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| Subject: | GISList: Re: clockwise-anticlockwise |
| Date: |
02/01/2003 10:22:07 AM |
| From: |
Peter Shary |
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----- I apologize for possible cross-posting -----
Hi all,
The topic on polygon check and its quality understanding
seems to be of general value. Please delete this message
if it does not fit your interests.
------ Original message -----
From : soumen roy <soumenroys@yahoo.com>
Reply-To : gisindia@yahoogroups.com
To : gisindia@yahoogroups.com
Subject : [GIS India] Re: clockwise-anticlockwise
Date : Fri, 24 Jan 2003 20:19:07 -0800 (PST)
hi all,
i am facing some typical problem in acad programming. there are many
polygons in an autocad drawing. i have to check all those polygons,
wheather drawn in clockwise direction or anticlockwise direction.
there are nearly 2500 polygons in each drawing. so it is impossible to
check it manually. i have to check it through autolisp/visual lisp
programming. but i am not getting any logic to check this by
programming! when a logic is working fine in 1st quadrant that is
not applicable in 2nd or 3rd quadrant. can anyone give me the
program, algorithm or logic for this job?
soumen
----- End of the original message -----
Soumen,
People have provided you with some advices, but they
seem to be partial cases of the general task, and will not
always work (that is, they generally will not work). I believe
your real task is somewhat more complex, provided that you
want to CHECK polygon, not simply to determine its
clockwise-anticlockwise orientation.
Tasks of general interest in this relation:
1) Points of some polygon may be MULTIPLE in
your original data, forming zero-length segments.
For example, I encountered situation when a triangle
was composed by 17 points: these points were located
at 3 positions. Your program should internally delete
extra points before further polygon treatment.
2) Polygon may be incorrect, that is, it may contain
SELF-INTERSECTION, for example, polygon that
looks like 8. Most GISs do not perform such check by
default. Your program should check this, because in
an opposite case it will provide unpredictable results in
polygon orientation determining. An idea to check
polygon for self-intersection is to check all pairs of
segments: they should not contain mutual points,
except for ends of adjacent segments.
Note 1: self-intersection is often formed by a very
small loop which is difficult to find manually and to
visualise.
Note 2: there is slightly different understanding of
what is self-intersection in different GISs, see for example
"durty" polygon description in ESRI's White Paper on
shapefile description.
3) For a polygon with no self-intersection, your
decision on its ORIENTATION (clockwise-
anticlockwise) may be based on the following
idea. When you pass the polygon from first point
to the last point and sum angles between adjacent
segments (let these angles be positive when you turn
from given point clockwise, and negative in an
opposite case), this sum will be (N-2)*PI for
clockwise polygon, and (2-N)*PI for counterclockwise
one, where N is number of points, PI=3.14...
Note that this theory is not applicable to polygons
that look like 8.
I have implemented these ideas in my Analytical
GIS Eco (see URL
http://members.fortunecity.com/eco4/giseco.html )
to automatically check hundreds of thousands of
polygons 'on fly'.
Good luck,
Peter
************************
Peter A. Shary,
scientific researcher and GIS developer
Institute of physical, chemical and biological problems of soil science
of the Russian Academy of Sciences
142290 Poushchino
Moscow region
Russian Federation
Phone: +7 (0967) 733604
Fax: +7 (0967) 790595
************************
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