gistfile1.js

module.exports = function gk2geo() {

    /* Copyright (c) 2006, HELMUT H. HEIMEIER
    Permission is hereby granted, free of charge, to any person obtaining a
    copy of this software and associated documentation files (the "Software"),
    to deal in the Software without restriction, including without limitation
    the rights to use, copy, modify, merge, publish, distribute, sublicense,
    and/or sell copies of the Software, and to permit persons to whom the
    Software is furnished to do so, subject to the following conditions:
    The above copyright notice and this permission notice shall be included
    in all copies or substantial portions of the Software.*/

    /* Die Funktion wandelt GK Koordinaten in geographische Koordinaten
    um. Rechtswert rw und Hochwert hw müssen gegeben sein.
    Berechnet werden geographische Länge lp und Breite bp
    im Potsdam Datum.*/

    // Rechtswert rw und Hochwert hw im Potsdam Datum
    var args = arguments[0] || {};
    var rw = args.rw,
        hw = args.hw;
    if (rw == "" || hw == "") {
        lp = "";
        bp = "";
        return;
    }
    rw = parseFloat(rw);
    hw = parseFloat(hw);

    //  Potsdam Datum
    // Große Halbachse a und Abplattung f
    a = 6377397.155;
    f = 3.34277321e-3;
    pi = Math.PI;

    // Polkrümmungshalbmesser c
    c = a / (1 - f);

    // Quadrat der zweiten numerischen Exzentrizität
    ex2 = (2 * f - f * f) / ((1 - f) * (1 - f));
    ex4 = ex2 * ex2;
    ex6 = ex4 * ex2;
    ex8 = ex4 * ex4;

    // Koeffizienten zur Berechnung der geographischen Breite aus gegebener
    // Meridianbogenlänge
    e0 = c * (pi / 180) * (1 - 3 * ex2 / 4 + 45 * ex4 / 64 - 175 * ex6 / 256 + 11025 * ex8 / 16384);
    f2 = (180 / pi) * (3 * ex2 / 8 - 3 * ex4 / 16 + 213 * ex6 / 2048 - 255 * ex8 / 4096);
    f4 = (180 / pi) * (21 * ex4 / 256 - 21 * ex6 / 256 + 533 * ex8 / 8192);
    f6 = (180 / pi) * (151 * ex6 / 6144 - 453 * ex8 / 12288);

    // Geographische Breite bf zur Meridianbogenlänge gf = hw
    sigma = hw / e0;
    sigmr = sigma * pi / 180;
    bf = sigma + f2 * Math.sin(2 * sigmr) + f4 * Math.sin(4 * sigmr) + f6 * Math.sin(6 * sigmr);

    // Breite bf in Radianten
    br = bf * pi / 180;
    tan1 = Math.tan(br);
    tan2 = tan1 * tan1;
    tan4 = tan2 * tan2;

    cos1 = Math.cos(br);
    cos2 = cos1 * cos1;

    etasq = ex2 * cos2;

    // Querkrümmungshalbmesser nd
    nd = c / Math.sqrt(1 + etasq);
    nd2 = nd * nd;
    nd4 = nd2 * nd2;
    nd6 = nd4 * nd2;
    nd3 = nd2 * nd;
    nd5 = nd4 * nd;

    //  Längendifferenz dl zum Bezugsmeridian lh
    kz = parseInt(rw / 1e6);
    lh = kz * 3
    dy = rw - (kz * 1e6 + 500000);
    dy2 = dy * dy;
    dy4 = dy2 * dy2;
    dy3 = dy2 * dy;
    dy5 = dy4 * dy;
    dy6 = dy3 * dy3;

    b2 = -tan1 * (1 + etasq) / (2 * nd2);
    b4 = tan1 * (5 + 3 * tan2 + 6 * etasq * (1 - tan2)) / (24 * nd4);
    b6 = -tan1 * (61 + 90 * tan2 + 45 * tan4) / (720 * nd6);

    l1 = 1 / (nd * cos1);
    l3 = -(1 + 2 * tan2 + etasq) / (6 * nd3 * cos1);
    l5 = (5 + 28 * tan2 + 24 * tan4) / (120 * nd5 * cos1);

    // Geographischer Breite bp und Länge lp als Funktion von Rechts- und Hochwert
    bp = bf + (180 / pi) * (b2 * dy2 + b4 * dy4 + b6 * dy6);
    lp = lh + (180 / pi) * (l1 * dy + l3 * dy3 + l5 * dy5);

    if (lp < 5 || lp > 16 || bp < 46 || bp > 56) {
        lp = "";
        bp = "";
    }
    return {
        lat : bp,
        lng : lp
    };

};