PDA

View Full Version : Map Projection Question



Sapiento
01-13-2013, 06:21 AM
I know we have some members which are experts in map projections and I need some support/help/info for a project I'm working on.

The goal is to create a world map. I decided for a map in a 2:1 ratio. And now the difficult part: I have two images of the poles as seen from space. These I want to get on the map in the north and south.

What is the best way to get a relatively good result for this? Which (free) software is able to do this?

I'm thankful for any suggestions.

AzurePlanet
01-13-2013, 05:39 PM
If your images of the poles have associated polar projections, you could change them to equirectangular, via a GIS software, like Quantum GIS (free).
The result could not be so good because of excessive distortions.

Sapiento
01-14-2013, 03:07 AM
If your images of the poles have associated polar projections, you could change them to equirectangular, via a GIS software, like Quantum GIS (free).
The result could not be so good because of excessive distortions.

Thank you. I never worked with this software, but I will give it a try.
Distortions are inevitable, no matter what I do.

vorropohaiah
01-14-2013, 03:25 AM
if you have an equirectangular projection of the world you can use the NASA G. Projector to convert it to a globe, though its probably unlikely that you have an equirectangular projection. and even so the poles are usually so distorted that youre better off drawing it flat and then adding the graticules on top.

Sapiento
01-14-2013, 07:18 AM
if you have an equirectangular projection of the world you can use the NASA G. Projector to convert it to a globe, though its probably unlikely that you have an equirectangular projection. and even so the poles are usually so distorted that youre better off drawing it flat and then adding the graticules on top.

Thanks. Half of this NASA Website is down at the moment, I will try the program once the problems are solved.

vorropohaiah
01-14-2013, 08:01 AM
Thanks. Half of this NASA Website is down at the moment, I will try the program once the problems are solved.

yeah, thought so. I was trying to give a direct link though it wasn't working earlier. its a pretty intuitive to use (I have no experience in GIS software at all, and dont even know if they're related at all)

aquarits
01-14-2013, 09:00 AM
If u have a equirectangular, guess Photoshop can do it too using 3D tools.
Not free :D and dunno if u used it for your map :P

Hai-Etlik
01-14-2013, 11:47 PM
The poles "as seen from space" would be approximately Orthographic projections. Is it actually as seen from space or is it just a polar aspect azimuthal projection in general? There are others it could be besides Orthographic which would be centred on a pole, but wouldn't be as seen from space. Georeferencing and reprojecting an orthographic projection probably isn't something you are going to find an easy tool for. Converting between Polar Equidistant Azimuthal and Plate Carree though is quite simple as it's just a matter of converting from polar to cartesian coordinates which many graphics tools can do. Stereographic is also a possibility which would probably be as much work as Orthographic, but would give better results in the end since it doesn't "squash" things.

Also, 2:1 is an odd aspect ratio for a world map. There are a number of projections that do project a globe to a 2:1 area, but none of them are well suited to the general reference maps popular on the guild. Plate Carree (Normal Tangent Equidistant Cylindrical/Equirectangular) is good if you want to easily convert to other projections or wrap it on a 3D globe, but it's a poor choice for a finished map for display, Mollweide, Sinusoidal, and Hammer are all equal area and work well for thematic maps but aren't common for reference maps. I think there should also be a parametrization of the Cylindrical Equal Area projection that has an aspect of 2:1, but I don't know the parameters, and wouldn't recommend using any parametrization of Cylindrical Equal Area (Including Gall-Peters and Hobo-Dyer) for anything really as it's horribly ugly.

jbgibson
01-15-2013, 02:40 AM
Don't know if archive.org collects 4.5 Mb zip files, but you could give one of their captures of the Goddard site a try: NASA GISS: G.Projector: Download for Windows (http://web.archive.org/web/20120305221738/http://www.giss.nasa.gov/tools/gprojector/download_win.html)

Softpedia seems to have a download of the Mac version (http://mac.softpedia.com/get/Math-Scientific/G-Projector.shtml).

Sapiento
01-15-2013, 08:17 AM
Thank you all for this very informative input. I will take it into account and try the software to find a solution.

Sapiento
01-15-2013, 02:57 PM
Ok, the GIS software is more difficult than I had expected.

This is one of the poles - flat picture, I don't know which projection. How do I turn it into a flat small band at the southern top of the map? I tried G.Projector, but couldn't load it in pole position.

waldronate
01-15-2013, 03:56 PM
http://www.jhendor.de/tutorials/cc2-ft.pdf describes how Ralf Schemann over at ProFantasy used a little toy I wrote a long time ago to do the sort of thing that you're asking. You would be particularly interested in the "Converting the image to an equirectangular projection" step, I'm guessing.

I haven't had a pressing need to use one of the projection tools out there, so I can't help you much with those, I'm afraid.

This image shows what the UI for ReprojectImage would look like to unproject a stereographic image
51261

And this is a 4096x2048 export from those settings.
51262

Other projections are pretty straightforward.

ReprojectImage is an ancient tool, but it sort of works for this kind of thing.

Sapiento
01-16-2013, 03:12 AM
Excellent! Thank you, waldronate, that's exactly what I needed.
Another proof of the excellent support and help in this forum. I took the freedom to donate a few bucks to the site.