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Thread: A Continental Map from a Spherical World Map?

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    Guild Member ScotlandTom's Avatar
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    Default A Continental Map from a Spherical World Map?

    I'm looking to create a continental map for one of the continents from my recently created World of Eldaron map. The continent in question is Auctoria. Since completing the world map I wrapped it to a sphere, which I'm going to assume to be completely accurate at this stage. The world is a sphere with a 25,000 mile circumference (just a touch larger than Earth). After wrapping the map to the sphere in Photoshop I've wound up with this:

    A Continental Map from a Spherical World Map?-world-eldaron-sphere.jpg

    My main question at this stage: How can I take what I have now (the spherical map, the original .psd that includes the original continental outlines) and create a new rectangular map of the continent of Auctoria with as little distortion as possible? I realize that with a continent this size distortion is inevitable, I'd simply like to minimize the distortion as best as I can. Ideally this continental map will be used as the basis for all the regional maps I intend to make.

    I have the following tools available to me:
    Photoshop CS4
    Illustrator
    Any other free/open source tools that might be of use

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      Lukc is offline
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    Well ... as far as I can tell, what you have here is already a projection of the 3D map onto a flat plane. I'm assuming you want a different projection?

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    Guild Member ScotlandTom's Avatar
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    Right... well I guess another question would be: What projection would be best to use? Is the one I've already got going to be useful for zooming in and creating regional maps with reasonably accurate scales? Honestly this is an area of mapping I am totally and completely unfamiliar with so any insight/advice is absolutely welcome.

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      Hai-Etlik is offline
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    Well it appears you are treating your original map as being Equidistant Cylindrical and covering the whole globe and including the borders. Based on the dimensions of the map, the standard parallels of the projection are at 41.41 degrees.

    So on your original map, the borders are actually covering up part of the map, everything between 41.41 N and 41.41 S is squashed horizontally, and everything outside that is stretched horizontally as drawn on the map. Unfortunately, it doesn't look like that, which is why it looks distorted on the globe with pinching at the poles and stretching at the equator. That's why those little peninsulas stretching up to the pole are pinched down to needles on the globe, and why they would still look so if you were to make a regional map in a suitable projection. If it's in equidistant cylindrical, then they really are needlelike.

    On the other hand, your original map also has rhumb lines and a compass rose, which imply that it is in Normal Mercator projection (The only projection that preserves compass bearings globally). Treating it as such could actually fix things.

    Treating it as Mercator means that the top and bottom edges would not be at the poles. You could shift it up and down a bit, but assuming the map is centred on the equator then after trimming off the borders, the top and bottom edges would be in the vicinity of 75 N/S

    I've overlayed it on a Mercator map of Earth (Going to 82 degrees N/S) and reprojected into Equidistant Cylindrical (With standard parallel at 0)
    A Continental Map from a Spherical World Map?-eqdcyl-eldaron.png
    You should be able to map this onto a sphere the same way you did with the original map.

    Now with all that out of the way, if you decide to treat it as a Mercator map from 75 N to 75 S, then the continent in question would probably fit well in a conic projection. Your three main choices are Lambert Conformal Conic, Albers Equal Area Conic, and Equidistant Conic. The first can be thought of as preserving shapes best, The second obviously preserves areas, and the last is sort of a middle ground, and also preserves north-south distances.

    For the version you have now where it covers a full 8th of the planet's surface, a conic could still be used, though with far more distortion. I might also consider Chamberlain Trimetric.

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      RobA is offline
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    Once you have done what Hai-Etlik suggests (getting a true equi-rectangular projection) in a 2:1 ratio pull it into flexprojector http://www.flexprojector.com/

    I'm personally a fan of the Winkel Tripel for this map:

    A Continental Map from a Spherical World Map?-wt.jpg

    -Rob A>

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      Hai-Etlik is offline
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    Quote Originally Posted by RobA View Post
    Once you have done what Hai-Etlik suggests (getting a true equi-rectangular projection) in a 2:1 ratio pull it into flexprojector http://www.flexprojector.com/

    I'm personally a fan of the Winkel Tripel for this map:

    Click image for larger version. 

Name:	wt.jpg 
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ID:	41740

    -Rob A>
    Winkel Tripel is a fine projection for a global map, but it's not a particularly good choice for larger scale maps. Flex Projector lacks any suitable projections for larger scale maps like conics, azimuthals, or transverse Mercator.

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      RobA is offline
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    Quote Originally Posted by Hai-Etlik View Post
    Winkel Tripel is a fine projection for a global map, but it's not a particularly good choice for larger scale maps. Flex Projector lacks any suitable projections for larger scale maps like conics, azimuthals, or transverse Mercator.
    I've also used Hugin to re-project equirectangular, but again, it is a limited subset. Can you suggest any raster re-projection software?

    -Rob A>

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    Guild Member ScotlandTom's Avatar
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    Wow, okay first, thanks for the lesson in different types of map projections and whatnot, Hai-Etlik. I will likely have to reference your post numerous times in the future because I'm sure I'll never remember it all.

    Second, I did some fiddling about with the map in Google Earth yesterday and came to the conclusion that my original world map (done in a 2:3 ratio) was not going to be assumed entirely accurate. I am now assuming that the original, unnamed cartographer was not aware of the actual distances across the oceans and seas. He also made a few errors in the island chains closer to the poles. From now on, the totally accurate Mercator map of Eldaron looks like this:

    A Continental Map from a Spherical World Map?-world-eldaron-bw.jpg

    Which then looks like this when I bring it into Google Earth:

    A Continental Map from a Spherical World Map?-eldaron-google-earth.jpg

    (Yes, I've started experimenting with height mapping.) So now my question is, how can I create a rectangular continental map of Auctoria (not the entire world, just the continent) with a viewing angle from over the continent at about 30º-35º latitude? Something more akin to this:

    A Continental Map from a Spherical World Map?-north-america-3.gif

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      Hai-Etlik is offline
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    The example image of NA looks like it's probably an azimuthal projection though I may be wrong. If so, it could be in Stereographic, Lambert Azimuthal Equal Area, or Equidistant Azimuthal. The Google Earth image is either Orthographic, or a perspective projection close to it (bbth also azimuthal projections)

    For an azimuthal projection, you would select a point of tangency at roughly the centre of the continent. Set up the projection software and then crop the result to a rectangle.

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      Hai-Etlik is offline
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    Quote Originally Posted by RobA View Post
    I've also used Hugin to re-project equirectangular, but again, it is a limited subset. Can you suggest any raster re-projection software?

    -Rob A>
    G.Projector if you want something easy to use, but fairly limited, it's likewise focused on full globe projections, and doesn't support vector data at all but does support conic and azimuthal projections. The command line tools for GDAL/OGR or a full GIS if you want a full power solution, though they are more complex and focused on larger scale data-sets in a way that can make them a bit of a pain when dealing with global data sets at times.

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