[postgis-users] ST_length_spheroid and ST_distance_spheroid

[Paul Ramsey]

You're such a Black Team member, Regina :)
I think the geography spheroid calculation falls back to a sphere
calculation if it is handed the 180 case, and probably the 180 degree
test is sufficiently fuzzy that it is picking up "pretty close"
results.
Worth a ticket, I'm sure we can make things a bit more consistent, for
the geography type too.

P

On Thu, Apr 22, 2010 at 10:24 AM, Paragon Corporation <lr at pcorp.us> wrote:
> Also not entirely true.
>
> If we set to 179.9999
>
> Answers are different
>
> Though 179 they are the same
>
> SELECT st_distance_spheroid(geomfromtext('POINT(0 0)',4326),
> geomfromtext('POINT(179
> 0)',4326),'SPHEROID["WGS84",6378137,298.257223563]');
>
> On 1.5 -  19926188.8494779
>
> On 1.4 -  19926188.851996
>
> However:
>
> SELECT st_distance_spheroid(geomfromtext('POINT(0 0)',4326),
> geomfromtext('POINT(179.999
> 0)',4326),'SPHEROID["WGS84",6378137,298.257223563]');
> On 1.4 - 20037397.0232984
> On 1.5 - 19903482.0714439
> -----Original Message-----
> From: postgis-users-bounces at postgis.refractions.net
> [mailto:postgis-users-bounces at postgis.refractions.net] On Behalf Of Paragon
> Corporation
> Sent: Thursday, April 22, 2010 1:18 PM
> To: 'PostGIS Users Discussion'
> Subject: Re: [postgis-users] ST_length_spheroid and ST_distance_spheroid
>
> I think it's a difference in versions
>
> On our PostGIS 1.4 windows the answers are the same (note Paul was showing a
> 1.4 answer I presume from his command prompt)
>
> On our PostGIS 1.5 its different.
>
> I suspect its because in
>
> PostGIS 1.5 the distance_spheroid piggy backs on the geography support to
> support more than just points.
>
> I suspect the length_spheroid logic wasn't changed to do the same which is
> why it gives the same answer as 1.4
>
> Hope that helps,
> Regina and Leo
> http://www.postgis.us
>
>
>
> -----Original Message-----
> From: postgis-users-bounces at postgis.refractions.net
> [mailto:postgis-users-bounces at postgis.refractions.net] On Behalf Of Olivier
> Courtin
> Sent: Thursday, April 22, 2010 1:07 PM
> To: PostGIS Users Discussion
> Subject: Re: [postgis-users] ST_length_spheroid and ST_distance_spheroid
>
>
> On Apr 22, 2010, at 6:54 PM, Paul Ramsey wrote:
>
>> If you change the 180 to 179 do the results match up again? (ie, do
>> any distance less than a perfect half-sphere?)
>
> Yes, right assertion.
> Values are the same below 180
>
> --
> Olivier
>
>
>
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