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Why projections may return latitude and longitude values greater than 90?
The formula used for some projections are defined so that it is possible to have latitudes > 90. In real world maps, every projection has an applicable range. For example, the Mercator projection is defined only between 80 degree south and 80 degree north. On latitudes greater that 80 degrees south or north, the distortion is too big to be really usable. Latitudes greater than 90 degrees are returned by inverse function of projections that are never used to picture pole areas. We chose in the lib to _not_ reduce the latitudes over 90 degrees, because it would slow the computation down for "correct" points: It wouldn't be smart if 99% of the computation is in the range under 90 degrees. Users can reduce the latitude range if they want to picture pole areas with a projection that is not made for.
For information, the projections that can give latitudes greater than 90 degrees are mainly transverse or oblique cylindrical projections. These projections are used mainly to represent the world between latitudes not greater than 80 (in general cases). There are only few cases where they are used to picture the whole world. In these cases, users have to reduce the latitude range.
In the other hand, we can guarantee that projections usually used to picture north and south regions (such as universal polar stereographic) do never return latitudes over 90.