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* How to use function PointN?
@ 2007-05-07 08:07 Nemo Terry <ntuser155@hotmail.com>
2007-05-07 13:13 ` Re: How to use function PointN? Michael Fuhr <mike@fuhr.org>
0 siblings, 1 reply; 2+ messages in thread
From: Nemo Terry @ 2007-05-07 08:07 UTC (permalink / raw)
To: pgsql-sql
select PointN(envelope(polyline),1) from highway;
return null,why?
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* Re: How to use function PointN?
2007-05-07 08:07 How to use function PointN? Nemo Terry <ntuser155@hotmail.com>
@ 2007-05-07 13:13 ` Michael Fuhr <mike@fuhr.org>
0 siblings, 0 replies; 2+ messages in thread
From: Michael Fuhr @ 2007-05-07 13:13 UTC (permalink / raw)
To: Nemo Terry <ntuser155@hotmail.com>; +Cc: pgsql-sql
On Mon, May 07, 2007 at 04:07:00PM +0800, Nemo Terry wrote:
> select PointN(envelope(polyline),1) from highway;
> return null,why?
PointN is a PostGIS function; you might get more help on the
postgis-users mailing list.
http://postgis.refractions.net/mailman/listinfo/postgis-users
See also the PostGIS documentation for Envelope and PointN:
http://postgis.refractions.net/docs/ch06.html
Envelope(geometry)
Returns a POLYGON representing the bounding box of the geometry.
PointN(geometry,integer)
Return the N'th point in the first linestring in the geometry.
Return NULL if there is no linestring in the geometry.
You've used Envelope to get a polygon but PointN expects a linestring.
Try using ExteriorRing on Envelope's polygon:
SELECT PointN(ExteriorRing(Envelope(polyline)), 1) FROM highway;
--
Michael Fuhr
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2007-05-07 08:07 How to use function PointN? Nemo Terry <ntuser155@hotmail.com>
2007-05-07 13:13 ` Michael Fuhr <mike@fuhr.org>
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