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From "Lance Norskog (JIRA)" <j...@apache.org>
Subject [jira] Issue Comment Edited: (SOLR-2155) Geospatial search using geohash prefixes
Date Sat, 16 Oct 2010 21:24:22 GMT

    [ https://issues.apache.org/jira/browse/SOLR-2155?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12921761#action_12921761
] 

Lance Norskog edited comment on SOLR-2155 at 10/16/10 5:23 PM:
---------------------------------------------------------------

The problem with Geohash is that it puts zeros in Greater London and at the Equator, so every
computation that uses it has to dodge at these points. More to the point, the Hamming distance
trick does not work, so a simple super-fast scan of an array of Lucene Trie-Integers does
not work.

On the Aleutian island of Unalaska, there is a longitude which goes through a mountainous
region with no roads. The longitude does not touch any other land north of Antarctica.

[Unalaska|http://maps.google.com/maps?q=65%C2%B037%E2%80%B221%E2%80%B3N+168%C2%B020%E2%80%B242%E2%80%B3W]

65°37′21″N 168°20′42″W

If you rotate the geohash frame of reference to latitude North Pole and a longitude through
Unalaska, you can cheerfully ignore all of the zero points, at the cost of alienating some
Inuit.

Seriously, if you limit Geohash accuracy to land-based services (like maps), with an accuracy
warning about comparing Nome and Vladivostok, sacrificing a road-less part of an Aleutian
island seems a small price to pay. 

      was (Author: lancenorskog):
    The problem with Geohash is that it puts zeros in Greater London and at the Equator, so
every computation that uses it has to dodge at these points. More to the point, the Hamming
distance trick does not work, so a simple super-fast scan of an array of Lucene Trie-Integers
does not work.

On the Aleutian island of Unalaska, there is a longitude which goes through a mountainous
region with no roads. The longitude does not touch any other land north of Antarctica.

[Unalaska|http://maps.google.com/maps?q=65%C2%B037%E2%80%B221%E2%80%B3N+168%C2%B020%E2%80%B242%E2%80%B3W]

65°37′21″N 168°20′42″W

If you rotate the geohash frame of reference to latitude North Pole and a longitude through
Unalaska, you can cheerfully ignore all of the zero points, at the cost of alienating some
Inuit.

  
> 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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