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Thread: mapping from a sphere

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    Default mapping from a sphere

    Hi (first post),

    Wonderful community, I've been browsing for the last few days. Here's my question: I want to convert a world map that has been sketched on the surface of a globe (whose original map has been painted over) into an image file that I can edit on the computer. Does anyone know of a practical approach? There are two issues: digitization, and projection. Once I have a digitized file I would like to explore different projections, but first I need to figure out how to get the sketch into the computer - just the rough outlines, which I will touch up, but I do want to respect the coastlines as drawn as much as possible.

    So I wonder: is there any software than can start with multiple photos of a spherical surface then stitch them together into one image? And do you know of any spherical-to-flat projection software for an image file, assuming I can make one from my globe?

    Thanks! Monty

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      Chashio is offline
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    There was an awesome little application floating around here somewhere, called Reproject Image, or something like that, that may have an orthographic-to-XXX projection capability. I can't remember where the thread was. :/ It would still require quite a bit of hands-on work... not an automagic type of thing, but it might help. If you can get it into equirectangular then you can play around with projections more easily in NASA's G-Project application.

    I don't know of anything else that might be useful to you... those are the only map projection programs I have experience with, limited though it is.

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      waldronate is offline
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    Any practical camera lens is going to give you a perspective projection, which requires the perspective analogue of the Orthographic projection (I'm ignoring a telecentric lens as impractical for this purpose). However, if you have a good telephoto and zoom lens, then backing up and zooming in will probably give a workable approximation to the Orthographic projection. Taking eight or ten images around the globe and one for each pole should give you enough to work with.

    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.

    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.

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    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.

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    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 :-)

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      Chashio is offline
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    What are these programs for stitching images to inside-out spheres, please?

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    The keywords are "spherical panorama software". There's a list of programs and some information here: panoguide: Creating spherical panoramas
    Chashio likes this.

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    I've also discovered the free program hugin, which (according to users on the google groups forum for the program - I haven't tried yet) will be able to do just what I want.

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    Guild Master Gracious Donor Midgardsormr's Avatar
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    Incidentally, for these little utilities that are Windows-only, I find that VirtualBox running TinyXP is quite useful. Get a Windows environment running inside your Mac OS, and it's free. Well, technically, you do need a Windows XP license to be legal, but those things are pretty easy to come by.
    Bryan Ray, visual effects artist
    http://www.bryanray.name

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    Quote Originally Posted by waldronate View Post
    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

    Quote Originally Posted by waldronate View Post
    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.

    Quote Originally Posted by Lingon View Post
    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...

    Quote Originally Posted by AvuncularMontague View Post
    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...

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