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Rasstik
04-05-2013, 10:11 PM
Hello, CG Forum.
I've not found a thread describing my problem exactly, and so I've joined to ask for assistance.

I've been given the shapes of continents and islands to eventually transfer to a tangible globe; I do not, however, know how to go about shaping them to correctly fit to a sphere. Many of the members here have already expressed and dealt with ways to warp images to sit tidily on a digital sphere, but it seems that there has been no discussion on how to preserve a certain shape and proportion when transferring that shape to a rectangle or to an "sinusoidal interrupted" map which can then be applied to an approximate globe.
I am particularly interested in how to preserve a shape around the poles.

I will provide two or more examples of the continental shapes in an upcoming post, or edit this one should no one have responded by then.

Hai-Etlik
04-11-2013, 01:12 AM
What you are going to need to do is decide where on the globe each continent is, then come up with an appropriate regional projection for that location. Then georeference the given image in that projection. Repeat that for each of the images you've been given. Then you can reproject them to a common projection and combine them, then reproject that to your interrupted sinusoidal projection. You're pretty much going to need a GIS for all that.

waldronate
04-11-2013, 03:11 AM
The most important point here is probably that you need to understand that there will be distortion in your images and it's likely that your source material wasn't carefully drawnin a particular projection. If it's just the traditional "flat map" that assumes that the world is truly flat, then you'll need to decide which is more important: relative shape or relative size. If you can go for some significant compromises in shape and area for large continents, then there are a couple of physical methods that should work nicely.

First, get a blank globe. There are discussions about getting a blank globe here on the forums, but the simplest way is to visit your local thrift store to buy a used globe and the hardware store for a can of white spray paint. The most basic method is just freehanding the shapes directly on the globe; that might be good enough. If you have a projector of some sort, then you can physically project the images onto your globe and trace them directly on the globe. Both of these techniques require a little artistic ability on your part.

Next on the list is to do as Hai-Etlik says, but there are a number of programs out there that don't require that you master GIS concepts. You'll need to understand that projections cause distortion, but other than that there's not much to do except decide where the continents go and work from there. I recommend using the Equirectangular projection for your intermediate projection because it's also the basic UV projection used in many 3D modeling program. If you can paste your images onto a sphere in a 3D modeling program, you can probably get it to generate what is effectively an equirectangular projection (and that is in fact method 3: use a 3D modeling program and use an orthographic projection to push the images down onto the virtual globe).

Long ago, I wrote a program called ReprojectImage ( http://www.ridgenet.net/~jslayton/ReprojectImage.zip ) that will let you take an image and drag a grid over it and see the results on an Equirectangular projection right next to it. Not perfectly accurate, but it's a pretty simple thing to operate. Reproject each of the images and then use something like Photoshop to assemble them and you'll have your equirectangular globe. Programs like Hugin will also do the same sort of thing ( How to create a little planet using Hugin | Ultrawide (http://ultrawide.wordpress.com/2008/11/20/how-to-create-a-little-planet-using-hugin/) shows how to make a panorama, but the idea is the same).

I also wrote a thing for ProFantasy called Fractal Terrains that will ingest (among other things) an Equirectangular image and can output (among other things) an image in other projections. The basic package includes a number of projections, including interrupted sinusoidal and interrupted stereographic. You can define your own if you've a mind to.

There are many other tools out there that will do this same sort of thing; G.Projector ( http://www.giss.nasa.gov/tools/gprojector/ ) is a popular one.

Once you have the gores, then it's just a paste job. I recommend making separate endcaps for the globe to avoid having to fit lots of tiny slivers at the poles. Alternately, use a daisy-type projection ( Maps and Globes | USGS Astrogeology Science Center (http://astrogeology.usgs.gov/maps/planetary-maps-and-globes) shows an example) and paste away from that. I keep meaning to teach Fractal Terrains that projection, but it keeps falling through the cracks in the too many things I have to do each year...

Anyhow, Hai-Etlik is a GIS person and naturally gravitates in that direction, with a focus on precision and accuracy. Most of the folks hereabouts gravitate more in the Photoshop and GIMP direction, with a focus on getting things to look good enough. I'm not saying that GIS things are bad, but most folks don't have the technical background or willingness to study long enough to get that background. For those folks, I've aimed more at simpler tools.

The way to preserve shape at the poles is to use a polar aspect of a projection. The attachment shows what a polar stereographic to equirectangular conversion might look like in the ReprojectImage program described above.
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Rasstik
05-04-2013, 11:07 AM
I apologize for my tardiness in replying, but there was a rather severe storm that interrupted electricity near my home.

Thank you both for what were prompt and detailed replies.
This forum is always a wonderful resource.

jbgibson
05-05-2013, 07:08 PM
When you pick a method, would you show us a bit of a work-in-progress as you go, please? Don't be shy about whether it turns out just as you want -- missteps and false starts teach as much as the straight path to perfection would (as if we ever reach that ;-) ).

WillP
05-06-2013, 11:40 AM
Waldronate:

I'm impressed with this little bit of software. I just plugged the continent I am working on into it and using the Mollyweide projection at 2.5 scale I got these.

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