[postgis-devel] Sphere or Spheroid

Fri Oct 30 16:25:28 PDT 2009

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
>>
> _______________________________________________
> postgis-devel mailing list
> postgis-devel at postgis.refractions.net
> http://postgis.refractions.net/mailman/listinfo/postgis-devel
>
>
> _______________________________________________
> postgis-devel mailing list
> postgis-devel at postgis.refractions.net
> http://postgis.refractions.net/mailman/listinfo/postgis-devel
>