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Bruce Chen edited this page May 22, 2013 · 7 revisions

以(39.92324, 116.3906)为例,介绍一下geohash的编码算法。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。 纬度范围 划分区间0 划分区间1 39.92324所属区间

(-90, 90) (-90, 0.0) (0.0, 90) 1

(0.0, 90) (0.0, 45.0) (45.0, 90) 0

(0.0, 45.0) (0.0, 22.5) (22.5, 45.0) 1

(22.5, 45.0) (22.5, 33.75) (33.75, 45.0) 1

(33.75, 45.0) (33.75, 39.375) (39.375, 45.0) 1

(39.375, 45.0) (39.375, 42.1875) (42.1875, 45.0) 0

(39.375, 42.1875) (39.375, 40.7812) (40.7812, 42.1875) 0

(39.375, 40.7812) (39.375, 40.0781) (40.0781, 40.7812) 0

(39.375, 40.0781) (39.375, 39.7265) (39.7265, 40.0781) 1

(39.7265, 40.0781) (39.7265, 39.9023) (39.9023, 40.0781) 1

(39.9023, 40.0781) (39.9023, 39.9902) (39.9902, 40.0781) 0

(39.9023, 39.9902) (39.9023, 39.9462) (39.9462, 39.9902) 0

(39.9023, 39.9462) (39.9023, 39.9243) (39.9243, 39.9462) 0

(39.9023, 39.9243) (39.9023, 39.9133) (39.9133, 39.9243) 1

(39.9133, 39.9243) (39.9133, 39.9188) (39.9188, 39.9243) 1

(39.9188, 39.9243) (39.9188, 39.9215) (39.9215, 39.9243) 1

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。 经度范围 划分区间0 划分区间1 116.3906所属区间

(-180, 180) (-180, 0.0) (0.0, 180) 1

(0.0, 180) (0.0, 90.0) (90.0, 180) 1

(90.0, 180) (90.0, 135.0) (135.0, 180) 0

(90.0, 135.0) (90.0, 112.5) (112.5, 135.0) 1

(112.5, 135.0) (112.5, 123.75) (123.75, 135.0) 0

(112.5, 123.75) (112.5, 118.125) (118.125, 123.75) 0

(112.5, 118.125) (112.5, 115.312) (115.312, 118.125) 1

(115.312, 118.125) (115.312, 116.718) (116.718, 118.125) 0

(115.312, 116.718) (115.312, 116.015) (116.015, 116.718) 1

(116.015, 116.718) (116.015, 116.367) (116.367, 116.718) 1

(116.367, 116.718) (116.367, 116.542) (116.542, 116.718) 0

(116.367, 116.542) (116.367, 116.455) (116.455, 116.542) 0

(116.367, 116.455) (116.367, 116.411) (116.411, 116.455) 0

(116.367, 116.411) (116.367, 116.389) (116.389, 116.411) 1

(116.389, 116.411) (116.389, 116.400) (116.400, 116.411) 0

(116.389, 116.400) (116.389, 116.394) (116.394, 116.400) 0

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。

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

解码算法与编码算法相反,先进行base32解码,然后分离出经纬度,最后根据二进制编码对经纬度范围进行细分即可,这里不再赘述。不过由于geohash表示的是区间,编码越长越精确,但不可能解码出完全一致的地址。

用途 & 缺点: geohash的最大用途就是附近地址搜索了。不过,从geohash的编码算法中可以看出它的一个缺点:位于格子边界两侧的两点,虽然十分接近,但编码会完全不同。实际应用中,可以同时搜索当前格子周围的8个格子,即可解决这个问题。

geohash wiki: http://en.wikipedia.org/wiki/Geohash

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