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From "Lance Norskog (JIRA)" <j...@apache.org>
Subject [jira] Commented: (SOLR-2155) Geospatial search using geohash prefixes
Date Thu, 21 Oct 2010 03:34:43 GMT

    [ https://issues.apache.org/jira/browse/SOLR-2155?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12923300#action_12923300
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Lance Norskog commented on SOLR-2155:
-------------------------------------

You're right, this had no context! 

>From the [geohash site|http://geohash.org/site/tips.html]: _Geohashes offer properties
like arbitrary precision, similar prefixes for nearby positions, and the possibility of gradually
removing characters from the end of the code to reduce its size (and gradually lose precision)._

If you store geohashes in a bitwise format, you get the "N leading bits" trick: the Manhattan
distance between any two hashes is the length of the first N matching bits. The more matching
bits starting from the highest or "hemisphere" bit, the closer two points are.

You can use this to search bounding boxes to a given Level Of Detail (LOD) by only comparing
the first N bits (The LOD is of course a power of 2). 

The core problem with this bitwise comparison trick is that one zero crossing is in Greenwich,
in Greater London. The other is at the equator. So this bitwise search trick works in most
of the world, just not in London or at the Equator.Street mapping and "find the nearest X"
are major use cases for geo-search. So, we have an ultra-fast bounding box search *that blows
up in London*.  (Of course, not just London everything at Longitude 0.00.) 

The longitude above goes through Unalaska in an area with no roads, giving a zero crossing
that blows up in a sparsely inhabited area. Then, instead of the Equator, use the North Pole
as the zero crossing. The longitude passes through the island where there are no roads, and
there are no streets (yet) at the North Pole.  _Street mapping applications would work perfectly
well with a Rotated Geohash._ Thus, rotating the geohash gives a variable-LOD bitwise search
that always works and is very very fast.

Super-fast Manhattan distance search may not be an interesting goal any more, since CPUs are
so fast. So, rotating the basis of the Geohash is probably not worthwhile. Also, it would
generate loads of confused traffic on solr-user.

Does this help?


> Geospatial search using geohash prefixes
> ----------------------------------------
>
>                 Key: SOLR-2155
>                 URL: https://issues.apache.org/jira/browse/SOLR-2155
>             Project: Solr
>          Issue Type: Improvement
>            Reporter: David Smiley
>         Attachments: GeoHashPrefixFilter.patch
>
>
> There currently isn't a solution in Solr for doing geospatial filtering on documents
that have a variable number of points.  This scenario occurs when there is location extraction
(i.e. via a "gazateer") occurring on free text.  None, one, or many geospatial locations might
be extracted from any given document and users want to limit their search results to those
occurring in a user-specified area.
> I've implemented this by furthering the GeoHash based work in Lucene/Solr with a geohash
prefix based filter.  A geohash refers to a lat-lon box on the earth.  Each successive character
added further subdivides the box into a 4x8 (or 8x4 depending on the even/odd length of the
geohash) grid.  The first step in this scheme is figuring out which geohash grid squares cover
the user's search query.  I've added various extra methods to GeoHashUtils (and added tests)
to assist in this purpose.  The next step is an actual Lucene Filter, GeoHashPrefixFilter,
that uses these geohash prefixes in TermsEnum.seek() to skip to relevant grid squares in the
index.  Once a matching geohash grid is found, the points therein are compared against the
user's query to see if it matches.  I created an abstraction GeoShape extended by subclasses
named PointDistance... and CartesianBox.... to support different queried shapes so that the
filter need not care about these details.
> This work was presented at LuceneRevolution in Boston on October 8th.

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