[postgis-devel] Sphere or Spheroid

[Paul Ramsey]

I am just now hooking up some spheroid calculations into SQL and the
choice is here: what is the default return for ST_Distance(geog, geog)
? I have an extra signature ST_Distance(geog, geog, bool) which allows
you to choose, *but* something has to be the default. So, slow and
accurate or fast and not-quite-right? For reference, here's some test
numbers:

     spheroid     |      sphere      |         diff         |
diff_pct
------------------+------------------+----------------------+-----------------------
 14.1244075108035 | 14.1394160043876 |  -0.0150084935841761 |
-0.00106146488507862
 14.6266188748041 | 14.5910056309084 |   0.0356132438956287 |
0.00244076692151968
 20.7346086743544 | 20.7259709082645 |  0.00863776608995082 |
0.000416760504402065
 21.9682444424626 | 21.9811452394388 |  -0.0129007969762469 |
-0.000586902858596291
 23.7383733187578 | 23.7519404190066 |  -0.0135671002487641 |
-0.000571199658193295
   24.71417232105 | 24.6751858115921 |   0.0389865094578177 |
0.00157998848541607
  26.060846145618 | 25.9957171978245 |   0.0651289477934469 |
0.00250537222334829
 29.1593418185795 | 29.0828389449609 |   0.0765028736186224 |
0.00263051601542076
 30.4726043383638 |  30.407477679772 |   0.0651266585918009 |
0.00214179746434954
 33.3089494734479 | 33.2231927324098 |    0.085756741038054 |
0.00258123118174602
 54.5278744382593 | 54.5740236709768 |  -0.0461492327175321 |
-0.000845626355054244
 56.6270596077484 | 56.4794616931023 |    0.147597914646141 |
0.00261330243280571
 58.0564235524421 | 57.9072919817022 |    0.149131570739875 |
0.00257535045477517
 61.5962238336604 | 61.6609971505153 |  -0.0647733168549323 |
-0.00105047468980788
 61.7015675105702 | 61.5433341315128 |    0.158233379057449 |
0.00257108883180295
 67.6094786823676 | 67.4707740811243 |    0.138704601243305 |
0.00205577308297266
 68.2569872101138 | 68.0830690889332 |    0.173918121180577 |
0.00255449884248604
 68.4943435865133 | 68.5554249222873 |  -0.0610813357739914 |
-0.000890977422184046
 68.5746705524887 | 68.4885568487924 |   0.0861137036962845 |
0.00125734440406453
 68.6758026445452 | 68.7123761613873 |  -0.0365735168421395 |
-0.000532269714501474
 73.4229241308488 | 73.4212994751178 |  0.00162465573099269 |
2.21278531244639e-05
 79.4484754792361 | 79.4536134385929 | -0.00513795935687256 |
-6.46661509088383e-05
 85.4392924899187 | 85.3402382287158 |   0.0990542612029799 |
0.00116069820355446
 85.4539811995932 | 85.3919394786419 |   0.0620417209512425 |
0.00072655242789936
 93.5788665631846 | 93.4985937971677 |   0.0802727660169325 |
0.000858545169043645
 96.3199573238008 | 96.1797387047942 |     0.14021861900666 |
0.00145788105577033
 101.328501281213 | 101.062654489096 |    0.265846792116648 |
0.00263051463926598
 101.649559586565 | 101.502533562595 |     0.14702602397027 |
0.00144849609965254
 108.809850830453 | 108.894277925073 |  -0.0844270946195422 |
-0.000775312497848917
 109.397307465226 | 109.316521134613 |   0.0807863306121561 |
0.000739013003466101
 110.654864554623 | 110.763015546595 |   -0.108150991971058 |
-0.000976417908426867
  113.60376179349 | 113.561772904965 |   0.0419888885251396 |
0.000369744919007898
 114.121155128581 | 113.828085749691 |    0.293069378889427 |
0.00257466667351227
 114.310532244975 | 114.431988777869 |   -0.121456532893603 |
-0.00106138619271374
 115.797340494535 | 115.877511025684 |  -0.0801705311490082 |
-0.000691855826375479
 116.509315790761 | 116.204507950436 |    0.304807840325097 |
0.00262302939620126
 116.944936442573 | 116.776885086999 |    0.168051355574235 |
0.00143908064895751
 118.802848975488 | 118.599760910854 |    0.203088064634358 |
0.00171238173732078
 124.752683002401 | 124.793829377275 |  -0.0411463748743159 |
-0.000329714819071083