緯度経度3点から大体の面積を求める

calcs.pl

use strict;
use warnings;
use utf8;

use Math::Trig;

use v5.12;

# lat 緯度 lng 経度

my $p1 = [ 34.678506,135.498941 ];
my $p2 = [ 34.6785,  135.499194 ];
my $p3 = [ 34.677377,135.499024 ];

# 長半径[m]
my $A = 6378137;
# 扁平率
#my $F = 1.0 / 298.257222101;

# 円周[m]
my $L = 2 * 3.14159 * $A;

say calcs($p1, $p2, $p3);
say calcs($p2, $p3, $p1);
say calcs($p3, $p1, $p2);

sub calcunit {
  my ($lat, $lng) = @_;

  my $ulat = $L/360.0;
  # 北緯xx度地点での半径
  my $r = $A * cos(deg2rad($lat));
  #say "r($lat) = $r";

  my $l = 2 * 3.14159 * $r;

  my $ulng = $l/360.0;

  return ($ulat, $ulng);
}

sub calcs {
  my ($p1, $p2, $p3) = @_;

  my ($ulat,$ulng) = calcunit(($p1->[0]+$p2->[0]+$p3->[0])/3.0, ($p1->[1]+$p2->[1]+$p3->[1])/3.0);

  #printf("p1 unit lat m/deg = %.2f\n", $ulat);
  #printf("p1 unit lng m/deg = %.2f\n", $ulng);
  #printf("p1-p2 lat diff = %f\n", abs($p2->[0]-$p1->[0]));
  #printf("p1-p2 lng diff = %f\n", abs($p2->[1]-$p1->[1]));

  my $P2 = [($p2->[0]-$p1->[0])*$ulat, ($p2->[1]-$p1->[1])*$ulng];
  my $P3 = [($p3->[0]-$p1->[0])*$ulat, ($p3->[1]-$p1->[1])*$ulng];

  #say join(",", map{int} @$P2);
  #say join(",", map{int} @$P3);

  0.5 * abs($P2->[0]*$P3->[1] - $P3->[0]*$P2->[1]);
}