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https://issues.apache.org/jira/browse/LUCENE-6191?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14285762#comment-14285762
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David Smiley commented on LUCENE-6191:
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I believe the % error of the facet results is the same as the resolution (grid
cell size) chosen + impacted by the resolution settings chosen when shapes were
indexed. Unfortunately, there isn't one error figure since at index time each
shape is independently turned into cells of a maximum resolution based on the
size of the shape (which can vary). So USSR is going to be approximated more
than a small country next to it would be. So if your heatmap is at a higher
resolution than the indexed shapes in the region in question, then you may
observe some blocky effects (e.g. all cells on one side of the heatmap could be
+1 due to matching an adjacent country). A "fix" could be to always index to a
maximum or fixed resolution... but that's not workable when the shape is large
-- it doesn't scale. I have an article on this subject:
http://opensourceconnections.com/blog/2014/04/11/indexing-polygons-in-lucene-with-accuracy/
> Spatial 2D faceting (heatmaps)
> ------------------------------
>
> Key: LUCENE-6191
> URL: https://issues.apache.org/jira/browse/LUCENE-6191
> Project: Lucene - Core
> Issue Type: New Feature
> Components: modules/spatial
> Reporter: David Smiley
> Assignee: David Smiley
> Fix For: 5.1
>
> Attachments: LUCENE-6191__Spatial_heatmap.patch
>
>
> Lucene spatial's PrefixTree (grid) based strategies index data in a way
> highly amenable to faceting on grids cells to compute a so-called _heatmap_.
> The underlying code in this patch uses the PrefixTreeFacetCounter utility
> class which was recently refactored out of faceting for NumberRangePrefixTree
> LUCENE-5735. At a low level, the terms (== grid cells) are navigated
> per-segment, forward only with TermsEnum.seek, so it's pretty quick and
> furthermore requires no extra caches & no docvalues. Ideally you should use
> QuadPrefixTree (or Flex once it comes out) to maximize the number grid levels
> which in turn maximizes the fidelity of choices when you ask for a grid
> covering a region. Conveniently, the provided capability returns the data in
> a 2-D grid of counts, so the caller needn't know a thing about how the data
> is encoded in the prefix tree. Well almost... at this point they need to
> provide a grid level, but I'll soon provide a means of deriving the grid
> level based on a min/max cell count.
> I recommend QuadPrefixTree with geo=false so that you can provide a square
> world-bounds (360x360 degrees), which means square grid cells which are more
> desirable to display than rectangular cells.
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