###Geohash的认知 Geohash的最简单的解释就是：将一个经纬度信息，转换成一个可以排序，可以比较的字符串编码. ###Geohash的特点
- 首先，geohash用一个字符串表示经度和纬度两个坐标。某些情况下无法在两列上同时应用索引 （例如MySQL 4之前的版本，Google App Engine的数据层等），利用geohash，只需在一列上应用索引即可。
- 其次，geohash表示的并不是一个点，而是一个矩形区域。比如编码wx4g0ec19，它表示的是一个矩形区域。使用者可以发布地址编码，既能表明自己位于北海公园附近，又不至于暴露自己的精确坐标，有助于隐私保护。
- 第三，编码的前缀可以表示更大的区域。例如wx4g0ec1，它的前缀wx4g0e表示包含编码wx4g0ec1在内的更大范围。这个特性可以用于附近地点搜索。首先根据用户当前坐标计算geohash（例如wx4g0ec1）然后取其前缀进行查询（SELECT * FROM place WHERE geohash LIKE 'wx4g0e%'），即可查询附近的所有地点。
当然，Geohash比直接用经纬度的高效很多。
###Geohash的原理 首先将纬度范围(-90, 90)平分成两个区间(-90,0)、(0, 90)，如果目标纬度位于前一个区间，则编码为0，否则编码为1。
由于39.92324属于(0, 90)，所以取编码为1。
然后再将(0, 90)分成 (0, 45), (45, 90)两个区间，而39.92324位于(0, 45)，所以编码为0。
以此类推，直到精度符合要求为止，得到纬度编码为1011 1000 1100 0111 1001。
经度也用同样的算法，对(-180, 180)依次细分，得到116.3906的编码为1101 0010 1100 0100 0100。
接下来将经度和纬度的编码合并，奇数位是纬度，偶数位是经度，得到编码 11100 11101 00100 01111 00000 01101 01011 00001。
最后，用0-9、b-z（去掉a, i, l, o）这32个字母进行base32编码，得到(39.92324, 116.3906)的编码为wx4g0ec1。
</tbody>
| 十进制 | 0 | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 | 11 | 12 | 13 | 14 | 15 |
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| base32 | 0 | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | b | c | d | e | f | g |
| 十进制 | 16 | 17 | 18 | 19 | 20 | 21 | 22 | 23 | 24 | 25 | 26 | 27 | 28 | 29 | 30 | 31 |
| base32 | h | j | k | m | n | p | q | r | s | t | u | v | w | x | y | z |
实现代码(php版)：
/** * Geohash generation class * http://blog.dixo.net/downloads/ * * This file copyright (C) 2008 Paul Dixon ([email protected]) * * 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; $icoding[$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
"; //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); } } ?>