[postgis-users] [postgis-devel] Sphere or Spheroid

[Paragon Corporation]

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 at postgis.refractions.net
[mailto:postgis-users-bounces at postgis.refractions.net] On Behalf Of
pcreso at pcreso.com
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 at cleverelephant.ca> wrote:

> From: Paul Ramsey <pramsey at cleverelephant.ca>
> Subject: Re: [postgis-users] [postgis-devel] Sphere or Spheroid
> To: "PostGIS Users Discussion" <postgis-users at postgis.refractions.net>
> 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 at gmail.com>
> wrote:
> > On Fri, Oct 30, 2009 at 5:48 PM, Paragon Corporation
> <lr at pcorp.us>
> 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 at gmail.com
> > www.greenwoodmap.com
> > _______________________________________________
> > postgis-users mailing list
> > postgis-users at postgis.refractions.net
> > http://postgis.refractions.net/mailman/listinfo/postgis-users
> >
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