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Thread: What am I missing?

  1. #11
      Lalaithion is offline
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    Quote Originally Posted by Hai-Etlik View Post
    Yes, it does matter. At the equator, the "box" bounded by a small equal angles of latitude and longitude is approximately a euclidean square. As you move into higher latitudes, it narrows and becomes more like a trapezoid, until you get a very narrow triangle at the poles.

    A map in a projection suitable to a restricted extent is going to try to minimize distortion within that extent so those shapes will be approximately preserved: At mid latitudes, a lat-long box with equal angles would be approximately a rectangle with an aspect ratio determined by the latitude. In a normal cylindrical projection it will be projected as precisely a rectangle.

    In tangent normal equidistant cylindrical, all such boxes are squares, but this means that they, and all other shapes, are being stretched out east-west. Which is a rather noticeable form of distortion. So my point stands, to get a square lat-long graticule, you can either be covering a small area near the equator in a projection that minimizes distortion within your extent. Or you can be in Plate Carree, in which case you can be anywhere, but would have significant and ugly distortion unless you are covering a small area near the equator.
    The equator is geographically significant, yes, but it mainly just is another invisible line on our map. You could center a equirectangular map anywhere in the world, effectively treating any great circle (or really, any circle at all!) as the equator and any two points as poles- here we see the cassini projection of our world:
    What am I missing?-cassini_blank.png
    Which is basically the equirectangular projection with the prime meridian as the equator (it is called a cassini projection, just like how a plate careé is just an equirectangular projection centered on the equator).

    (I agree with you that it is anachronisctic, but we can make exceptions in order to provide our readers/players/others clearer maps.)
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  2. #12
      Hai-Etlik is offline
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    Quote Originally Posted by Lalaithion View Post
    The equator is geographically significant, yes, but it mainly just is another invisible line on our map. You could center a equirectangular map anywhere in the world, effectively treating any great circle (or really, any circle at all!) as the equator and any two points as poles- here we see the cassini projection of our world:
    Click image for larger version. 

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    Which is basically the equirectangular projection with the prime meridian as the equator (it is called a cassini projection, just like how a plate careé is just an equirectangular projection centered on the equator).

    (I agree with you that it is anachronisctic, but we can make exceptions in order to provide our readers/players/others clearer maps.)
    My comments specifically regard a projection that produces square graticules. That was the entire point of my post. If the graticule hadn't been a square grid I wouldn't have said anything.

    I suppose you might have some other coordinate system. I even mentioned that in my original post. But that would be extremely anachronistic and out of place on a map like this.

    If you took that transverse projection and plotted a graticule on it, you'd get something very wonky looking. If you come up with some other angular coordinate system not aligned to the poles, and then align a tangent equidistant cylindrical projection to it, and the area you are mapping happens to be near the circle of tangency (effectively the equator of the new non-rotation defined coordinate system). Then sure, you could make this sort of work out. This is however such an incredibly bizarre and unlikely thing to do, and so much more complex and unintuitive to explain compared the already hard to grasp concept of map projections in the first place, that I stuck with the simple "equator" rather than trying to explain what a "circle of tangency" was so that I could then use the term to be more precise and cover this insane possibility.

  3. #13
      ravells is offline
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    This is a really interesting and educational discussion for me (apologies to imaginer of darkness for the hijack of thread).

    The conclusions that I'm drawing from it so far are these:

    Assuming a spherical planet which has night and day means that the planet is spinning around an axis and orbiting a star.

    Where the axis intersects the planet are the poles ( call them: North and South, but they could be called anything). The great circle which bisects the planet in half and is equidistant from the poles is the equator.

    Graticules are drawn to serve a purpose. One purpose of lines of longitude are to measure distance in relation to time (which is why we have minutes and seconds of arc). Because lines of longitude are 'parallel' (so far as an arc can be parallel to a line) to the axis, they cannot be put anywhere else and still serve the same purpose - if you put them anywhere else they would not be parallel to the axis. Lines of lat are lines drawn at right-angles to lines of long and tell you how far you are from the poles.

    So this means (to me anyway) that once a planet is spinning, the equator, lines of long and lines of lat (to be used for the purposes we use them for) can only be in one place.

    Is that about right?

  4. #14
      Lalaithion is offline
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    Let me try and explain this without going into really advanced cartography. If you take a map of an area, and you draw "square miles" on it, and then you place geographical and human features on it while trying to keep things as accurate as possible: imagine that you don't know that the earth is a sphere-- in this case you will end up with an equirecrangular projection with either a different "standard parallel" than the equator or a different great circle acting as the equator.

    And you're right: that's what lat/long lines are for. But you could decide randomly to choose any antipodal points as your poles instead of the axis of rotation: these new points would create new, arbitrary lat/long lines with a new, arbitrary equator.

  5. #15
      jbgibson is offline
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    Another use for a grid on a map is strictly location. Particularly if the cartographer labeled the lines in a way that did not imply lat/lon, one could put a square grid across any projection. Now, the present instance (desperately trying to be germane :-) ) is almost the exact opposite of that, presuming that one would want to find things on land more than on water. But even here one could easily imagine a set of cartographic standards where for whatever use this map was made (in-character) one puts the locator grid 'at sea' where nothing'll be obscured, and the user is left to infer the connecting lines. Plenty of atlas maps do that, where the grid is nothing more than tic-marks around the periphery - labeled, say, A-Z at top & bottom, 1-n on left and right. I mention atlases, because the locator-use-case is good for where you have tabular or textual info you want to relate to the map, but either not enough space for on-map labelling, or else the text is indexed in some way that would not be easy to depict graphically -- say, alphabetized cities or hydro features. Sure, one could use lat/lon for that, and whatever curvy grid suited the projection.... but while more exact, lat/lon might be a bit too 'wordy' for a compact table. And it's easier for a layman to trace imaginary straight lines up and down a map than a set of curved lines of varying spacing.

    So Hai-Etlik is exactly correct that we would infer a certain class of projections from a simple square grid, but you could steer the user away from that inference by non-lat/lon labeling*, and/or by somehow removing any possibility that you're trying to show lat/lon - say by running a real lat/lon line or two askew through the view, or pointing north off-axis. Yes, I know a compass is way misleading when 'real' north varies across a map sheet. A period cartographer might look at you oddly for such an objection, if of course compass indications only count right at the capital city - don't you know the Emperor's Directional Mapping Decree of 1415 j.l.? :-) <shrug> There'll always be some mismatch somewhere, even if you hand out globes or sections thereof. Much the same as you'd avoid coloring land blue and ocean tan by convention, to avoid confusion, getting across some indication of the projection in use can minimize confusion.

    Unless the in-character cartographer WANTS to confuse... lying with maps is a whole 'nother subject.


    * mind you, other cultures could label their lat/lon with letters one way and integers the other :-). at some point ya gotta quit making your fantasy/ fictional world too different from earth, lest you bog down your game/ book/ movie with constant explanations.

  6. #16
      jbgibson is offline
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    As for the original question of general improvements to suggest - think why the Eleut is all desert. Winds carry moisture. Yes, prevailing winds could 'scrape off' moisture on high mountains (the rain-shadow effect), but then the whole downwind side of the range would be as dry as the flatlands downwind. So that river would still not have much of a source, even if tweaked over to arise in the mountians. Plus, the range is hardly extensive enough to shelter the whole peninsula. If there's no single direction of prevailing wind, but the more usual case of seasonal variation, you might have scrubby grasslands instead of bone-dry desert. Now, latitudes in the 30-ish north or south range tend to be dryer based on overall vertical as well as horizontal airflow patterns, but if you get wind across ocean onto land, you're going to see some rainfall. I'd buy the whole peninsula being savannah, maybe, with the northern area (IF downwind of the mountains) as being outright desert. Or if by desert you mean "useless wilderness, somewhat dry" you could call the whole thing desert. The whole middle of North America once was referred to as desert, even places that now have grain and cattle. You might have to lose the southernmost saguaro symbols though :-).

    Run even some low hills across the north of that peninsula, and you have a visual reason for water to flow south. And if you figure the far NE to have a bit of rain, you get some water to flow - maybe bend the headwaters over that way? Would it suit your story for the lower reaches of the river to be intermittent?

    How about another mountain range down the whole east coast? That would make a dandy rain-barrier, again postulating certain wind patterns. The seaward side of that range might be extremely wet. That would make the river even less plausible though.

    I do like the map, by the way - plenty of detail to be useful, neatly done, clear.
    Last edited by jbgibson; 10-08-2012 at 03:46 PM.

  7. #17
      Zipster is offline
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    For not having any map-making background or anything, that's quite impressive.

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