# [postgis-users] order of points in ST_ConvexHull

Armin Burger armin.burger at gmx.net
Thu Feb 26 14:01:43 PST 2009

```Thanks, I tried your suggestion and it looks like the following works:

Max(x + y) => upper right
Min(x + y) => Lower Left
Max(abs(x - y)) => Upper Left
Min(abs(x - y)) => Lower Right

Armin

On 26/02/2009 22:11, Martin Davis wrote:
> Or perhaps more simply, you can test the points using the following
> criteria:
>
> Max(x + y) => upper right
> Min(x + y) => Lower Left
> Max(abs(x - y)) => Lower Right
> Min(abs(x - y)) => Lower Left
>
> Sufficool, Stanley wrote:
>> Pull the centroid of the geometry and use generate_series() / pointn()
>> to "explode" the points and test each point for above & right of
>> centroid, above and left of centroid, etc...
>>
>>
>>> -----Original Message-----
>>> From: postgis-users-bounces at postgis.refractions.net
>>> [mailto:postgis-users-bounces at postgis.refractions.net] On Behalf Of
>>> Armin Burger
>>> Sent: Thursday, February 26, 2009 11:57 AM
>>> To: PostGIS Users Discussion
>>> Subject: [postgis-users] order of points in ST_ConvexHull
>>>
>>>
>>> Hello
>>>
>>> I need to find a possibility to identify upper-left, upper-right,
>>> etc. corners of +/- rectangular polygons. I.e. polygons with
>>> guranteed just 4 corners, but with a shape that is typically between
>>> a rectangle and a rhomb. The polygons define the geometry of image
>>> boundaries ("image footprints"). But it cannot be guaranteed which
>>> point in the polygon corresponds to which corner since the order of
>>> points during geometry creation is unknown.
>>>
>>> One idea was to use ST_ConvexHull(geometry) since for this very
>>> simple polygons the convex hull seems to be identical with the
>>> geometry. It looked to me that the order in this convex hull was:
>>>    lower-right, lower-left, upper-left, upper-right
>>> Does anybody know if this order is always like that or can this order
>>> change? Would anybody know another method to identify which point of
>>> the polygon corresponds to which corner?
>>>
>>> Regards
>>>
>>> Armin
>>> _______________________________________________
>>> postgis-users mailing list postgis-users at postgis.refractions.net
>>> http://postgis.refractions.net/mailman/listinfo/postgis-users
>>>
>>>
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>

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