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From "David Smiley (JIRA)" <>
Subject [jira] Commented: (SOLR-2155) Geospatial search using geohash prefixes
Date Tue, 26 Oct 2010 15:46:23 GMT


David Smiley commented on SOLR-2155:

bq. Using the canonical geohash gives facet values that can be copy&pasted with other
software. Thinking again, this is a great feature. Would it be worth optimizing geohash with
a Trie version? Trie fields (can be made to) show up correctly in facets.

The geohash usage is purely internal to the implementation; users don't see it when they use
this field.  And even if they were exposed, they can be generated on-demand.  There's even
javascript code I've seen to do this.  So I'm not married to using geohashes -- it's the underlying
heirarchical/gridded nature of them that is key.  I'm not sure how a "trie version" of geohash
is developed.  I already spoke of further refining the implementation to index the geohashes
at each grid level and I think that is very similar to what trie does for numbers.

Thanks for the suggestion of using OpenStreetMaps to get locations; I'll look into that. 
I want to put together a useful data set -- using real data as much as possible is good. 
I'll need to synthesize a one-to-many document to points mapping, randomly, however.  And
I'll need to come up with various random lat-lon box queries to perform.  I'd like to use
Lucene's benchmark contrib module as a framework to develop the performance test.  I read
about it in LIA2 and it seems to fit the bill.

> Geospatial search using geohash prefixes
> ----------------------------------------
>                 Key: SOLR-2155
>                 URL:
>             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 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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