com.graphhopper.geohash

Class SpatialKeyAlgo

java.lang.Object

com.graphhopper.geohash.SpatialKeyAlgo

All Implemented Interfaces:

KeyAlgo

public class SpatialKeyAlgo
extends Object
implements KeyAlgo

This class implements the idea of a geohash but in 'binary form' - to avoid confusion this is called 'spatial key'. The idea of mixing the latitude and longitude is also taken to allow removing the insignificant (right side) bits to make a geo-query or the coordinate less precise. E.g. for a 3 bit precision the spatial key would need 6 bits and look like:

lat0 lon0 | lat1 lon1 | lat2 lon2

This works similar to how BIG endianness works for bytes to int packing. Detailed information is available in this blog post: http://karussell.wordpress.com/2012/05/23/spatial-keys-memory-efficient-geohashes/

The bits are usable as key for hash tables like our SpatialKeyHashtable or for a spatial tree like QuadTreeSimple. Also the binary form makes it relative simple for implementations using this encoding scheme to expand to arbitrary dimension (e.g. shifting n-times if n would be the dimension).

A 32 bit representation has a precision of approx 600 meters = 40000/2^16

There are different possibilities how to handle different precision and order of bits. Either:

lat0 lon0 | lat1 lon1 | lat2 lon2

0 0 | lat0 lon0 | lat1 lon1

as it is done now. Advantage: A single shift is only necessary to make it less precise. Or:

lat2 lon2 | lat1 lon1 | lat0 lon0

0 0 | lat1 lon1 | lat0 lon0

Advantage: the bit mask to get lat0 lon0 is simple: 000..0011 and independent of the precision! But when stored e.g. as int one would need to (left) shift several times if precision is only 3bits.

Author:

Peter Karich

Constructor Summary

Constructors

Constructor and Description
SpatialKeyAlgo(int allBits)

Method Summary

Modifier and Type	Method and Description
SpatialKeyAlgo	bounds(BBox box)
void	decode(long spatialKey, GHPoint latLon) This method returns latitude and longitude via latLon - calculated from specified spatialKey
long	encode(double lat, double lon) Take latitude and longitude as input.
long	encode(GHPoint coord)
int	getBits()
int	getExactPrecision()
SpatialKeyAlgo	setBounds(double minLonInit, double maxLonInit, double minLatInit, double maxLatInit) Sets the bounds of the underlying key algorithm.
protected void	setWorldBounds()
String	toString()

Methods inherited from class java.lang.Object

clone, equals, finalize, getClass, hashCode, notify, notifyAll, wait, wait, wait

Constructor Detail

SpatialKeyAlgo

public SpatialKeyAlgo(int allBits)

Parameters:

allBits - how many bits should be used for the spatial key when encoding/decoding

Method Detail

getBits

public int getBits()

Returns:

the number of involved bits

getExactPrecision

public int getExactPrecision()

bounds

public SpatialKeyAlgo bounds(BBox box)

setBounds

public SpatialKeyAlgo setBounds(double minLonInit,
                                double maxLonInit,
                                double minLatInit,
                                double maxLatInit)

Description copied from interface: KeyAlgo

Sets the bounds of the underlying key algorithm.

Specified by:

setBounds in interface KeyAlgo

setWorldBounds

protected void setWorldBounds()

encode

public long encode(GHPoint coord)

Specified by:

encode in interface KeyAlgo

encode

public final long encode(double lat,
                         double lon)

Take latitude and longitude as input.

Specified by:

encode in interface KeyAlgo

Returns:

the spatial key

decode

public final void decode(long spatialKey,
                         GHPoint latLon)

This method returns latitude and longitude via latLon - calculated from specified spatialKey

Specified by:

decode in interface KeyAlgo

Parameters:

spatialKey - is the input

toString

public String toString()

Overrides:

toString in class Object