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From "David Smiley (JIRA)" <j...@apache.org>
Subject [jira] Commented: (SOLR-2155) Geospatial search using geohash prefixes
Date Fri, 22 Oct 2010 17:28:04 GMT

    [ https://issues.apache.org/jira/browse/SOLR-2155?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12923925#action_12923925
] 

David Smiley commented on SOLR-2155:
------------------------------------

Lance, did you look at my patch or just skim the issue description?  There seems to be a big
disconnect in what I'm doing and what you think I'm doing.

First of all, I'm re-using the existing geohash field support in Solr which indexes the lat-lons
as actual geohashes (i.e. the character representation), not in a bitwise fashion.  But that
doesn't really matter -- it would be a worthwhile optimization to index them in that fashion
as it would be more compact.

Secondly, as stated in the issue description, this filter finds _multiple_ geohash grid squares
to cover the queried area.  It doesn't matter where boundaries are, 0, 90, 180, Unalaska,
North pole, whatever -- it doesn't matter.  A search over the London or Greenwhich areas will
yield grid squares that have no prefixes in common, but that doesn't matter; +each grid square
is subsequently searched independently against the user's query+.

Thirdly, the *only* distance measurements in this patch are against _resolved_ latitude-longitudes
points (e.g. decoded geohashes) in the index against the user's query *if* that query is point-radius
(vs lat-lon bounding box which need not calculate distance).  This uses haversine bu  I'm
not using geohashes for distance calculation.

If you still insist there is a shortcoming of my implementation, then I challenge you to add
a unit test proving that my implementation here doesn't work.  The existing tests I used in
unit

> 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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