Closed Polygon

[Rick Levine]

Ed,

Again good points all.  We wouldn't be having this discussion if I wasn't
the original nitpicker.

Thanks,

Rick

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                      "Ed McNierney"
                      <ed at topozone.com>        To:       <Richard_D_Levine at Raytheon.com>, <MAPSERVER-USERS at LISTS.UMN.EDU>
                                               cc:
                      01/18/2005 11:11         Subject:  RE: [UMN_MAPSERVER-USERS] Closed Polygon
                      AM






Rick -

Not to continue to pick at this nit <G>, but the concept "closed
polygon" is by no means restricted to the "GIS domain".  It's just a
polygon, which might be used in the latest video game renderer.  It's
really just a specification question - does the format under discussion
require polygons to re-specify the first point as the last, or is that
specification assumed?  If the system is using that "hint" to decide
whether the object is an (un-closed) polyline or a (closed) polygon then
it's important.  If the object itself is flagged as being a polygon or a
polyline, then it's redundant but harmless.  I wouldn't claim either way
is "right", nor is either way the "GIS way", since it's just as relevant
to the definition of a polygon in an Adobe Acrobat document describing
an advertisement for your new GIS product as it is to the GIS product
itself.  In the Adobe Acrobat/PostScript world, yet another mechanism
(the use of the closepath operator) is used to determine whether a path
is closed (i.e like a polygon) or not.

             - Ed

Ed McNierney
President and Chief Mapmaker
TopoZone.com / Maps a la carte, Inc.
73 Princeton Street, Suite 305
North Chelmsford, MA  01863
ed at topozone.com
(978) 251-4242

-----Original Message-----
From: UMN MapServer Users List [mailto:MAPSERVER-USERS at LISTS.UMN.EDU] On
Behalf Of Rick Levine
Sent: Tuesday, January 18, 2005 8:30 AM
To: MAPSERVER-USERS at LISTS.UMN.EDU
Subject: Re: [UMN_MAPSERVER-USERS] Closed Polygon

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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