Question

I’m using the Core Data Framework to manage a set of accounts which also include geospatial (GPS) coordinate data for each account. How can I query against this data based on position of the device to get a list of accounts within x feet and list them in order of distance?

Answer 1

to get you started, here’s a method i use in my iOS app that returns the distance in meters between two CLLocationCoordinate2D locations, assuming the Google Spherical Mercator Projection (if you want to use another projection, you can specify the appropriate flattening ratio value (f) and semi-major axis value (a). if you want the forward and backward azimuth values between the coordinates, you can uncomment and return the faz and baz values along with the distance by defining your own struct. this method can be used to add the distance to each of your ‘account’ objects and the current location being reported by your CLLocationManager object, then you could easily sort and filter an array of account objects based on their distances.

based on code by Gerald Evenden located here: http://article.gmane.org/gmane.comp.gis.proj-4.devel/3478

#define PI 3.141592653589793238462643
#define EPS 5e-14
#define DEG_TO_RAD 0.0174532925199432958

// returns the geodesic distance in meters between two coordinates based on the google spherical mercator projection.
- (int) geodesicDistanceFromCoordinate: (CLLocationCoordinate2D) fromCoord toCoordinate: (CLLocationCoordinate2D) toCoord {
    double c, d, e, r, x, y, sa, cx, cy, cz, sx, sy, c2a, cu1, cu2, su1, tu1, tu2, ts, phi1, lam1, phi2, lam2, f, baz, faz, s, a;

    phi1 = fromCoord.latitude * DEG_TO_RAD;
    lam1 = fromCoord.longitude * DEG_TO_RAD;
    phi2 = toCoord.latitude * DEG_TO_RAD;
    lam2 = toCoord.longitude * DEG_TO_RAD;
    f = 0;  //google's spherical mercator projection has no flattening
    a = 6378137;  //earth's axis in meters used in google's projection

    r = 1. - f;
    tu1 = r * tan(phi1);
    tu2 = r * tan(phi2);
    cu1 = 1. / sqrt(tu1 * tu1 + 1.);
    su1 = cu1 * tu1;
    cu2 = 1. / sqrt(tu2 * tu2 + 1.);
    ts = cu1 * cu2;
    baz = ts * tu2;
    faz = baz * tu1;
    x = lam2 - lam1;

    do {
        sx = sin(x);
        cx = cos(x);
        tu1 = cu2 * sx;
        tu2 = baz - su1 * cu2 * cx;
        sy = sqrt(tu1 * tu1 + tu2 * tu2);
        cy = ts * cx + faz;
        y = atan2(sy, cy);
        sa = ts * sx / sy;
        c2a = -sa * sa + 1.;
        cz = faz + faz;
        if (c2a > 0.)
            cz = -cz / c2a + cy;
        e = cz * cz * 2. - 1.;
        c = ((c2a * -3. + 4.) * f + 4.) * c2a * f / 16.;
        d = x;
        x = ((e * cy * c + cz) * sy * c + y) * sa;
        x = (1. - c) * x * f + lam2 - lam1;
    } while (fabs(d - x) > EPS);

//forward azimuth    faz = atan2(tu1, tu2);
//backward azimuth    baz = atan2(cu1 * sx, baz * cx - su1 * cu2) + PI;
    x = sqrt((1. / r / r - 1.) * c2a + 1.) + 1.;
    x = (x - 2.) / x;
    c = (x * x / 4. + 1.) / (1. - x);
    d = (x * .375 * x - 1.) * x;
    s = ((((sy * sy * 4. - 3.) * (1. - e - e) * cz * d / 6. - e * cy) * d / 4. + cz) * sy * d + y) * c * r;
    return (int)(s * a);
}