http://en.wikipedia.org/wiki/Mercator_projection "A Mercator map can never fully show the polar areas, since linear scale becomes infinitely high at the poles. (...snip...) At latitudes higher than 70° north or south, the Mercator projection is practically unusable." The "throw Exception" is there as a guard and even the 85° limit (before throwing an exception) is a lot. If you see a world map with latitude up to 90°N or 90°S, then this is not a Mercator projection. It may be a Plate Carre or something else, but *not* a Cylindrical Mercator. Martin
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