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 <[email protected]> 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 <[email protected] > > wrote: > >> Thanks for your reply Paul. >> 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 <[email protected]> >> 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 < >>> [email protected]> 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 >>>> >>>> >>>> >>>> >>>> >>>> >>>> >>>> _______________________________________________ >>>> postgis-users mailing list >>>> [email protected] >>>> https://lists.osgeo.org/mailman/listinfo/postgis-users >>>> >>> >>> >>> _______________________________________________ >>> postgis-users mailing list >>> [email protected] >>> https://lists.osgeo.org/mailman/listinfo/postgis-users >>> >> >> >> >> _______________________________________________ >> postgis-users mailing list >> [email protected] >> https://lists.osgeo.org/mailman/listinfo/postgis-users >> > > > _______________________________________________ > postgis-users mailing list > [email protected] > https://lists.osgeo.org/mailman/listinfo/postgis-users > --
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