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GeoHash PHP Class
Provides a class for generating and decoding geohashes as
documented at http://en.wikipedia.org/wiki/Geohash
Developed by Paul Dixon and licenced for reuse via the
GNU General Public Licence – see LICENCE.txt for details
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UPDATES
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The latest version of this class is available from
HISTORY
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0.10 – 27th Feb 2008 – First release
EXAMPLES
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$geohash=new Geohash();
#decode a hash
$coords=$geohash->decode(“mh7w”);
echo “Lat {$coords[0]} Long {$coords[1]}
“;
#encode a hash
$hash=$geohash->encode(-62.5, 23.4);
echo “Hash is $hash
“;
<?php /** * Geohash generation class * http://blog.dixo.net/downloads/ * * This file copyright (C) 2008 Paul Dixon (paul@elphin.com) * * This program is free software; you can redistribute it and/or * modify it under the terms of the GNU General Public License * as published by the Free Software Foundation; either version 3 * of the License, or (at your option) any later version. * * This program is distributed in the hope that it will be useful, * but WITHOUT ANY WARRANTY; without even the implied warranty of * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the * GNU General Public License for more details. * * You should have received a copy of the GNU General Public License * along with this program; if not, write to the Free Software * Foundation, Inc., 59 Temple Place - Suite 330, Boston, MA 02111-1307, USA. */ /** * Encode and decode geohashes * */ class Geohash { private $coding="0123456789bcdefghjkmnpqrstuvwxyz"; private $codingMap=array(); public function Geohash() { //build map from encoding char to 0 padded bitfield for($i=0; $i<32; $i++) { $this->codingMap[substr($this->coding,$i,1)]=str_pad(decbin($i), 5, "0", STR_PAD_LEFT); } } /** * Decode a geohash and return an array with decimal lat,long in it */ public function decode($hash) { //decode hash into binary string $binary=""; $hl=strlen($hash); for($i=0; $i<$hl; $i++) { $binary.=$this->codingMap[substr($hash,$i,1)]; } //split the binary into lat and log binary strings $bl=strlen($binary); $blat=""; $blong=""; for ($i=0; $i<$bl; $i++) { if ($i%2) $blat=$blat.substr($binary,$i,1); else $blong=$blong.substr($binary,$i,1); } //now concert to decimal $lat=$this->binDecode($blat,-90,90); $long=$this->binDecode($blong,-180,180); //figure out how precise the bit count makes this calculation $latErr=$this->calcError(strlen($blat),-90,90); $longErr=$this->calcError(strlen($blong),-180,180); //how many decimal places should we use? There's a little art to //this to ensure I get the same roundings as geohash.org $latPlaces=max(1, -round(log10($latErr))) - 1; $longPlaces=max(1, -round(log10($longErr))) - 1; //round it $lat=round($lat, $latPlaces); $long=round($long, $longPlaces); return array($lat,$long); } /** * Encode a hash from given lat and long */ public function encode($lat,$long) { //how many bits does latitude need? $plat=$this->precision($lat); $latbits=1; $err=45; while($err>$plat) { $latbits++; $err/=2; } //how many bits does longitude need? $plong=$this->precision($long); $longbits=1; $err=90; while($err>$plong) { $longbits++; $err/=2; } //bit counts need to be equal $bits=max($latbits,$longbits); //as the hash create bits in groups of 5, lets not //waste any bits - lets bulk it up to a multiple of 5 //and favour the longitude for any odd bits $longbits=$bits; $latbits=$bits; $addlong=1; while (($longbits+$latbits)%5 != 0) { $longbits+=$addlong; $latbits+=!$addlong; $addlong=!$addlong; } //encode each as binary string $blat=$this->binEncode($lat,-90,90, $latbits); $blong=$this->binEncode($long,-180,180,$longbits); //merge lat and long together $binary=""; $uselong=1; while (strlen($blat)+strlen($blong)) { if ($uselong) { $binary=$binary.substr($blong,0,1); $blong=substr($blong,1); } else { $binary=$binary.substr($blat,0,1); $blat=substr($blat,1); } $uselong=!$uselong; } //convert binary string to hash $hash=""; for ($i=0; $i<strlen($binary); $i+=5) { $n=bindec(substr($binary,$i,5)); $hash=$hash.$this->coding[$n]; } return $hash; } /** * What's the maximum error for $bits bits covering a range $min to $max */ private function calcError($bits,$min,$max) { $err=($max-$min)/2; while ($bits--) $err/=2; return $err; } /* * returns precision of number * precision of 42 is 0.5 * precision of 42.4 is 0.05 * precision of 42.41 is 0.005 etc */ private function precision($number) { $precision=0; $pt=strpos($number,'.'); if ($pt!==false) { $precision=-(strlen($number)-$pt-1); } return pow(10,$precision)/2; } /** * create binary encoding of number as detailed in http://en.wikipedia.org/wiki/Geohash#Example * removing the tail recursion is left an exercise for the reader */ private function binEncode($number, $min, $max, $bitcount) { if ($bitcount==0) return ""; #echo "$bitcount: $min $max<br>"; //this is our mid point - we will produce a bit to say //whether $number is above or below this mid point $mid=($min+$max)/2; if ($number>$mid) return "1".$this->binEncode($number, $mid, $max,$bitcount-1); else return "0".$this->binEncode($number, $min, $mid,$bitcount-1); } /** * decodes binary encoding of number as detailed in http://en.wikipedia.org/wiki/Geohash#Example * removing the tail recursion is left an exercise for the reader */ private function binDecode($binary, $min, $max) { $mid=($min+$max)/2; if (strlen($binary)==0) return $mid; $bit=substr($binary,0,1); $binary=substr($binary,1); if ($bit==1) return $this->binDecode($binary, $mid, $max); else return $this->binDecode($binary, $min, $mid); } } ?>
test.php
<?php /** * Geohash generation class test harness * http://blog.dixo.net/downloads/ * * This file copyright (C) 2008 Paul Dixon (paul@elphin.com) * * This program is free software; you can redistribute it and/or * modify it under the terms of the GNU General Public License * as published by the Free Software Foundation; either version 3 * of the License, or (at your option) any later version. * * This program is distributed in the hope that it will be useful, * but WITHOUT ANY WARRANTY; without even the implied warranty of * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the * GNU General Public License for more details. * * You should have received a copy of the GNU General Public License * along with this program; if not, write to the Free Software * Foundation, Inc., 59 Temple Place - Suite 330, Boston, MA 02111-1307, USA. */ require_once('geohash.class.php'); $geohash=new Geohash; //these test hashes were made on geohash.org //and test various combinations of precision //and range $tests=array( "ezs42"=>array(42.6,-5.6), "mh7w"=>array(-20, 50), "t3b9m"=>array(10.1, 57.2), "c2b25ps"=>array(49.26, -123.26), "80021bgm"=>array(0.005, -179.567), "k484ht99h2"=>array(-30.55555, 0.2), "8buh2w4pnt"=>array(5.00001, -140.6), ); foreach($tests as $actualhash=>$coords) { $computed_hash=$geohash->encode($coords[0], $coords[1]); echo "Encode {$coords[0]}, {$coords[1]} as $actualhash : "; if ($computed_hash==$actualhash) { echo "OK<br>"; } else { echo "FAIL (got $computed_hash)<br>"; } echo "<hr>"; $computed_coords=$geohash->decode($actualhash); echo "Decode $actualhash as {$coords[0]}, {$coords[1]} : "; if (($computed_coords[0]==$coords[0]) && ($computed_coords[1]==$coords[1])) { echo "OK<br>"; } else { echo "FAIL (got {$computed_coords[0]}, {$computed_coords[1]})<br>"; } echo "<hr>"; } ?>