[jira] [Created] (LUCENE-5271) A slightly more accurate SloppyMath distance

["Gilad Barkai (JIRA)" <ji...@apache.org>]

Gilad Barkai created LUCENE-5271:
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             Summary: A slightly more accurate SloppyMath distance
                 Key: LUCENE-5271
                 URL: https://issues.apache.org/jira/browse/LUCENE-5271
             Project: Lucene - Core
          Issue Type: Improvement
          Components: modules/other
            Reporter: Gilad Barkai
            Priority: Minor


SloppyMath, intriduced in LUCENE-5258, uses earth's avg. (according to WGS84) ellipsoid radius as an approximation for computing the "spherical" distance. (The TO_KILOMETERS constant).

While this is pretty accurate for long distances (latitude wise) this may introduce some small errors while computing distances close to the equator (as the earth radius there is larger than the avg.)

A more accurate approximation would be taking the avg. earth radius at the source and destination points. But computing an ellipsoid radius at any given point is a heavy function, and this distance should be used in a scoring function.. So two optimizations are optional - 
* Pre-compute a table with an earth radius per latitude (the longitude does not affect the radius)
* Instead of using two point radius avg, figure out the avg. latitude (exactly between the src and dst points) and get its radius.




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