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<TITLE>Re: [postgis-users] Why doesn't startpoint work? (and X() for that matter)</TITLE>
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<DIV dir=ltr><FONT face=Arial color=#000000 size=2>Ok got it. I think the
documentation should be changed from "find an return the X coordinate of the
first point in the geometry" to "...first point in a linestring"</FONT></DIV>
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<DIV dir=ltr><FONT face=Arial size=2>-e</FONT></DIV></DIV>
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<FONT face=Tahoma size=2><B>From:</B> strk@refractions.net
[mailto:strk@refractions.net]<BR><B>Sent:</B> Tue 12/28/2004 2:16
AM<BR><B>To:</B> Ethan Alpert<BR><B>Cc:</B> PostGIS Users
Discussion<BR><B>Subject:</B> Re: [postgis-users] Why doesn't startpoint work?
(and X() for that matter)<BR></FONT><BR></DIV>
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<P><FONT size=2>On Mon, Dec 27, 2004 at 05:14:51PM -0700, Ethan Alpert
wrote:<BR>><BR>><BR>> >From the docs:<BR>><BR>>
X(geometry)<BR>> Find and return the X coordinate of the first point in the
geometry.<BR>> Return NULL if there is no point in the
geometry.<BR>><BR>> StartPoint(geometry)<BR>> Returns the first point
of the geometry as a point.<BR>><BR>> spatialdb=> select
startpoint(the_geom),X(the_geom),the_geom from snaps<BR>> where catalogid
='101001000379CE01';<BR>> -[ RECORD 1]<BR>> startpoint |<BR>>
x |<BR>>
the_geom | SRID=4326;POLYGON((73.2565444444444<BR>>
45.141725,73.4717277777778 45.1421555555556,73.4707527777778<BR>>
44.9756666666667,73.257175 44.975625,73.2565444444444
45.141725))<BR>><BR>> So what gives?! Why do these return NULL? Is there a
better reference<BR>> doc somewhere?<BR><BR>These are OGC functions.
StartPoint/EndPoint should only work with<BR>LineString inputs (postgis actually
extends this looking for the<BR>first LineString in a complex geometry, but that
'extension' should<BR>probably be dropped for OGC *strictness*). X/Y/Z/M should
only<BR>work with Point geometries.<BR><BR>Your query should be:<BR><BR>=>
SELECT StartPoint(OuterRing(the_geom)),
X(StartPoint(OuterRing(the_geom)))<BR> FROM snaps WHERE catalogid =
'101001000379CE01';<BR><BR>> While we're on the subject how can I convert
between geometry types? I'd<BR>> like to turn some polygons into
lines.<BR><BR>There is no function available for this. You should script
it<BR>with calls to ExteriorRing(), NumInteriorRing() and
InteriorRingN().<BR><BR>--strk;<BR><BR>><BR>> -e<BR>>
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