Re: Miidpoint between two long/lat points? (earthdistance?)

[Merlin Moncure <mmoncure@xxxxxxxxx>]

On Wed, May 25, 2011 at 12:47 PM, Carlo Stonebanks
<stonec.register@xxxxxxxxxxxx> wrote:
> I need to calculate the long/lat values between a line demarcated by two
> long/lat points.
>
>
>
> The points will be very close, but there is the 180 degree problem to
> consider, so a simple average won’t work.
>
>
>
> Does anyone know of a function or have a formula that will work using geo
> long/lat values? I don’t see anything obvious in the earthdistance module.

Conerted from javascript from here: http://en.wikipedia.org/wiki/Atan2

btw i'm not huge fan of earthdistance module either -- it's easier to
just rig functions to do what you want.

merlin

create or replace function midpoint(
  lat1 float8,
  lon1 float8,
  lat2 float8,
  lon2 float8,
  lat_mid out float8,
  lon_mid out float8) returns record as
$$
   select
     atan2(sin(lat1)+sin(lat2),
                    sqrt( (cos(lat1)+Bx)*(cos(lat1)+Bx) + By*By) ) * 57.2957795,
     (lon1 + atan2(By, cos(lat1) + Bx)) * 57.2957795
     from
     (
        select
          lat1,
          lat2,
          lon1,
          cos(lat2) * cos(dlon) as bx,
          cos(lat2) * sin(dlon) as by
          from
          (
            select
            $1 * 0.0174532925 as lat1,
            $3 * 0.0174532925 as lat2,
            $2 * 0.0174532925 as lon1,
            ($4 - $2) * 0.0174532925 as dlon
           ) q
      ) q;
$$ language sql immutable;

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