[postgis-devel] Sphere or Spheroid

Fri Oct 30 16:29:30 PDT 2009

Ah okay, maybe that was why we decided against it last time.  You should put
that in the notes.  Anyrate even if we had GUC, I would still go with the
multiple signatures as well since you may want different behavior for a
specific query or when you are comparing numbers (you want both).

-----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 7:25 PM
To: PostGIS Development Discussion
Subject: Re: [postgis-devel] Sphere or Spheroid

I did GUC research this morning and even did a GUC implementation! I don't
think it makes this decision any less relevant, because frankly the default
will rule 95% of the users. So choosing the "right"
default is a good idea.

Anyhow, on the GUC, a module-controlled GUC has an issue, in that the GUC
can't exist until it's initialized by the module, and the initialization
routine for the module is not executed until the first time a module
function is called. So, running "SET geography.use_spheroid = true" will
return an error if it's the first thing you do, but will work if you first
do something like "SELECT ST_AsText('POINT(0 0)'::geography)" to force the
module to load

In all, a non-happy situation, so I have opted for function signatures for
now, unless someone can tell me I should do otherwise.

P.

On Fri, Oct 30, 2009 at 4:10 PM, Paragon Corporation <lr at pcorp.us> wrote:
> 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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