Re: [postgis-devel] Sphere or Spheroid
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Re: [postgis-devel] Sphere or Spheroid
by Paul Ramsey-3
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It looks like, for basic usage, the performance difference is
relatively small (which is probably because most of the calculations are on the sphere, only the last few are on the spheroid). For cases line point/point tests the differences will be most stark. But here's a medium sized spatial join on line/polygon. geog=# select count(*) from buildings b, streets s where st_dwithin(b.geog, s.geog, 50000) and s.gid = 100; count -------- 144783 (1 row) Time: 6185.445 ms geog=# select count(*) from buildings b, streets s where st_dwithin(b.geog, s.geog, 50000, false) and s.gid = 100; count -------- 144795 (1 row) Time: 5879.593 ms geog=# On Fri, Oct 30, 2009 at 4:48 PM, Paragon Corporation <lr@...> wrote: > > The choice of default sphere vs. spheroid is probably something we should > ask the general PostGIS users group so cc'ing them since more people > frequent that list that would have strong opinions on this subject. > > So question folks -- is which do you prefer as default behavior for new > geography data type > > faster sphere calculation (with 0.05% - 1% margin error) (I vaguely recall > that's what my studies suggested around the globe compared to UTM) > > or > slower more accurate spheroid calculation > > Keep in mind -- this is just default behavior without an extra arg to the > functions, you can use both in your work. > > details of current functions below > > http://www.postgis.org/documentation/manual-svn/ch08.html#PostGIS_GeographyF > unctions > > Thanks, > Regina > -----Original Message----- > From: postgis-devel-bounces@... > [mailto:postgis-devel-bounces@...] On Behalf Of Paul > Ramsey > Sent: Friday, October 30, 2009 7:41 PM > To: PostGIS Development Discussion > Subject: Re: [postgis-devel] Sphere or Spheroid > > Oh, BTW, we could just as easily have a utility function > geography_set_spheroid(bool) that you can call from SQL that will manipulate > the global variable. That's all that the SET command ends up doing in the > end anyways. > > P. > > On Fri, Oct 30, 2009 at 4:29 PM, Paragon Corporation <lr@...> wrote: >> 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@... >> [mailto:postgis-devel-bounces@...] 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@...> 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@... >>> [mailto:postgis-devel-bounces@...] 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@...> 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@... >>> http://postgis.refractions.net/mailman/listinfo/postgis-devel >>> >>> >>> _______________________________________________ >>> postgis-devel mailing list >>> postgis-devel@... >>> http://postgis.refractions.net/mailman/listinfo/postgis-devel >>> >> _______________________________________________ >> postgis-devel mailing list >> postgis-devel@... >> http://postgis.refractions.net/mailman/listinfo/postgis-devel >> >> >> _______________________________________________ >> postgis-devel mailing list >> postgis-devel@... >> http://postgis.refractions.net/mailman/listinfo/postgis-devel >> > _______________________________________________ > postgis-devel mailing list > postgis-devel@... > http://postgis.refractions.net/mailman/listinfo/postgis-devel > > > _______________________________________________ > postgis-devel mailing list > postgis-devel@... > http://postgis.refractions.net/mailman/listinfo/postgis-devel > postgis-users mailing list postgis-users@... http://postgis.refractions.net/mailman/listinfo/postgis-users |
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Re: [postgis-devel] Sphere or Spheroid
by pcreso
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My 02c,
The simplest invocation generates the simplest/fastest result. A more complex invocation (specifying spheroid) generates a more accurate/complex/slower result. I'm not sure I really understand the question, as a query: ST_distance(g,g) returns a sphere based result, while ST_distance(g,g,spheroid) returns an spheroidal result. I think my preferred approach for this would be; If users can pre-set a default spheroid to use, then ST_distance(g,g) is still spherical, but ST_distance(g,g,'default') uses the selected default spheroid, otherwise the spheroid must be specified explicitly. So a spheroidal calculation is only applied when a spheroid is explicitly supplied in the function, otherwise use a spherical calculation. Cheers, Brent Wood --- On Sat, 10/31/09, Paragon Corporation <lr@...> wrote: > From: Paragon Corporation <lr@...> > Subject: Re: [postgis-users] [postgis-devel] Sphere or Spheroid > To: "'PostGIS Development Discussion'" <postgis-devel@...>, "'PostGIS Users Discussion'" <postgis-users@...> > Date: Saturday, October 31, 2009, 12:48 PM > > The choice of default sphere vs. spheroid is probably > something we should > ask the general PostGIS users group so cc'ing them since > more people > frequent that list that would have strong opinions on this > subject. > > So question folks -- is which do you prefer as default > behavior for new > geography data type > > faster sphere calculation (with 0.05% - 1% margin error) (I > vaguely recall > that's what my studies suggested around the globe compared > to UTM) > > or > slower more accurate spheroid calculation > > Keep in mind -- this is just default behavior without an > extra arg to the > functions, you can use both in your work. <snip> _______________________________________________ postgis-users mailing list postgis-users@... http://postgis.refractions.net/mailman/listinfo/postgis-users |
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Re: [postgis-devel] Sphere or Spheroid
by Richard Greenwood
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On Fri, Oct 30, 2009 at 5:48 PM, Paragon Corporation <lr@...> wrote:
> > The choice of default sphere vs. spheroid is probably something we should > ask the general PostGIS users group so cc'ing them since more people > frequent that list that would have strong opinions on this subject. > > So question folks -- is which do you prefer as default behavior for new > geography data type > > faster sphere calculation (with 0.05% - 1% margin error) (I vaguely recall > that's what my studies suggested around the globe compared to UTM) > > or > slower more accurate spheroid calculation I think that the default should be to "do it right" and that "doing it fast" should be optional. -- Richard Greenwood richard.greenwood@... www.greenwoodmap.com _______________________________________________ postgis-users mailing list postgis-users@... http://postgis.refractions.net/mailman/listinfo/postgis-users |
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Re: [postgis-devel] Sphere or Spheroid
by Paul Ramsey-3
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My feeling as well, particularly when comparing the spheroid distance
to the sphere distance to the distance on a properly projected plane. The spheroid and projected plane distances are very very close, the sphere is as much as 0.5% off them, depending on how long a distance you measure. P. On Fri, Oct 30, 2009 at 5:17 PM, Richard Greenwood <richard.greenwood@...> wrote: > On Fri, Oct 30, 2009 at 5:48 PM, Paragon Corporation <lr@...> wrote: >> >> The choice of default sphere vs. spheroid is probably something we should >> ask the general PostGIS users group so cc'ing them since more people >> frequent that list that would have strong opinions on this subject. >> >> So question folks -- is which do you prefer as default behavior for new >> geography data type >> >> faster sphere calculation (with 0.05% - 1% margin error) (I vaguely recall >> that's what my studies suggested around the globe compared to UTM) >> >> or >> slower more accurate spheroid calculation > > I think that the default should be to "do it right" and that "doing it > fast" should be optional. > > -- > Richard Greenwood > richard.greenwood@... > www.greenwoodmap.com > _______________________________________________ > postgis-users mailing list > postgis-users@... > http://postgis.refractions.net/mailman/listinfo/postgis-users > postgis-users mailing list postgis-users@... http://postgis.refractions.net/mailman/listinfo/postgis-users |
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Re: [postgis-devel] Sphere or Spheroid
by pcreso
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And I assume that while the count(*) difference will be less, it may still vary depending on which spheroid you use?
Brent --- On Sat, 10/31/09, Paul Ramsey <pramsey@...> wrote: > From: Paul Ramsey <pramsey@...> > Subject: Re: [postgis-users] [postgis-devel] Sphere or Spheroid > To: "PostGIS Development Discussion" <postgis-devel@...> > Cc: "PostGIS Users Discussion" <postgis-users@...> > Date: Saturday, October 31, 2009, 1:00 PM > It looks like, for basic usage, the > performance difference is > relatively small (which is probably because most of the > calculations > are on the sphere, only the last few are on the spheroid). > For cases > line point/point tests the differences will be most stark. > But here's > a medium sized spatial join on line/polygon. > > geog=# select count(*) from buildings b, streets s where > st_dwithin(b.geog, s.geog, 50000) and s.gid = 100; > count > -------- > 144783 > (1 row) > > Time: 6185.445 ms > geog=# select count(*) from buildings b, streets s where > st_dwithin(b.geog, s.geog, 50000, false) and s.gid = 100; > count > -------- > 144795 > (1 row) > > Time: 5879.593 ms > geog=# > > > > On Fri, Oct 30, 2009 at 4:48 PM, Paragon Corporation <lr@...> > wrote: > > > > The choice of default sphere vs. spheroid is probably > something we should > > ask the general PostGIS users group so cc'ing them > since more people > > frequent that list that would have strong opinions on > this subject. > > > > So question folks -- is which do you prefer as default > behavior for new > > geography data type > > > > faster sphere calculation (with 0.05% - 1% margin > error) (I vaguely recall > > that's what my studies suggested around the globe > compared to UTM) > > > > or > > slower more accurate spheroid calculation > > > > Keep in mind -- this is just default behavior without > an extra arg to the > > functions, you can use both in your work. > > > > details of current functions below > > > > http://www.postgis.org/documentation/manual-svn/ch08.html#PostGIS_GeographyF > > unctions > > > > Thanks, > > Regina > > -----Original Message----- > > From: postgis-devel-bounces@... > > [mailto:postgis-devel-bounces@...] > On Behalf Of Paul > > Ramsey > > Sent: Friday, October 30, 2009 7:41 PM > > To: PostGIS Development Discussion > > Subject: Re: [postgis-devel] Sphere or Spheroid > > > > Oh, BTW, we could just as easily have a utility > function > > geography_set_spheroid(bool) that you can call from > SQL that will manipulate > > the global variable. That's all that the SET command > ends up doing in the > > end anyways. > > > > P. > > > > On Fri, Oct 30, 2009 at 4:29 PM, Paragon Corporation > <lr@...> > wrote: > >> 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@... > >> [mailto:postgis-devel-bounces@...] > 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@...> > 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@... > >>> [mailto:postgis-devel-bounces@...] > 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@...> > 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@... > >>> http://postgis.refractions.net/mailman/listinfo/postgis-devel > >>> > >>> > >>> > _______________________________________________ > >>> postgis-devel mailing list > >>> postgis-devel@... > >>> http://postgis.refractions.net/mailman/listinfo/postgis-devel > >>> > >> _______________________________________________ > >> postgis-devel mailing list > >> postgis-devel@... > >> http://postgis.refractions.net/mailman/listinfo/postgis-devel > >> > >> > >> _______________________________________________ > >> postgis-devel mailing list > >> postgis-devel@... > >> http://postgis.refractions.net/mailman/listinfo/postgis-devel > >> > > _______________________________________________ > > postgis-devel mailing list > > postgis-devel@... > > http://postgis.refractions.net/mailman/listinfo/postgis-devel > > > > > > _______________________________________________ > > postgis-devel mailing list > > postgis-devel@... > > http://postgis.refractions.net/mailman/listinfo/postgis-devel > > > _______________________________________________ > postgis-users mailing list > postgis-users@... > http://postgis.refractions.net/mailman/listinfo/postgis-users > postgis-users mailing list postgis-users@... http://postgis.refractions.net/mailman/listinfo/postgis-users |
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Re: [postgis-devel] Sphere or Spheroid
by Paul Ramsey-3
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Yes, though right now you are hard-coded into WGS84. Funding
gratefully accepted to allow arbitrary spheroids :) P. On Fri, Oct 30, 2009 at 5:32 PM, <pcreso@...> wrote: > And I assume that while the count(*) difference will be less, it may still vary depending on which spheroid you use? > > Brent > > --- On Sat, 10/31/09, Paul Ramsey <pramsey@...> wrote: > >> From: Paul Ramsey <pramsey@...> >> Subject: Re: [postgis-users] [postgis-devel] Sphere or Spheroid >> To: "PostGIS Development Discussion" <postgis-devel@...> >> Cc: "PostGIS Users Discussion" <postgis-users@...> >> Date: Saturday, October 31, 2009, 1:00 PM >> It looks like, for basic usage, the >> performance difference is >> relatively small (which is probably because most of the >> calculations >> are on the sphere, only the last few are on the spheroid). >> For cases >> line point/point tests the differences will be most stark. >> But here's >> a medium sized spatial join on line/polygon. >> >> geog=# select count(*) from buildings b, streets s where >> st_dwithin(b.geog, s.geog, 50000) and s.gid = 100; >> count >> -------- >> 144783 >> (1 row) >> >> Time: 6185.445 ms >> geog=# select count(*) from buildings b, streets s where >> st_dwithin(b.geog, s.geog, 50000, false) and s.gid = 100; >> count >> -------- >> 144795 >> (1 row) >> >> Time: 5879.593 ms >> geog=# >> >> >> >> On Fri, Oct 30, 2009 at 4:48 PM, Paragon Corporation <lr@...> >> wrote: >> > >> > The choice of default sphere vs. spheroid is probably >> something we should >> > ask the general PostGIS users group so cc'ing them >> since more people >> > frequent that list that would have strong opinions on >> this subject. >> > >> > So question folks -- is which do you prefer as default >> behavior for new >> > geography data type >> > >> > faster sphere calculation (with 0.05% - 1% margin >> error) (I vaguely recall >> > that's what my studies suggested around the globe >> compared to UTM) >> > >> > or >> > slower more accurate spheroid calculation >> > >> > Keep in mind -- this is just default behavior without >> an extra arg to the >> > functions, you can use both in your work. >> > >> > details of current functions below >> > >> > http://www.postgis.org/documentation/manual-svn/ch08.html#PostGIS_GeographyF >> > unctions >> > >> > Thanks, >> > Regina >> > -----Original Message----- >> > From: postgis-devel-bounces@... >> > [mailto:postgis-devel-bounces@...] >> On Behalf Of Paul >> > Ramsey >> > Sent: Friday, October 30, 2009 7:41 PM >> > To: PostGIS Development Discussion >> > Subject: Re: [postgis-devel] Sphere or Spheroid >> > >> > Oh, BTW, we could just as easily have a utility >> function >> > geography_set_spheroid(bool) that you can call from >> SQL that will manipulate >> > the global variable. That's all that the SET command >> ends up doing in the >> > end anyways. >> > >> > P. >> > >> > On Fri, Oct 30, 2009 at 4:29 PM, Paragon Corporation >> <lr@...> >> wrote: >> >> 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@... >> >> [mailto:postgis-devel-bounces@...] >> 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@...> >> 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@... >> >>> [mailto:postgis-devel-bounces@...] >> 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@...> >> 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@... >> >>> http://postgis.refractions.net/mailman/listinfo/postgis-devel >> >>> >> >>> >> >>> >> _______________________________________________ >> >>> postgis-devel mailing list >> >>> postgis-devel@... >> >>> http://postgis.refractions.net/mailman/listinfo/postgis-devel >> >>> >> >> _______________________________________________ >> >> postgis-devel mailing list >> >> postgis-devel@... >> >> http://postgis.refractions.net/mailman/listinfo/postgis-devel >> >> >> >> >> >> _______________________________________________ >> >> postgis-devel mailing list >> >> postgis-devel@... >> >> http://postgis.refractions.net/mailman/listinfo/postgis-devel >> >> >> > _______________________________________________ >> > postgis-devel mailing list >> > postgis-devel@... >> > http://postgis.refractions.net/mailman/listinfo/postgis-devel >> > >> > >> > _______________________________________________ >> > postgis-devel mailing list >> > postgis-devel@... >> > http://postgis.refractions.net/mailman/listinfo/postgis-devel >> > >> _______________________________________________ >> postgis-users mailing list >> postgis-users@... >> http://postgis.refractions.net/mailman/listinfo/postgis-users >> > _______________________________________________ > postgis-users mailing list > postgis-users@... > http://postgis.refractions.net/mailman/listinfo/postgis-users 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Re: [postgis-devel] Sphere or Spheroid
by pcreso
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Some questions:
Which spheroid is used by default? (I commonly use 3 different ones) Does the geography datatype require a SRID (or spheroid?) to be specified, and reject a null one? (I'm not familiar with the geography implementation details) If there is no default spheroid available does ST_Distance() then fail unless it is explicitly told to use spherical? As I see it, the issue is less about providing the right (or best) answer by default, & more about protecting users from their ignorance. I'm a bit cautious about making assumptions about which spheroid should be applied by default for users who do not fully understand the issues. As long as the issue is well covered in the documentation, and both spherical & spheroidal versions are available, I don't have a strong preference either way. Cheers, Brent --- On Sat, 10/31/09, Paul Ramsey <pramsey@...> wrote: > From: Paul Ramsey <pramsey@...> > Subject: Re: [postgis-users] [postgis-devel] Sphere or Spheroid > To: "PostGIS Users Discussion" <postgis-users@...> > Date: Saturday, October 31, 2009, 1:28 PM > My feeling as well, particularly when > comparing the spheroid distance > to the sphere distance to the distance on a properly > projected plane. > The spheroid and projected plane distances are very very > close, the > sphere is as much as 0.5% off them, depending on how long a > distance > you measure. > > P. > > On Fri, Oct 30, 2009 at 5:17 PM, Richard Greenwood > <richard.greenwood@...> > wrote: > > On Fri, Oct 30, 2009 at 5:48 PM, Paragon Corporation > <lr@...> > wrote: > >> > >> The choice of default sphere vs. spheroid is > probably something we should > >> ask the general PostGIS users group so cc'ing them > since more people > >> frequent that list that would have strong opinions > on this subject. > >> > >> So question folks -- is which do you prefer as > default behavior for new > >> geography data type > >> > >> faster sphere calculation (with 0.05% - 1% margin > error) (I vaguely recall > >> that's what my studies suggested around the globe > compared to UTM) > >> > >> or > >> slower more accurate spheroid calculation > > > > I think that the default should be to "do it right" > and that "doing it > > fast" should be optional. > > > > -- > > Richard Greenwood > > richard.greenwood@... > > www.greenwoodmap.com > > _______________________________________________ > > postgis-users mailing list > > postgis-users@... > > http://postgis.refractions.net/mailman/listinfo/postgis-users > > > _______________________________________________ > postgis-users mailing list > postgis-users@... > http://postgis.refractions.net/mailman/listinfo/postgis-users > postgis-users mailing list postgis-users@... http://postgis.refractions.net/mailman/listinfo/postgis-users |
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Re: [postgis-devel] Sphere or Spheroid
by Paragon Corporation-2
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I think for the functions, we don't need to specify the spheroid
even looking forward when we support more than WGS 84 The options will be a) Use sphere b) Use spheroid data is stored in (you can only store your data in WGS 84 when using geography) Is WGS 84 especially bad for some regions (I'm not as up on this as I should be?) In which case one could argue that for certain cases both are wrong anyway though sphere is more wrong than WGS 84 spheroid. The timings are not quite as different as I thought, so I'm leaning on the side of indifference (or switching my sides) . The main reason I would lean toward the using spheroid approach is I believe that is what the other databases do. E.g SQL Server measures spheroid, I believe Oracle does as well, and not sure about IBM DB2 I can only assume so. So I suppose for sake of sticking with the crowd and comfort of crowds that would be what to choose. Thanks, Regina -----Original Message----- From: postgis-users-bounces@... [mailto:postgis-users-bounces@...] On Behalf Of pcreso@... Sent: Friday, October 30, 2009 8:49 PM To: PostGIS Users Discussion Subject: Re: [postgis-users] [postgis-devel] Sphere or Spheroid Some questions: Which spheroid is used by default? (I commonly use 3 different ones) Does the geography datatype require a SRID (or spheroid?) to be specified, and reject a null one? (I'm not familiar with the geography implementation details) If there is no default spheroid available does ST_Distance() then fail unless it is explicitly told to use spherical? As I see it, the issue is less about providing the right (or best) answer by default, & more about protecting users from their ignorance. I'm a bit cautious about making assumptions about which spheroid should be applied by default for users who do not fully understand the issues. As long as the issue is well covered in the documentation, and both spherical & spheroidal versions are available, I don't have a strong preference either way. Cheers, Brent --- On Sat, 10/31/09, Paul Ramsey <pramsey@...> wrote: > From: Paul Ramsey <pramsey@...> > Subject: Re: [postgis-users] [postgis-devel] Sphere or Spheroid > To: "PostGIS Users Discussion" <postgis-users@...> > Date: Saturday, October 31, 2009, 1:28 PM My feeling as well, > particularly when comparing the spheroid distance to the sphere > distance to the distance on a properly projected plane. > The spheroid and projected plane distances are very very close, the > sphere is as much as 0.5% off them, depending on how long a distance > you measure. > > P. > > On Fri, Oct 30, 2009 at 5:17 PM, Richard Greenwood > <richard.greenwood@...> > wrote: > > On Fri, Oct 30, 2009 at 5:48 PM, Paragon Corporation > <lr@...> > wrote: > >> > >> The choice of default sphere vs. spheroid is > probably something we should > >> ask the general PostGIS users group so cc'ing them > since more people > >> frequent that list that would have strong opinions > on this subject. > >> > >> So question folks -- is which do you prefer as > default behavior for new > >> geography data type > >> > >> faster sphere calculation (with 0.05% - 1% margin > error) (I vaguely recall > >> that's what my studies suggested around the globe > compared to UTM) > >> > >> or > >> slower more accurate spheroid calculation > > > > I think that the default should be to "do it right" > and that "doing it > > fast" should be optional. > > > > -- > > Richard Greenwood > > richard.greenwood@... > > www.greenwoodmap.com > > _______________________________________________ > > postgis-users mailing list > > postgis-users@... > > http://postgis.refractions.net/mailman/listinfo/postgis-users > > > _______________________________________________ > postgis-users mailing list > postgis-users@... > http://postgis.refractions.net/mailman/listinfo/postgis-users > postgis-users mailing list postgis-users@... http://postgis.refractions.net/mailman/listinfo/postgis-users _______________________________________________ postgis-users mailing list postgis-users@... http://postgis.refractions.net/mailman/listinfo/postgis-users |
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Re: [postgis-devel] Sphere or Spheroid
by Paul Ramsey-3
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On Fri, Oct 30, 2009 at 5:48 PM, <pcreso@...> wrote:
> Which spheroid is used by default? (I commonly use 3 different ones) WGS84 > Does the geography datatype require a SRID (or spheroid?) to be specified, and reject a null one? (I'm not familiar with the geography implementation details) Unlike geometry, if you don't specify an SRID, you'll get 4326 set by default. The default user is assumed to be modern. The implementation will be SRID-based not magic-text-string based, as the current _spheroid functions in geometry are. > If there is no default spheroid available does ST_Distance() then fail unless it is explicitly told to use spherical? The default behaviour is what we're on about, but there's no reason to prefer sphere over spheroid on the basis of a missing SRID, since the values of WGS84 are well-known. > As I see it, the issue is less about providing the right (or best) answer by default, & more about protecting users from their ignorance. And most users of geography will in fact be entirely ignorant of issues of spheroid. > I'm a bit cautious about making assumptions about which spheroid should be applied by default for users who do not fully understand the issues. I'm the opposite. Most folks will be using lat/lons they got from who-knows-where. A WGS84 assumption is the best thing we can provide. > As long as the issue is well covered in the documentation, and both spherical & spheroidal versions are available, I don't have a strong preference either way. It'll be documented. P. _______________________________________________ postgis-users mailing list postgis-users@... http://postgis.refractions.net/mailman/listinfo/postgis-users |
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Re: [postgis-devel] Sphere or Spheroid
by Paul Ramsey-3
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On Fri, Oct 30, 2009 at 6:04 PM, Paragon Corporation <lr@...> wrote:
> The main reason I would lean toward the using spheroid approach is I believe > that is what the other databases do. E.g SQL Server measures spheroid, I > believe Oracle does as well, and not sure about IBM DB2 I can only assume > so. Indeed, one of my testers has been pointing out differences between postgis and oracle which are arising precisely because the postgis implementation is sphere based (or rather, *was* sphere based). I think I have satisfied the question entirely in my mind, spheroid should be the default. If people want to hack for speed, they have that option. P. _______________________________________________ postgis-users mailing list postgis-users@... http://postgis.refractions.net/mailman/listinfo/postgis-users |
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