More About the Orthographic Maps
A true orthographic map would require our viewpoint to be an infinite distance from the globe,
and the field of view to be infinitely small. Computers have difficulties with infinities, and we
have to specify finite values for these parameters. This means that our maps are actually “near-face vertical
perspective” projections rather than true orthographic projections. However, the practical difference from a true
orthographic view is so small that we feel comfortable using the orthographic label.
Let’s take the example of our free Globe Views. The minimum field of view our software can achieve is
0.8 degrees. With this field of view, the viewpoint is situated 904,683 km above the globe surface at the centre of
the view (this is more than twice the Moon’s mean orbit of 378,000 km from the Earth).
A bit of geometry will show that the Globe View spheres cover 179.2 degrees edge to edge, rather
than a true orthographic value of 180 degrees. Starting from the actual globe diameter of 2700 pixels, a bit more
first-order geometry shows that the diameter of a true orthographic globe would be about 2700.19 pixels in diameter.
A difference, certainly, but a small one, given that pixels are indivisible.
We did think of using the word “vertographic” for our projection, but that sounds like we’ve invented something
new, which we haven’t. The following sketch (which is not to any scale) summarizes the situation:
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