Ordnance Survey and Map reference point calculations

Functions example from a Well House Consultants training course
More on Functions [link]

This example is described in the following article(s):
• A lazy programmer is a good programmer - [link]

If you're searching for a page where you can try this code, select here

Source code: near.php Module: H105


<?php

/* Distance calculations from grid reference to kms
East and North from a single fixed point, and distance
calculations between two points. */

/*  function calls:

array $result = offsets($grid);
        Converts an Ordnance Survey grid reference to
        East and North distances from a fixed point

        $grid - the 8 character grid reference such as
                SU007875
        $result[0] - distance (units of 100 metres) east
                of a fixed point off Cornwall
        $result[1] - distance (units of 100 metres) north
                of a fixed point off Cornwall

float $result = dist($first, $second);
        Calculates the straight distance between two points
        and returns the rounded distance. Sets a compass point
        direction variable (8 point compass)

        $first - an array of the format returned by offsets,
                being the point from which to measure
        $second - an array of the format returned by offsets,
                being the point to which to measure
        $result - distance from point $first to point $second.
                rounded to nearest 1/10 km, 1km or 10km
                depending on the distance

        ALSO SETS global variable $dirn to "N", "NE", "NW",
        "E", "W", "SE", "SW" or "S" depending on the direction
        from $first to $second

string $html = nb6($gridref,$nrows);
        Returns an HTML table of the nearest places to the
        given grid reference that are defined in a MySQL
        database

        $gridref - the grid reference of the point of interest
        $nrows - the number of rows of nearby places to list
        $html - an HTML table with links to nearby places

        ALSO USES the global variable $dirn set by the dist
        function, and the "sharedata" database from Well House
        Consultant's Wiki.

The code in the offsets and dist functions can be cut and
pasted into other applications, or this file imported, and they
will work well in a stand alone mode. The nb6 function is specific
to the Well House Consultants wiki code and serves as an example
here - you'll need to alter the code if you wish to use it
elsewhere.

Code written by Graham Ellis of Well House Consultants
graham@wellho.net / +44 (0) 1225 708225

*/

function offsets($from = "ST907633") {

$gridrefs = <<< GRIDREFS

# Grid references - bottom left corner of square

# This file contains the two letter codes used by the British
# map reference systems.    For example, ST914628 is:
# From the West 300 x 10 + 914 = 3914
# From the South 100 x 10 + 628 = 1628
# Distances measured in tenths of a kilometre.

0 0 SV SW SX SY SZ TV
100 100 SR SS ST SU TQ TR
100 200 SM SN SO SP TL TM
200 300 SH SJ SK TF TG
200 400 SC SD SE TA
100 500 NW NX NY NZ OV
100 600 NR NS NT NU GX
0 700 NL NM NN NO
0 800 NF NG NH NJ NK
0 900 NA NB NC ND
100 1000 HW HX HY HZ
300 1100 HT HU
400 1200 HP
GRIDREFS;
###############################################################

foreach (explode("\n",$gridrefs) as $line) {
        $line = preg_replace("/#.*/","",trim($line));
        $fields = explode(" ",$line);
        if (count($fields) < 3) continue;
        for ($k=2; $k<count($fields); $k++) {
                $twolettersx[$fields[$k]] = $fields[0] + 100 * ($k-2);
                $twolettersy[$fields[$k]] = $fields[1] ;
                }
        }

# define a point we want to highlight on the map and convert it to km from (0,0)

$highpoint = $from; #"ST914628";
preg_match("/(..)(...)(...)/",$highpoint,$parts);
$xlocn = $twolettersx[$parts[1]] * 10 + $parts[2];
$ylocn = $twolettersy[$parts[1]] * 10 + $parts[3];
return array($xlocn,$ylocn);
}

function dist($from, $to) {
        global $dirn;
        $dx = $from[0] - $to[0];
        $dy = $from[1] - $to[1];
        $di1= "E"; $di2 = "N";
        if ($dx > 0) $di1 = "W";
        if ($dy > 0) $di2 = "S";
        $dirn = (abs($dx) > abs($dy)) ? $di1 : $di2;
        if (abs(abs($dx)-abs($dy))/(abs($dx)+abs($dy)) < 0.6)
                $dirn = "$di2$di1";
        $d = sqrt($dx*$dx+$dy*$dy) / 10.0;
        $rv = round($d,1);
        if ($d > 5.0) $rv = round ($d,0);
        if ($d > 50.0) $rv = round ($d,-1);
        return $rv;
        }

function nb6($base = "ST907633",$nln = 5) {
global $dirn;
$bxy = offsets($base);
$already = array();

# mysql_pconnect("www.wellho.net","login","password");
# mysql_select_db("wellho");

$rset = mysql_query("select * from sharedata");
while ($row = mysql_fetch_assoc($rset)) {
        if (ereg('\[map=(........)\]([^\[]*)\[/map\]',$row[info],$gotten)) {
                if ($gotten[1] == $base) continue;
                if (ereg("area",$row[pagename])) continue;
                if ($already[$gotten[1]]) continue;
                $txy = offsets($gotten[1]);
                $distance = dist ($bxy,$txy);
                $dhash[$row[pagename]] = $distance;
                $way[$row[pagename]] = $dirn;
                $pnhash[$row[pagename]] = $gotten[2];
                $already[$gotten[1]] = 1;
        }
}
asort($dhash);
$prev = "";
$html = "<br><table width=100% cellpadding=2><tr class=generaltext>".
        "<th>Pages nearby ...</th>";
$count = 0;
$perline = 3;
foreach (array_keys($dhash) as $pname) {
        if ($prev == $pnhash[$pname]) continue;
        $html .= "<td width=33%><a href=/share/$pname.html>$pnhash[$pname]</a><br>";
        $html .= "$dhash[$pname] kms $way[$pname]</td>";
        $prev = $pnhash[$pname];
        $count++;
        if ($count == $nln * $perline - 1) break;
        if ($count%$perline == $perline-1)
                        $html .= "</tr><tr class=generaltext>";
}
$html .= "</tr></table>";
return $html;
}
?>

Learn about this subject

This module and example are covered on the following public courses:
* Learning to program in PHP
* PHP Programming
* Object Oriented Programming in PHP
* Beginning PHP - weekend course / hobby / club / leisure users
* PHP Programming
* Learning to program in PHP
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