# [postgis-users] Geography point to polygon st_distance for a polygon crossing the equator

Sebastien Delaux s.delaux at metocean.co.nz
Mon Feb 20 11:56:43 PST 2017

```Thanks Paul, I get it now.
So I guess, it all comes from the fact that an arc is defined as the
shortest path between 2 points which in the geometry case is a straight
line and in the geography case is a great circle.
This also mean that "the polygon I had in mind" cannot be defined as a
geography polygon.

S.

On Tue, Feb 21, 2017 at 6:03 AM, Paul Ramsey <pramsey at cleverelephant.ca>
wrote:

> :) :) :)
>
> Your second polygon has the same problem as the first, except in
> reverse... Your second polygon basically covers no area at all, since it
> consists of two arcs that both do a direct southerly run to the south pole.
>
> P.
>
>
> On Sun, Feb 19, 2017 at 8:09 PM, Sebastien Delaux <s.delaux at metocean.co.nz
> > wrote:
>
>> Yes, that's what I thought. That would explain why (0,-82) is inside the
>> polygon.
>> Nevertheless, if my first segment runs through the south pole, then I
>> would expect both of the queries mentioned in my initial post to return 0
>> which is not the case.
>>
>> Sebastien
>>
>> On Mon, Feb 20, 2017 at 4:56 PM, Paul Ramsey <pramsey at cleverelephant.ca>
>> wrote:
>>
>>> I don't the polygon you've draw means what you think it does.
>>>
>>> POLYGON((-90 -80, 90 -80, 90 10, -90 10, -90 -80))
>>>
>>> For example, you probably figure the first segment, -90 -80, 90 -80
>>> runs east-west between two points close to the south pole. In fact, it runs
>>> directly over the south pole, so actually to the south of your point of
>>> interest.
>>>
>>> P
>>>
>>>
>>> On Sun, Feb 19, 2017 at 7:36 PM, Sebastien Delaux <
>>> s.delaux at metocean.co.nz> wrote:
>>>
>>>> Hi all,
>>>>
>>>> I am new to postgis and I am interested in finding all the points that
>>>> are located at x metres or less of a polygon that are stored in my postgres
>>>> database.
>>>> As I am working with data located all over the world and want to work
>>>> with distances in metres I decided to use the geography type.
>>>>
>>>> I am trying to understand why the following query returns 0 when the
>>>> point is clearly not included in the polygon:
>>>> SELECT ST_Distance(ST_GeographyFromText('SRID=4326;POINT(0.
>>>> -82.)'),ST_GeographyFromText('SRID=4326;POLYGON((-90 -80, 90 -80, 90
>>>> 10, -90 10, -90 -80))'));
>>>>
>>>> I suspect this has something to do with the polygon crossing the
>>>> equator as
>>>> SELECT ST_Distance(ST_GeographyFromText('SRID=4326;POINT(0.
>>>> -82.)'),ST_GeographyFromText('SRID=4326;POLYGON((-90 -80, 90 -80, 90
>>>> -10, -90 -10, -90 -80))'));
>>>> returns a plausible distance.
>>>>
>>>> Would anybody know whether my polygon is violating some assumption or
>>>> whether there is any other reason that I am getting those results?
>>>>
>>>> Thanks
>>>>
>>>> Sebastien
>>>>
>>>>
>>>>
>>>>
>>>>
>>>>
>>>>
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>>
>>
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