[postgis-users] Re: Comparing Geometries with Different SRIDs
Paragon Corporation
lr at pcorp.us
Sun Sep 28 21:58:23 PDT 2008
Brent,
Ah but in SQL server with dictionary sort order -
select 'Regina' union select 'REGINA';
Is
Regina
(1 row)
I'm with Paul on this one though
> I would expect A = B to return false if SRIDs are not the same, but
>ST_Equals(A,B) to error out if SRIDs are not the same. I'm that screwed up.
Blame my parents.
ST_Equals(different srids) should return an error. Its not that its unknown
its that we are too lazy to calculate it. Its quite known if we had the
energy.
We should go with binary equal rather than geometric equality. I think that
would satisfy Charlie too. I guess I have my parents to blame too :)
Thanks,
Regina
-----Original Message-----
From: postgis-users-bounces at postgis.refractions.net
[mailto:postgis-users-bounces at postgis.refractions.net] On Behalf Of Brent
Wood
Sent: Sunday, September 28, 2008 6:40 PM
To: PostGIS Users Discussion
Subject: RE: [postgis-users] Re: Comparing Geometries with Different SRIDs
After a belated case of RTFM...
It seems UNION does an implict distinct on all rows, unless UNION ALL is
applied (How many Regina's do you want :-)
I guess it makes sense to drop duplicates unless otherwise specified.
stations=# select 1 union select 1;
?column?
----------
1
(1 row)
stations=# select 1 union all select 1;
?column?
----------
1
1
(2 rows)
stations=# select 'Regina' union select 'REGINA'; ?column?
----------
Regina
REGINA
(2 rows)
stations=# select 'Regina' union select 'REGINA' union all select 'Regina';
?column?
----------
Regina
REGINA
Regina
(3 rows)
Cheers
Brent Wood
--- On Mon, 9/29/08, Paragon Corporation <lr at pcorp.us> wrote:
> From: Paragon Corporation <lr at pcorp.us>
> Subject: RE: [postgis-users] Re: Comparing Geometries with Different
> SRIDs
> To: "'PostGIS Users Discussion'"
> <postgis-users at postgis.refractions.net>
> Date: Monday, September 29, 2008, 11:13 AM Brent, Ah never mind. For
> some reason I was under the false assumption that in SQL Server with
> dictionary sort order that even though
>
> REGINA = Regina
>
> That I would get 2 Reginas when I union them. That is not the case.
> I just get one.
>
> I'm not sure why I thought that or maybe it was like that a long time
> ago.
>
> Anyrate - so I guess I am back to square one.
>
> By the way - I think we should be talking about ST_OrderingEquals not
> ST_Equals.
>
> I'm not sure which if any of those PostGIS is using to do the check,
> but it should be ST_OrderingEquals since using ST_Equals introduces a
> whole lot more philosophical can of worms. Is space empty or is Empty
> space.
>
> Thanks,
> Regina
>
>
>
>
> -----Original Message-----
> From: Brent Wood [mailto:pcreso at yahoo.com]
> Sent: Sunday, September 28, 2008 3:19 PM
> To: lr at pcorp.us
> Subject: RE: [postgis-users] Re: Comparing Geometries with Different
> SRIDs
>
> Hi Regina,
>
> I think that is more an encoding/language issue than case in such an
> analogy.
>
> To wax metaphysical, does a muslim worship a different god because
> they spell it allah? (Similarly Jews with Jehovah & French with
> dieu...)
>
> So is st_equals() a cross-language thesaurus, or is it restricted to
> the specified SRID? :-)
>
>
>
>
> Spotcha,
>
> Brent Wood
>
>
> --- On Mon, 9/29/08, Paragon Corporation <lr at pcorp.us> wrote:
>
> > From: Paragon Corporation <lr at pcorp.us>
> > Subject: RE: [postgis-users] Re: Comparing Geometries
> with Different
> > SRIDs
> > To: "'PostGIS Users Discussion'"
> > <postgis-users at postgis.refractions.net>
> > Date: Monday, September 29, 2008, 8:04 AM Charlie,
> >
> > On second thought not sure how DB II deals with
> relational Union of
> > geometries. Does anybody have that by chance to test?
> >
> > I suppose we could say this is the same deal as
> difference in casing.
> >
> > SELECT 'Regina'
> > UNION
> > SELECT 'REGINA'
> > UNION
> > SELECT 'Regina'
> >
> > Even if both geometries are the same which SRID would
> you choose? So
> > we should not be doing a geometry compare, but a
> binary compare.
> >
> > SELECT ST_AsEWKB('SRID=4269;POINT(1 3)') =
> > ST_AsEWKB('SRID=4326;POINT(1 3)')
> >
> >
> > Thanks,
> > Regina
> >
> > -----Original Message-----
> > From: postgis-users-bounces at postgis.refractions.net
> > [mailto:postgis-users-bounces at postgis.refractions.net]
> On Behalf Of
> > Paragon Corporation
> > Sent: Sunday, September 28, 2008 2:48 PM
> > To: 'PostGIS Users Discussion'
> > Subject: RE: [postgis-users] Re: Comparing Geometries
> with Different
> > SRIDs
> >
> > I feel your pain Charlie, but I'm on Reid's
> side.
> > Given that some spatial
> > databases do support cross compare between geometries
> of different
> > SRID (e.g. DB II), not sure about Oracle or SQL Server
> 2008, I would
> > prefer an answer that is in compliance or throws an
> exception when it
> > can't be sure.
> > Its annoying from a debugging perspective to deal
> with things that
> > fail silently. Returning false is a fail silently in
> my book.
> >
> > If we deal with this as a special case - why
> wouldn't we deal with
> > everything else the same with every other relational
> compare we do?
> > Its just wrong, but granted doing the wrong thing
> would be
> > conveniently useful in many cases.
> >
> > Thanks,
> > Regina
> >
> >
> >
> > -----Original Message-----
> > From: postgis-users-bounces at postgis.refractions.net
> > [mailto:postgis-users-bounces at postgis.refractions.net]
> On Behalf Of
> > Charlie Savage
> > Sent: Sunday, September 28, 2008 2:10 PM
> > To: PostGIS Users Discussion
> > Subject: Re: [postgis-users] Re: Comparing Geometries
> with Different
> > SRIDs
> >
> >
> >
> > Reid Priedhorsky wrote:
> > > Charlie Savage wrote:
> > >>> The 2 point could be spatially equal
> given
> > different SRIDS and
> > >>> coordinates if they were projected to a
> common
> > SRID. So should
> > >>> geometry operators silently Call
> st_transform
> > to make the righthand
> > >>> match the lefthand before comparing? This
> > would be quite the
> > >>> expensive operation.
> > >>
> > >> No. You can't automatically transform
> between
> > different SRID values
> > >> Think of the case of one geometry with an
> SRID
> > value of 4326 and one
> > >> with an SRID value of -1 (no coordinate
> system).
> > >
> > > Exactly.
> > >
> > >> So different SRID values, then the geometries
> are
> > not equal.
> > >
> > > No -- as Stanley said, the geometries could be in
> fact
> > equal, but
> > > expressed in different SRS. So if ST_Equals()
> returned
> > False, it would
> > > be wrong.
> >
> > Maybe. But returning "Operation permitted"
> is even worse because it
> > means you can't do natural things like this
> (without extra annoying
> > SRID checking
> > code) in plpgsql:
> >
> > IF (geom1 == geom2) ... END IF;
> >
> > Or the example with the union earlier posted
> >
> >
> > >> It is up to the user to transform geometries
> to
> > the same SRID before
> > >> calling ST_EQUALS.
> > >
> > > Exactly. ;)
> >
> > So, I still vote that st_equals will not blow up when
> comparing two
> > geometries with different SRID values, it just will
> return false.
> >
> > Charlie
> >
> >
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