Hello Keith,
There's no easy way I'm afraid but it possible to compute which polygons of
a region form holes ("-ve") and which are actual ("+ve"). There is also a
work-around which is the option I have chosen for most data development
projects.
The problem with identifying holes is that MapInfo regions consist of any
number of polygons and the order tells you nothing! The first polygon could
well be a hole, etc. There are also cases where you can programmatically
build bad regions which have polygons sitting across another polygon and are
therefore "half fill and half hole". The way to compute the +ve and -ve
polygons is to follow the fill rules used when drawing regions. This is
known as the odd-even rule and you can find a good explanation of it at the
following URL:
http://www.daimi.au.dk/~beta/Manuals/latest/bifrost-ref/bifrost-ref-12.html
A polygons centroid is a good place to make tests as it's guaranteed to be
within a single-polygon region. So, I guess one test could be... For each
polygon within a region, extract the polygon (using ExtractNodes()) and then
test if the point is inside each polygon in the region (including the
polygon it came from) and count the number of "in's". If the count is odd,
it's a +ve (an inclusive area), if even it's a -ve (exclusive area, donut
hole, etc.). I've never tried this(!) but I think it would work assuming you
have well-formed regions (no self/inter-polygon intersections).
One thing that another respondent forgot is that you can have inclusive
areas within holes within exclusive areas, etc. For example imagine a three
polygon region depicting some land (+ve), with a lake (-ve) and an island
(+ve). Only the "count" can really help you.
Now the workaround. I decided sometime ago to normalise all my regions so
that the major +ve polygon was always first and subsequent polygons were
only holes. If I needed to "refill" part of a donut hole, I use another
record (i.e. In my example above, the island would be a separate record).
It's easy to test that polygons 2+ are in polygon 1 and some simple MapBasic
tools give me the ability to reorder polygons in a region. I do other things
in normalisation too - such as polygon orientation - but that's another
story!
Hope that helps.
Regards,
Warren Vick
Europa Technologies Ltd. (U.K.)
http://www.europa-tech.com
-----Original Message-----
From: Campbell, Keith A [mailto:[EMAIL PROTECTED]
Sent: 19 November 2003 14:06
To: [EMAIL PROTECTED]
Subject: MI-L Identifying Regions with Holes
Hi all,
Is there any way to identify, by a query or other straightforward process,
all regions that have a hole or holes in them? The ObjectInfo function will
enable multi-polygon regions to be identified, but I can't see that it will
provide what I want. Any suggestions?
TIA,
Keith
----------------------------------------------
Keith Campbell
GIS Consultant
ATKINS ENVIRONMENT
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