[postgis-devel] Sphere or Spheroid

[Paragon Corporation]

Paul,
My vote would be fast and not quite right. 

Though would be helpful if you could better quantify your  "slow" vs. "fast"
definition since I think that would be more of a deciding decision.

I assume it would depend on the number of points, size of geometry and holes
in a geometry but pick a reference geometry size and tell us the differences
in speed.

Probably 2 or 3 sets of the same geometry positioned across the globe (worst
case geom, average geom, point)  (and for each give some relative timings).

I'm still dreaming of the GUC by the way.
http://trac.osgeo.org/postgis/ticket/267 to make this decision less
relevant.

What is bool (will that mean True if you want measured in sphere?).
Regardless of what default you decide, that boolean has to mean the same
thing I think. (but I guess that's not too important)

Thanks,
Regina

-----Original Message-----
From: postgis-devel-bounces at postgis.refractions.net
[mailto:postgis-devel-bounces at postgis.refractions.net] On Behalf Of Paul
Ramsey
Sent: Friday, October 30, 2009 4:41 PM
To: PostGIS Development Discussion
Subject: Re: [postgis-devel] Sphere or Spheroid

I should add that in general, the differences seem to be about 0.1% to 0.2%
for my Oregon test area.

P.

On Fri, Oct 30, 2009 at 1:39 PM, Paul Ramsey <pramsey at opengeo.org> wrote:
> 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
>
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