Re: earthdistance compass bearing

On 6/18/2013 11:16 AM, Jeff Herrin wrote:
> I don't need it to be too accurate. We're pushing hotel info into the
> GDS (sabre, expedia, orbitz, etc). They require airport info relative
> to the hotel. Example: DFW is 25 miles NW of the property. I thought
> about just faking it...comparing the hotel's lat/long from the
> airports. I can probably get N,S,E,W reliably enough, but i'm not sure
> at what point N becomes NW, etc. That just seems like a really crude
> bad way to do it, but the alternatives seem unnecessarily complex. I
> found some examples that use bearing but they all take headings in
> degrees (which im not seeing in earthdistance). I guess I'm going to
> have to either setup postGIS or brush up on my trig.

half the difference.   like, if NW is 45 degrees, then its anything +/-
22.5 degrees of that.

if you're willing to accept the error in cartesian vs spherical, its
fairly simple.     theta is asin(X/R) where X is the delta longitude,
and R is the radius, aka your earthdistance.   asin returns radians, so
multiply by 180/pi() to get degrees.   divide the degrees by 16/360 and
truncate to an integer, and you should get 0-15, index the string array
('N','NW','NW','W','W','SW','SW','S','S','SE','SE','E','E','NE','NE','N') to
get compass direction points.    verify you got the polarity right on
the delta longitude such that 0 == North and 90 degrees == West.



--
john r pierce                                      37N 122W
somewhere on the middle of the left coast