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12/12/2007 at 09:35AM PST, ID: 23018885
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8.6

Initial Bearing Calculation error in PHP

Asked by mdougan in PHP Scripting Language, Algorithms

Tags: bearing, calculate, heading, php

I had some navigation code written in VB that I tried to convert to PHP.  It seemed fine until I started seeing errors when using Lat/Lons that appeared in the Eastern or Southern hemispheres.  

The output of the code sample below produces these results:

heading east in the northern/western hemisphere: 90
heading west in the northern/western hemisphere: 270
heading north in the northern/western hemisphere: 339
heading south in the northern/western hemisphere: nan

heading east in the northern/eastern hemisphere: 270
heading west in the northern/eastern hemisphere: 90
heading north in the northern/eastern hemisphere: 339
heading south in the northern/eastern hemisphere: nan

heading east in the southern/western hemisphere: 90
heading west in the southern/western hemisphere: 270
heading north in the southern/western hemisphere: nan
heading south in the southern/western hemisphere: 204

heading east in the southern/eastern hemisphere: 270
heading west in the southern/eastern hemisphere: 90
heading north in the southern/eastern hemisphere: nan
heading south in the southern/eastern hemisphere: 204

It appears my East/West calculations get reversed in the Eastern Hemisphere.

In all cases, I'd want to see either 0 or 360 for the North and 180 for the South bearings, I don't understand the readings that I'm getting here at all... and not sure whether "nan" is another way of saying 0?

I tried looking up alternative ways to calculate the initial bearing, and found this website, that describes a method using JavaScript.  

http://www.movable-type.co.uk/scripts/latlong.html

However, I tried converting that to PHP (found in the HeadingNew function in the code sample) and it gives me this output:

heading east in the northern/western hemisphere: 90
heading west in the northern/western hemisphere: -90
heading north in the northern/western hemisphere: 0
heading south in the northern/western hemisphere: 0

heading east in the northern/eastern hemisphere: 90
heading west in the northern/eastern hemisphere: -90
heading north in the northern/eastern hemisphere: 0
heading south in the northern/eastern hemisphere: 0

heading east in the southern/western hemisphere: 90
heading west in the southern/western hemisphere: -90
heading north in the southern/western hemisphere: 0
heading south in the southern/western hemisphere: 0

heading east in the southern/eastern hemisphere: 90
heading west in the southern/eastern hemisphere: -90
heading north in the southern/eastern hemisphere: 0
heading south in the southern/eastern hemisphere: 0

That is almost correct, except for the South calculation (and, I'm not sure if the North calculation is actually working or not).  If I used the new heading function, I'd have to find a way to convert the results to a compass bearing too.

Can anyone help me get either routine working, or do you have a sample of a completely different routine that will work in PHP?    

Thanks! 
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<?php
 
	    function HeadingNew($lat1, $long1, $lat2, $long2) {
	       // Takes lat/longs as expressions in Degrees
	       $rLat1 = 0.0;
	       $rLong1 = 0.0;
	       $rLat2 = 0.0;
	       $rLong2 = 0.0;
	       $d = 0.0;
	       $h = 0.0;
	       $result = 0.0;
 
	       $rLat1 = deg2rad($lat1);
	       $rLong1 = deg2rad($long1);
 
	       $rLat2 = deg2rad($lat2);
	       $rLong2 = deg2rad($long2);
 
 
                $d = deg2rad($long2 - $long1);
                $y = sin($d) * cos($rlat2);
                $x = cos($rlat1) * sin($rlat2) - sin($rlat1) * cos($rlat2) * cos($d);
                $h = atan2($y, $x);
                //echo "D = ".$d." X = ".$x." Y = ".$y." H = ".$h."<br>";
 
 	        $result = rad2deg($h);
 
 
	       return($result);
	    }
 
	    function Heading($lat1, $long1, $lat2, $long2) {
	       // Takes lat/longs as expressions in Degrees
	       $pi = 3.14159265358979;
	       $rLat1 = 0.0;
	       $rLong1 = 0.0;
	       $rLat2 = 0.0;
	       $rLong2 = 0.0;
	       $d = 0.0;
	       $h = 0.0;
	       $result = 0.0;
 
	       $rLat1 = deg2rad($lat1);
	       $rLong1 = deg2rad($long1);
 
	       $rLat2 = deg2rad($lat2);
	       $rLong2 = deg2rad($long2);
 
 
	       // Distance in Meters Using Radians
	       // Less subject to rounding errors for small distances
	        $d = ( 2 * asin(sqrt(pow(sin(($rLat1 - $rLat2) / 2), 2) + cos($rLat1) * cos($rLat2) * pow(sin(($rLong1 - $rLong2) / 2),2))));
 
	       if (sin(ABS($rLong2) - ABS($rLong1)) < 0) {
                  //echo "top calc <br>";
	          $h = acos(((sin($rLat2) - sin($rLat1)) * cos($d)) / (sin($d) * cos($rLat1)));
	          }
	       else {
                  //echo "bottom calc <br>";
	          $h = 2 * $pi - acos(((sin($rLat2) - sin($rLat1)) * cos($d)) / (sin($d) * cos($rLat1)));
	          }
 
	       $result = rad2deg($h);
 
 
	       return($result);
	    }
 
 
	 $dHeading = number_format(Heading(30, -30, 30, -20));
         echo "heading east in the northern/western hemisphere: ".$dHeading."<br>";
	 $dHeading = number_format(Heading(30, -20, 30, -30));
         echo "heading west in the northern/western hemisphere: ".$dHeading."<br>";
	 $dHeading = number_format(Heading(30, -30, 40, -30));
         echo "heading north in the northern/western hemisphere: ".$dHeading."<br>";
	 $dHeading = number_format(Heading(40, -30, 30, -30));
         echo "heading south in the northern/western hemisphere: ".$dHeading."<p>";
 
 
	 $dHeading = number_format(Heading(30, 30, 30, 40));
         echo "heading east in the northern/eastern hemisphere: ".$dHeading."<br>";
	 $dHeading = number_format(Heading(30, 40, 30, 30));
         echo "heading west in the northern/eastern hemisphere: ".$dHeading."<br>";
	 $dHeading = number_format(Heading(30, 30, 40, 30));
         echo "heading north in the northern/eastern hemisphere: ".$dHeading."<br>";
	 $dHeading = number_format(Heading(40, 30, 30, 30));
         echo "heading south in the northern/eastern hemisphere: ".$dHeading."<p>";
 
 
	 $dHeading = number_format(Heading(-50, -50, -50, -40));
         echo "heading east in the southern/western hemisphere: ".$dHeading."<br>";
	 $dHeading = number_format(Heading(-50, -40, -50, -50));
         echo "heading west in the southern/western hemisphere: ".$dHeading."<br>";
	 $dHeading = number_format(Heading(-50, -50, -40, -50));
         echo "heading north in the southern/western hemisphere: ".$dHeading."<br>";
	 $dHeading = number_format(Heading(-40, -50, -50, -50));
         echo "heading south in the southern/western hemisphere: ".$dHeading."<p>";
 
 
	 $dHeading = number_format(Heading(-50, 50, -50, 60));
         echo "heading east in the southern/eastern hemisphere: ".$dHeading."<br>";
	 $dHeading = number_format(Heading(-50, 60, -50, 50));
         echo "heading west in the southern/eastern hemisphere: ".$dHeading."<br>";
	 $dHeading = number_format(Heading(-50, 50, -40, 50));
         echo "heading north in the southern/eastern hemisphere: ".$dHeading."<br>";
	 $dHeading = number_format(Heading(-40, 50, -50, 50));
         echo "heading south in the southern/eastern hemisphere: ".$dHeading."<br>";
 
 
     ?>
[+][-]12/12/07 05:07 PM, ID: 20461584

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[+][-]12/13/07 02:06 PM, ID: 20468161

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About this solution

Zones: PHP Scripting Language, Algorithms
Tags: bearing, calculate, heading, php
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Solution Provided By: AlexanderR
Participating Experts: 1
Solution Grade: A
 
 
[+][-]12/13/07 02:08 PM, ID: 20468185

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[+][-]12/14/07 07:51 AM, ID: 20472260

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