[WriteLog] Re: calculating distances between grids (makrothen)

[Steve Woodruff]

You asked....  makrothen calculates distance this way:

double dist(double lat1, double lon1, 
                double lat2, double lon2) {
  // convert from degrees to rad
  double a = lat1 * (PI/180);
  double b = lon1 * (PI/180);
  double c = lat2 * (PI/180);
  double d = lon2 * (PI/180);

  // same grid is 100km but we'll calc that later
  if((a==c) && (b==d)) return 0;

  // yow.
  return acos(cos(a)*cos(b)*cos(c)*cos(d)+cos(a) *    

    sin(b)*cos(c)*sin(d)+sin(a)*sin(c)) * 6378.16;
}

Should give you the dist in km between 2 grids.

The 6378.16 km distance is from the makrothen rules.
Some other calculators use a different value for the
diameter of the earth at the equator.  Whatever. 

Aren't you glad you asked?  :)

/steve n9oh

--- writelog-request at contesting.com wrote:
> From: "Dave W7DPW" <w7dpw at comcast.net>
> I would like to find the formula for calculating the
> distance between
> two gridSquares.
> 
> Can some one point me in the right direction ?
> 
> Dave W7DPW
> 


=====
----------------------------------------------------------
 Steve Woodruff, N9OH @ 42.18N 88.34W, sjwoodr at yahoo.com

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