com.graphhopper.geohash

Class SpatialKeyAlgo

java.lang.Object

com.graphhopper.geohash.SpatialKeyAlgo

public class SpatialKeyAlgo
extends Object

This class implements the idea of a geohash but without a string representation - to avoid confusion, this is called 'spatial key'. Detailed information is available in this blog post: http://karussell.wordpress.com/2012/05/23/spatial-keys-memory-efficient-geohashes/

The hash can be used as a key for hash tables. When you organize the grid as a quad tree, it resembles the path down the tree to reach the cell that it encodes. That's how it is used in LocationIndexTree.

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

Implementation: - From the query point and the grid parameters, calculate (integer) coordinates (x,y) of the cell the query point is in, using simple arithmetics. - Use a lookup table to interleave the bits of (x,y) to get the cell number, which is the spatial key. See the drawing below. This is called a Z-order curve (because of the path you get when you follow increasing cell numbers through the grid), or Morton code.

Author:

Peter Karich, Michael Zilske

Constructor Summary

Constructors

Constructor and Description
SpatialKeyAlgo(int allBits, BBox bounds)

Method Summary

Modifier and Type	Method and Description
int[]	decode(long z)
long	down(long z)
long	encode(int x, int y)
long	encodeLatLon(double lat, double lon)
int	getBits()
long	left(long z)
long	right(long z)
long	up(long z)
int	x(double lon)
int	y(double lat)

Methods inherited from class java.lang.Object

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

Constructor Detail

SpatialKeyAlgo

public SpatialKeyAlgo(int allBits,
                      BBox bounds)

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

encodeLatLon

public final long encodeLatLon(double lat,
                               double lon)

y

public int y(double lat)

x

public int x(double lon)

encode

public long encode(int x,
                   int y)

decode

public int[] decode(long z)

up

public long up(long z)

down

public long down(long z)

right

public long right(long z)

left

public long left(long z)