mapping from a sphere

[AvuncularMontague]

Thanks for the replies so far. Looks like ReprojectImage would have been good, but it's not for a Mac (and I'm not sure it would be able to deal with multiple overlapping views instead of just two hemispheres 180 degrees apart). The NASA G Projector looks very cool for trying projections once I have a file. And Lingon, I had also thought of ways to do it by hand: basically trace out meridians and circles of latitude on the globe, then use one of the graticules available on this site or elsewhere and draw it in by hand, mapping by eye from the grid on the globe to the graticule on the flat screen.

But I'm still holding out for a lazy way! There are after all many programs that will take multiple images from the same viewpoint and stitch them together into a spherical panorama... viewed from the inside! I need the same but viewed from the outside. Probably just needs one bit flipped in the software :-)

[su_liam]

Any practical camera lens is going to give you a perspective projection...
Chashio mentioned ReprojectImage ( http://www.ridgenet.net/~jslayton/ReprojectImage.zip ) as an option. It would probably work for this action, but it doesn't have the vertical perspective projections that allow for non-infinite distance perspective projections.

If you kept all the layers visible on a large number of approximately projected images tied together into a panorama, you could use that as a guide to hand-draw(e.g. with a wacom tablet) a very good approximation of the original lines.^1

It's possible that Hugin might work for this. I'm not sure if Flex Projector or G.Projector have the inverse Vertical Perspective projection as options.

Dunno 'bout Hugin. I'm pretty sure Flex Projector and G.Projector don't have any inverse projection options at all. You either feed 'em plate caree or fuggedaboudit.

Turning a globe into an equirectangular map can be done by hand too. Been thinking a little about it as I'm planning to do something similar… sometime… in an undefined future…
I think the easiest way – at least if the globe only has rough shapes on it – would be to take a paper with 1:2 dimensions, and draw one line in the vertical middle and one in the horizontal middle. These are the equator and prime meridian. Each important point on the globe is then transferred to the paper, using 1 degree = 1/360 of the paper's width. It'd be time consuming, but not actually hard, and if I've understood the equirectangular projection correctly it'd give an exact map and not an approximation. The equirectangular map can then be scanned/photographed and turned into other projections using G.Projector or something similar.

This would be similar to point [^1] above. This is a lot more difficult if you're globe doesn't have a proper graticule printed on it. A lot more difficult...

Thanks for the replies so far. Looks like ReprojectImage would have been good, but it's not for a Mac (and I'm not sure it would be able to deal with multiple overlapping views instead of just two hemispheres 180 degrees apart). The NASA G Projector looks very cool for trying projections once I have a file. And Lingon, I had also thought of ways to do it by hand: basically trace out meridians and circles of latitude on the globe, then use one of the graticules available on this site or elsewhere and draw it in by hand, mapping by eye from the grid on the globe to the graticule on the flat screen.

But I'm still holding out for a lazy way! There are after all many programs that will take multiple images from the same viewpoint and stitch them together into a spherical panorama... viewed from the inside! I need the same but viewed from the outside. Probably just needs one bit flipped in the software :-)

Another Mac guy? Yay!
I'm trying to avoid booting up Windows myself. I've had some success getting data into GRASS, lately. I may see if I can figure out how to apply orthophoto reconstruction tools to the purpose. If it can be done and I figure it out, I'll post it.

Now if I can only find a good replacement for Wilbur on the Mac...