Closed Polygon

[Rick Levine]

Thanks for the link Ken, I inserted it into the debate.

I agree with all of your points completely Ed.  My contribution to the
debate has been that the terminology *closed polygon* is meaningless, or at
least redundant as you said, in the mathematical domain.  It only has
meaning in the GIS domain, where information systems are exchanging data.
In the GIS domain, it means the first point equals the last.  Again, as you
point out, that's how a polygon is distinguished from a polyline (I thought
it was multi-line, but that may just be an Oracle Spatialism.)

Thanks guys for the thoughtful responses.

Rick

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                      Ed McNierney
                      <ed at TOPOZONE.COM>           To:       MAPSERVER-USERS at LISTS.UMN.EDU
                      Sent by: UMN                cc:
                      MapServer Users List        Subject:  Re: [UMN_MAPSERVER-USERS] Closed Polygon
                      <MAPSERVER-USERS at LIS
                      TS.UMN.EDU>


                      01/17/2005 08:31 PM
                      Please respond to Ed
                      McNierney






Ken -

Yes, that defines what a polygon means - in an ESRI shapefile.

Rick, the problem is not "defining" a closed polygon so much as
"specifying" one.  "Closed" is a little redundant here, since a polygon
has to be closed in the sense I think you're using it.  If it's not
closed, it's a polyline.

I think your question is more a matter of agreement over the XML schema
used and its interpretation.

        - Ed

Ed McNierney
TopoZone.com

-----Original Message-----
From: UMN MapServer Users List [mailto:MAPSERVER-USERS at LISTS.UMN.EDU] On
Behalf Of Ken Lord
Sent: Monday, January 17, 2005 6:04 PM
To: MAPSERVER-USERS at LISTS.UMN.EDU
Subject: Re: [UMN_MAPSERVER-USERS] Closed Polygon

I knew that BCIT course was good for something ..

bango! ...

http://www.esri.com/library/whitepapers/pdfs/shapefile.pdf

Page 12 defines what a polygon is according to the ESRI Shapefile
format.

"A polygon consists of one or more rings. A ring is a connected
sequence of four or more points that form a closed,
non-self-intersecting loop."

Think of it this way, the simplest polygon is a triangle, but it takes
the four points to describe a polygon in a shapefile, therefore the
start and end point must be the same.

Page 13 lists some important points about polygons, one of which
explicitly states that the start and end points must be the same.

Cheers,
Ken Lord
Vancouver BC




On Mon, 17 Jan 2005 15:09:06 -0500, Rick Levine
<Richard_D_Levine at raytheon.com> wrote:
> Can anyone point me to a URL that defines a close polygon as one in
which
> the last point equals the first?
>
> I'm in the middle of a raging debate (tempest in a teapot) of what a
closed
> polygon is.  I'm getting polygons in XML from other systems where the
XML
> schema defines polygons as closed.  Some of the other systems disagree
> about what this means.
>
> BTW: I Googled and looked at opengis.org, but it seems no one bothers
to
> define something so obvious.
>
> Thanks, and sorry for being a little off topic.
>
> Rick
>
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