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Thread: Map Projections

  1. Top | #11
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    Naming the inventors of various map projections, Johann Lambert might top the list with several different projections. This amazing polymath was one of the greatest 18th-century geniuses, but his fame is drowned out by names like Lagrange and Gauss (who each also designed map projections).

    Prior to Lambert, inventors of map projections include three ancient Greeks — Thales, Hipparchus, and Ptolemy — and, from the 11th century in the Golden Age of Islamic Science, Abu Rayhan Mohammed ibn Ahmad Al-Biruni. Like Lambert, al-Biruni was a great polymath who is often overlooked. He is credited with the azimuthal equidistant projection and the Nicolosi polyconic method.

  2. Top | #12
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    The projection that tells us what we really need to know...


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    There is another kind of distance-preserving map projection, the all-distance one:

    Two-point equidistant projection

    One chooses two reference points and then uses the great-circle distances to those points.

    Chamberlin trimetric projection

    Uses three reference points, but since one cannot get an exact match of distances, one calculates the intersection for each pair of reference points then the average of those.


    The previously-mentioned kind was a distance-angle one.

  4. Top | #14
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    China, vietnam, and the two Koreas put together look like a giant chicken.

  5. Top | #15
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    The azimuthal and cylindrical equidistant projections are special cases of Riemannian or geodesic normal coordinates, a generalization of polar and spherical coordinates.

    Consider finding the coordinates of some point X.

    One starts with some reference point O and then finds the geodesic between O and X, leaving aside the question of its uniqueness. The geodesic-normal coordinates are thus (distance from O to X along the geodesic, direction along the geodesic at O).

    I tried to analyze that case, and I found it difficult, much more difficult than being area- or volume-preserving, shape-preserving, or geodesic-preserving. One can analyze it near the reference point, but that requires expanding in a series near it, and doing so becomes *very* difficult *very* quickly. The space metric turns out to be a power series in the position relative to the reference point, with the coefficients being functions of the space curvature that get very complicated very quickly.

  6. Top | #16
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    Creating a more accurate flat map of the Earth
    noting
    The Most Accurate Flat Map of Earth Yet - Scientific American

    "Previously, Goldberg and I identified six critical error types a flat map can have: local shapes, areas, distances, flexion (bending), skewness (lopsidedness) and boundary cuts."

    It is mathematically impossible for a sphere-to-plane projection to get everything right, so one must compromise.
    The object here is to find map projections that minimize the sum of the squares of the errors—a technique that dates back to the mathematician Carl Friedrich Gauss. The Goldberg-Gott error score (sum of squares of the six normalized individual error terms) for the Mercator projection is 8.296. The lower the score, the smaller the errors and the better the map. A globe of the Earth would have an error score of 0.0. We found that the best previously known flat map projection for the globe is the Winkel tripel used by the National Geographic Society, with an error score of 4.563. It has straight pole lines top and bottom with bulging left and right margins marking its 180 degree boundary cut in the middle of the Pacific.
    It's double-sided, a squashed-globe map.
    This double-sided map has a Goldberg-Gott error score of only 0.881 versus 4.563 for the Winkel tripel. It beats the Winkel tripel in each of the six error terms! It has zero boundary cut error since continents and oceans are continuous over the circular edge. It has a remarkable property no single-sided flat map possesses: distance errors between pairs of points (such as cities) are bounded, being off by only at most plus or minus 22.2 percent. In the Mercator and Winkel tripel projections, distance errors blow up as one approaches the poles and boundary cuts.

  7. Top | #17
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    New World Map Tries to Fix Distorted Views of Earth - The New York Times

    Princeton astrophysicists re-imagine world map, designing a less distorted, 'radically different' way to see the world

    Curvature in Map Projections
    How do you take a sphere and best project it onto a flat surface so that you can carry it around on a sheet of paper, or, better yet, view it on your monitor. The answer certainly depends on what you want to use your map for. Historically, people have used the Tissot Indicatrix in order to provide guidance. The method is simple: Imagine painting small circles on the earth (or any other planet -- above, we project the surface of Jupiter), and project the surface of the earth (and the circles) onto the map. Some applications will require that all of the circles still be circular (conformal projections). Some will require that all circles be of equal area. No map projection can meet both criteria.

    J. Richard Gott and I wanted to take this analysis another step forward. What happens when you project large bodies: the US-Canada Border, for example, or Australia, or the bands of Jupiter, onto a particular map projection? Are the features faithfully reproduced? To that end, we have introduced a formal measure of the flexion and skewness of a map.

    This can be visualized simply via the Goldberg-Gott Indicatrices. Pick a point on the earth and drive north 12 degrees (about 1300 kilometers). Even if you hit the north pole, don't turn your steering wheel. Follow a geodesic. Do the same thing but heading west, east, and south. From your perspective, you've drawn a big plus sign on the ground. When projected onto a map, this indicatrix immediately reveals the curvature of the projection. Moreover, if you connect the dots and close the indicatrix, you get an ellipse -- the Tissot ellipse.

    Rich and I have evaluated about 20 different projections and measured them with respect to area preservation, ellipticity of the Tissot, flexion (the bending of geodesics), skewness (the rate of change of speed on a map), boundary cuts, and interruptions between random points. All in all, we found that the best two overall projections are (respectively), the Winkel-Tripel (left), and the Kavrayskiy VII (right). See if you agree with our numerical assessment.
    But these articles don't give the math behind the projections and the error measures.

  8. Top | #18
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    [2102.08176] Flat Maps that improve on the Winkel Tripel
    Goldberg & Gott (2008) developed six error measures to rate flat map projections on their verisimilitude to the sphere: Isotropy, Area, Flexion, Skewness, Distances, and Boundary Cuts. The first two depend on the metric of the projection, the next two on its first derivatives. By these criteria, the Winkel Tripel (used by National Geographic for world maps) was the best scoring of all the known projections with a sum of squares of the six errors of 4.563, normalized relative to the Equirectangular in each error term. We present here a useful Gott-Wagner variant with a slightly better error score of only 4.497. We also present a radically new class of flat double-sided maps (like phonograph records) which have correct topology and vastly improved error scores: 0.881 for the azimuthal equidistant version. We believe it is the most accurate flat map of Earth yet. We also show maps of other solar system objects and sky maps.
    Gott and Goldberg earlier wrote
    [astro-ph/0608501] Flexion and Skewness in Map Projections of the Earth
    Tissot indicatrices have provided visual measures of local area and isotropy distortions. Here we show how large scale distortions of flexion (bending) and skewness (lopsidedness) can be measured. Area and isotropy distortions depend on the map projection metric, flexion and skewness, which manifest themselves on continental scales, depend on the first derivatives of the metric. We introduce new indicatrices that show not only area and isotropy distortions but flexion and skewness as well. We present a table showing error measures for area, isotropy, flexion, skewness, distances, and boundary cuts allowing us to compare different world map projections. We find that the Winkel-Tripel projection (already adopted for world maps by the National Geographic), has low distortion on most measures and excellent quality overall.

  9. Top | #19
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    Tissot's indicatrix - This is a measure of map-projection quality, doing so by showing the projection's amount of distortion at each point.

    (offsets on projection) = T . (offsets on sphere)

    For latitude b and longitude p to flat coordinates x and y:

    T11 = dx/db
    T12 = 1/cos(b) * dx/dp
    T21 = dy/db
    T22 = 1/cos(b) * dy/dp

    Area ~ det(T), so an equal-area transform keeps det(T) constant.

    The indicatrix is, as far as I can tell, D = transpose(T) . T

    Conformal: D = (some function) * I (the identity matrix)

    Let's find T and D for some projections.

    Azimuthal: x = r(b)*cos(p), y = r(b)*sin(p)

    T = {{r'*cos(p), - r/cos(b)*sin(p)}, {r'*sin(p), r/cos(b)*cos(p)}}
    det(T) = r'*r/cos(b)
    D = {{(r')^2, 0}, {0, (r/cos(b))^2}}

    Cylindrical: x = p, y = h(b)

    T = {{0, 1/cos(b)}, {h', 0}}
    det(T) = - h'/cos(b)
    D = {(h')^2, 0}, {1, (1/cos(b))^2}}

    Pseudocylindrical: x = p*w(b), y = h(b)

    T = {{p*w', w/cos(b)}, {h', 0}}
    det(T) = - h'/cos(b)
    D = {{(h')^2 + (w')^2, p*w*w'/cos(b)}, {p*w*w'/cos(b), (p*w')^2}}

  10. Top | #20
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    I read the Gott-Goldberg paper again, and the true value seems to be

    T = D . transpose(D)

    For azimuthal projections, the matrix has eigenvalues and eigenvectors
    • (r')^2 - {cos(p), sin(p)}
    • (r/cos(b))^2 - {-sin(p), cos(p)}


    For cylindrical projections, the matrix is
    {{(1/cos(b))^2, 0}, {0, (h')^2}}

    For pseudocylindrical projections, the matrix is
    {{(w/cos(b))^2 + (p*w')^2, p*w'*h'}, {p*w'*h', (h')^2}}

    Equal-area: det(T) = constant -> det(D) = constant
    Conformal: T = (scalar function) * (identity matrix)

    Tissot's indicatrix is customarily graphed as an ellipse with major axes sqrt(the matrix's eigenvalues), axes that point along the matrix's eigenvectors.

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