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Thread: Shifting an equirectangular map north/south

  1. #1
    Guild Adept acrosome's Avatar
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    Default Shifting an equirectangular map north/south

    Here I have a fine world map:

    Click image for larger version. 

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    It is Venus, churned through Wilbur and rotated 180 degrees (for reasons).

    Let's say that during the terraforming process the planetary engineers stopped Venus's rotation, and then later re-spun it but on a slightly different axis. Is there software out there that will let me manipulate this image to change the axis of rotation? Sort of shift everything north by 15 degrees, while appropriately deforming things to keep the equirectangular projection?

    Actually, I would want to do this with the original Venus grayscale heightmap. But is there a straightforward way to do this?
    My decade-long worldbuilding project: https://cartographersguild.com/showt...=25569&page=10

  2. #2

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    I don't know this for sure, so please don't kill me if it isn't possible, but I think you may be able to do that in NASAs GProjector.

    If you haven't already got it, its here.

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    Software Dev/Rep Hai-Etlik's Avatar
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    Quote Originally Posted by Mouse View Post
    I don't know this for sure, so please don't kill me if it isn't possible, but I think you may be able to do that in NASAs GProjector.

    If you haven't already got it, its here.
    No, G.Projector can't do oblique projections. GDAL should be able to do it but it'll involve learning how to set up a custom oblique projection in Proj.4 syntax.

    The energy requirements for altering a planet's rotation like that are up in "if you can do it, you don't need to" territory.

  4. #4
    Guild Grand Master Azélor's Avatar
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    Stopping the rotation of the planet would destroy everything on the surface except the landscape.
    They might not need to stop the planet to change the axis.

  5. #5
    Guild Artisan Charerg's Avatar
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    Actually you can shift the continents northwards 15 degrees with GProjector while preserving their shapes. Just pick the "Equirectangular Oblique" and set the center at -15N.

    Looks like this for Earth, as an example:
    Click image for larger version. 

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    I assume this is what you wanted to do?

    Edit: I should clarify that this shifts the "center of the map" to -15 N (aka 15 S), so if the "center" is located at 0 E (over the Atlantic), then areas around the Pacific will actually shift southwards (see Alaska and Eastern Siberia in the example map).
    Last edited by Charerg; 12-18-2017 at 05:44 PM.

  6. #6
    Guild Grand Master Azélor's Avatar
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    It doesn't preserve the latitudes. It bends.

  7. #7
    Guild Artisan Charerg's Avatar
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    Quote Originally Posted by Azelor View Post
    It doesn't preserve the latitudes. It bends.
    Not the original latitudes, no. What it effectively does is that it shifts the location of the poles. The shapes of the continents are preserved, however, and the resulting map can projected to a globe no problem.

    Here's a view from the south pole from the example map:
    Click image for larger version. 

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    As I understand it, this is what acrosome wanted to do, to shift the rotational axis?
    Last edited by Charerg; 12-18-2017 at 06:11 PM.

  8. #8
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    G.Plates will do it, and so will fractal terrains, I don't know how to load a custom image into fractal terrains but it's very simple to get a raster image into G.Plates and start dragging things around.

  9. #9

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    Resolution is quite low on the demo version of Fractal Terrains

  10. #10
    Guild Adept acrosome's Avatar
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    Quote Originally Posted by Charerg View Post
    I assume this is what you wanted to do?
    Yes! That's what I wanted to do! Awesome. (I already have GProjector, actually, I just didn't know it could do that.)

    EDIT--- But, dammit, the largest image that GProjector will export is 7500 px in the longest dimension and my original is 8192 px. I'd hate to lose the resolution.
    Last edited by acrosome; 12-18-2017 at 08:30 PM.
    My decade-long worldbuilding project: https://cartographersguild.com/showt...=25569&page=10

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