contrib/earthdistance/earthdistance.c

   1 /* contrib/earthdistance/earthdistance.c */
   2
   3 #include "postgres.h"
   4
   5 #include <math.h>
   6
   7 #include "utils/geo_decls.h"    /* for Point */
   8
   9 /* X/Open (XSI) requires <math.h> to provide M_PI, but core POSIX does not */
  10 #ifndef M_PI
  11 #define M_PI 3.14159265358979323846
  12 #endif
  13
  14 PG_MODULE_MAGIC;
  15
  16 /* Earth's radius is in statute miles. */
  17 static const double EARTH_RADIUS = 3958.747716;
  18 static const double TWO_PI = 2.0 * M_PI;
  19
  20
  21 /******************************************************
  22  *
  23  * degtorad - convert degrees to radians
  24  *
  25  * arg: double, angle in degrees
  26  *
  27  * returns: double, same angle in radians
  28  ******************************************************/
  29
  30 static double
  31 degtorad(double degrees)
  32 {
  33         return (degrees / 360.0) * TWO_PI;
  34 }
  35
  36 /******************************************************
  37  *
  38  * geo_distance_internal - distance between points
  39  *
  40  * args:
  41  *       a pair of points - for each point,
  42  *         x-coordinate is longitude in degrees west of Greenwich
  43  *         y-coordinate is latitude in degrees above equator
  44  *
  45  * returns: double
  46  *       distance between the points in miles on earth's surface
  47  ******************************************************/
  48
  49 static double
  50 geo_distance_internal(Point *pt1, Point *pt2)
  51 {
  52         double          long1,
  53                                 lat1,
  54                                 long2,
  55                                 lat2;
  56         double          longdiff;
  57         double          sino;
  58
  59         /* convert degrees to radians */
  60
  61         long1 = degtorad(pt1->x);
  62         lat1 = degtorad(pt1->y);
  63
  64         long2 = degtorad(pt2->x);
  65         lat2 = degtorad(pt2->y);
  66
  67         /* compute difference in longitudes - want < 180 degrees */
  68         longdiff = fabs(long1 - long2);
  69         if (longdiff > M_PI)
  70                 longdiff = TWO_PI - longdiff;
  71
  72         sino = sqrt(sin(fabs(lat1 - lat2) / 2.) * sin(fabs(lat1 - lat2) / 2.) +
  73                                 cos(lat1) * cos(lat2) * sin(longdiff / 2.) * sin(longdiff / 2.));
  74         if (sino > 1.)
  75                 sino = 1.;
  76
  77         return 2. * EARTH_RADIUS * asin(sino);
  78 }
  79
  80
  81 /******************************************************
  82  *
  83  * geo_distance - distance between points
  84  *
  85  * args:
  86  *       a pair of points - for each point,
  87  *         x-coordinate is longitude in degrees west of Greenwich
  88  *         y-coordinate is latitude in degrees above equator
  89  *
  90  * returns: float8
  91  *       distance between the points in miles on earth's surface
  92  ******************************************************/
  93
  94 PG_FUNCTION_INFO_V1(geo_distance);
  95
  96 Datum
  97 geo_distance(PG_FUNCTION_ARGS)
  98 {
  99         Point      *pt1 = PG_GETARG_POINT_P(0);
 100         Point      *pt2 = PG_GETARG_POINT_P(1);
 101         float8          result;
 102
 103         result = geo_distance_internal(pt1, pt2);
 104         PG_RETURN_FLOAT8(result);
 105 }