[postgis-users] ST_length_spheroid and ST_distance_spheroid

[Paul Ramsey]

Explication http://a.mongers.org/clueful/20020402-peopleware-blackteam

On Thu, Apr 22, 2010 at 10:48 AM, Paul Ramsey <pramsey at cleverelephant.ca> wrote:
> 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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