[LUCENE-1815] Geohash encode/decode floating point problems

• Type: Bug
• Status: Reopened
• Priority: Minor
• Resolution: Unresolved
• Affects Version/s: 2.9
• Component/s: modules/spatial
• Lucene Fields: New
• Assignee: Unassigned
• Reporter: Wouter Heijke
• Created: 17/Aug/09 08:54
• Updated: 28/Aug/22 12:06

Description

i'm finding the Geohash support in the spatial package to be rather unreliable.
Here is the outcome of a test that encodes/decodes the same lat/lon and geohash a few times.
the format:
action geohash=(latitude, longitude)

the result:
encode u173zq37x014=(52.3738007,4.8909347)
decode u173zq37x014=(52.373799999999996,4.890934)
encode u173zq37rpbw=(52.373799999999996,4.890934)
decode u173zq37rpbw=(52.373799999999996,4.8909329999999995)
encode u173zq37qzzy=(52.373799999999996,4.8909329999999995)

if I now change to the google code implementation:

encode u173zq37x014=(52.3738007,4.8909347)
decode u173zq37x014=(52.37380061298609,4.890934377908707)
encode u173zq37x014=(52.37380061298609,4.890934377908707)
decode u173zq37x014=(52.37380061298609,4.890934377908707)
encode u173zq37x014=(52.37380061298609,4.890934377908707)

Note the differences between the geohashes in both situations and the lat/lon's!
Now things get worse if you work on low-precision geohashes:

decode u173=(52.0,4.0)
encode u14zg429yy84=(52.0,4.0)
decode u14zg429yy84=(52.0,3.999999)
encode u14zg429ywx6=(52.0,3.999999)

and google:

decode u173=(52.20703125,4.5703125)
encode u17300000000=(52.20703125,4.5703125)
decode u17300000000=(52.20703125,4.5703125)
encode u17300000000=(52.20703125,4.5703125)

We are using geohashes extensively and will now use the google code version unfortunately.