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Geohash in mapping applications

December, 2011

Igor Luzhanov

Points selection task

Mapping application with brazillions of points

Need to select points in some rectangle

Common approach:

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if (point.lat > bounds.S && point.lat < bounds.N && point.lon > bounds.W && point.lon < bounds.E) { ...

}

Common approach

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Common approach problems

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Solution - Geohash

46.4845830 N, 30.7326000 E = u8mb7q6b36r6x

Invented by Gustavo Niemeyer, 2008, public domain

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How it works 1/2

Example: decoding from base 32 for ezs42

Base-32 binary

ezs42 0110111111110000010000010

Even – longitude, odd – latitude (start from 0, left to right)

lon = 0111110000000

lat = 101111001001

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How it works 2/2 Latitude = 101111001001 (~42.6 °)

All latitude range: from [+90°, -90°], middle = 0°

First bit = 1, so select upper interval: [+90°, 0°], middle =

45°

Continue for all bits, getting more detailed range

Geohash define box, not point!

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How you can use it – 1D In rectangular area (except "faultlines") points placed on

contiguous slices

Points can be requested by single parameter:

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SELECT * FROM myPoints WHERE

geohash >= u85 AND

geohash <= u8x

How you can use it – 2D

Save all slices for point in DB (u, u8, u8m, u8mb… etc)

Detect all visible top-level slices

Make a query:

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SELECT p FROM PointEntity p

JOIN p.slices s

WHERE s IN (:boundSlices)

Limitations Geohash define box, not point! Close points can have

different geohashes

1D queries return incorrect results near “faultlines”.

Solution – subqueries.

Close points not always have

common prefix

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Java implementations

javageomodel - http://code.google.com/p/javageomodel

geospatialweb - http://code.google.com/p/geospatialweb/

geohash-java - https://github.com/kungfoo/geohash-java

jGeohash - http://sourceforge.net/projects/jgeohash

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Thank you!

?

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