Added interface for conversion

jtejido · Apr 21, 2018 · ff96117 · ff96117
1 parent a94a0ea
commit ff96117
Showing 1 changed file with 9 additions and 1 deletion.
diff --git a/src/Geodesy/Distance/VincentyFormula.php b/src/Geodesy/Distance/VincentyFormula.php
@@ -75,10 +75,18 @@ public function distance()
         }
 
         $uSq = $cosSqAlpha * (pow($a, 2)- pow($b, 2)) / ($b * $b);
-        $k1 = (sqrt(1 + $uSq) - 1) / (sqrt(1 + $uSq) + 1);
         $A = 1 + $uSq / 16384
                 * (4096 + $uSq * (-768 + $uSq * (320 - 175 * $uSq)));
         $B = $uSq / 1024 * (256 + $uSq * (-128 + $uSq * (74 - 47 * $uSq)));
+
+        /**
+         * Alternatively, you can use Helbert's expansion for A and B
+         *
+         * $k1 = (sqrt(1 + $uSq) - 1) / (sqrt(1 + $uSq) + 1);
+         * $A = 1 + ((1/4) * pow($k1, 2)) / (1 - $k1);
+         * $B = $k1 * (1 - ((3/8) * pow($k1, 2)));
+         */
+
         $deltaSigma = 
                     $B * $sinSigma
                         * ($cos2SigmaM + $B / 4