Postcode distance calculation

Question

Hey,

I have just been developing a postcode distance calculator for my Dads company, where all our customers are kept on file and every time a new potential customer makes an enquiry the system will check against all other customers' postcodes. The problem is that it doesn't yet do the distance calculation. Typing in a DT1 postcode with the distance set to 5 miles says that DT1, 2 and 3 are close by. DT2 says DT1 and 2, DT3 says DT1, 3 and 4. This doesn't make any sense. Yet if I put in BH2 with a distance of 50 miles it will bring back Bournemouth, Dorchester, Portsmouth, Bath, Southampton and a couple of other towns. These are all correct - however I do not know what is going on with the DT postcodes. DT11 doesn't bring back any (under 5 miles), nor does DT10 (except for themselves).

I first of all used the root of (x * x + y * y) - I believe this is called Pythagoras' theorem.. not sure since it have been years since I used that term!!

I have also tried the following:

  $sinstuff = sin(deg2rad($latitude))*sin(deg2rad($latitude2));
  $cosstuff = cos(deg2rad($latitude))*cos(deg2rad($latitude2))*cos(deg2rad($longitude-$longitude2));
  $sumstuff = $sinstuff + $cosstuff;
  $acosstuff = rad2deg(acos($sumstuff));
  $distance = $acosstuff*60*1.1515; //dunno what the 60 and 1.1515 is all about - I got this formula off a website.

This is the formula which seems to be bringing up the weird results. I also notice that the exact distance between DT11 and DT11 is something like +/- 0.00000989{and lots of other numbers}. This also seems a bit odd since, surely, the distance between DT11 and itself is 0...

Has anyone ever done anything like this successfully before?

I do have a massive batch of files - about 158MB in total - containing every UK postcode in full, with its corresponding latitude and longitude. My aim is, rather than comparing the specified full postcode to every other full postcode, to work out which area codes are less than a certain distance away then compare the specified postcode to all the full postcodes which are within these areas. Is this efficient? Is there a more efficient way?

Any help with this would be greatly appreciated.

Regards,

Richard

PS I know there are some sites out there which tell you how to work out distances but I dont seem to be able to get any of them working correctly (as you can tell from above).

PPS I know I can use the Google API for this - but this is not an option since this is going to be an offline application running on a local installation of wamp server. It is for use at trade shows where we cannot guarantee an internet connection.

Answer 1

You're use the haversine formula:

$l1 ==> latitude1
$o1 ==> longitude1
$l2 ==> latitude2
$o2 ==> longitude2

(taken from http://www.go4expert.com/forums/showthread.php?t=339)

function haversine ($l1, $o1, $l2, $o2)

{

    $l1 = deg2rad ($l1);

    $sinl1 = sin ($l1);

    $l2 = deg2rad ($l2);

    $o1 = deg2rad ($o1);

    $o2 = deg2rad ($o2);

    return (7926 - 26 * $sinl1) * asin (min (1, 0.707106781186548 * sqrt ((1 - (sin ($l2) * $sinl1) - cos ($l1) * cos ($l2) * cos ($o2 - $o1)))));

}  

That should give you the distance between both points.