Don,
I have been doing as you suggested and looked into the package you mentioned and some of the areas coming back for each shapeRecord, even ones with 1 ring only, are negative. I don't know that the area should be negative. I know that there is a possibility that the area initially can be negative depending on which direction you calculate but in this case you should always take the absolute value of it.
Let me know your thoughts on this function. thank you
Chad
--- On Tue, 11/30/10, Don Dietrick <dfdietrick_at_gmail.com> wrote:
> From: Don Dietrick <dfdietrick_at_gmail.com>
> Subject: Re: [OpenMap Users] shapefiles - getting the outerRing
> To: "C S" <usmsci_at_yahoo.com>
> Cc: openmap-users_at_bbn.com
> Date: Tuesday, November 30, 2010, 7:09 PM
> Hi CS,
>
> Have you tried rendering these shape files with all these
> holes? Do
> they render properly? Seems like that would be the easiest
> way to tell
> if the code was working like you hope it is.
>
> I think your area calculation is OK for estimations, and
> should be OK
> for relative comparisons between parts, but it's not an
> accurate
> answer - I'd check out the
> com.bbn.openmap.geo.GeoArray.area() method
> if precision really matters.
>
> As long as each part starts and ends on the same
> coordinate, and the
> coordinates are in the right order, it should render
> properly. If you
> need analysis done at a mathematical level, I think you
> need to do
> what you are doing.
>
> Hope this helps,
>
> Don
>
> On Mon, Nov 29, 2010 at 2:20 PM, C S <usmsci_at_yahoo.com>
> wrote:
> > Don,
> >
> > thanks for the response. My shapefile that i have
> been dealing with had a large number of shapes and for each
> shape many of them are made up of more than 1 ring. I wrote
> a little function to tell me which ring had the biggest area
> by calculating the area of an irregular polygon and
> sometimes the first ring is not bigger than some of the
> other rings.
> >
> > The idea was that, assuming that all inner rings
> should have a smaller area than the outer ring
ne of
> the other rings touch one another. Also rings shouldn't
> cross one another -- all this according to the definition of
> a valid shapefile on ESRI's website. I also believe that
> according to ESRI the order of the rings for each shape
> doesn't have any special meaning, leading me to believe that
> the outer ring did not have to be the first one.
> >
> > The area of an irregular polygon is defined on wiki
> here: http://en.wikipedia.org/wiki/Polygon
> under the Area and Centroid section, the 3rd algorithm. As
> stated before there are various times where the first ring
> for each shape is not the largest according to this
> calculation.
> >
> >
> > So gathering from your response i need to verify that
> the first ring should be the biggest and if it isn't then
> either that area calculation somehow is wrong or the
> shapefile wasnt created correctly.
> >
> > If there is no particular sequence to the rings then
> you have an optimization problem when calculating the area
> of a polygon with internal rings which can only be correctly
> done once you subtract out all the areas of the inner rings.
> You first have to get the outer ring which potentially could
> be time consuming if you have a polygon, in my case, that
> has 2000 rings total. calculating the area of each one to
> find out the biggest one could be very costly. Also finding
> the ring that is clockwise cannot be determined if the rings
> are irregular until you have read most, if not all, the
> points which is also time consuming and costly. Perhaps
> there is a function to check for only clockwise rings?
> >
> > I am fairly confident that my area function works
> correctly. Below i will leave you with the code. Perhaps you
> could verify or someone could that has a shapefile handy.
> The only argument is a openmap Shape, code in java.
> >
> > protected double getArea(Shape shape) {
> > double area = 0.0;
> > double prev_x = 0;
> > double prev_y = 0;
> > PathIterator i =
> shape.getPathIterator(null);
> >
> >
double[] points = new
> double[6];
> > int shapeType =
> i.currentSegment(points);
> >
> > if(shapeType ==
> PathIterator.SEG_MOVETO)
> > {
> > prev_x =
> points[0];
> > prev_y =
> points[1];
> > }
> > else if(shapeType ==
> PathIterator.SEG_LINETO)
> > {
> > area += ( prev_x * points[1] )
> - ( prev_y * points[0]);
> > prev_x =
> points[0];
> > prev_y =
> points[1];
> >
> > }
> >
> > i.next();
> >
> >
> > }
> > area =
> java.lang.Math.abs(area);
> > return area * 0.5f;
> > }
> >
> >
> > Would love to hear your feedback to anyone interested
> - thanks a lot in advance!
> >
> >
> >
> > --- On Tue, 11/23/10, Don Dietrick <dfdietrick_at_gmail.com>
> wrote:
> >
> >> From: Don Dietrick <dfdietrick_at_gmail.com>
> >> Subject: Re: [OpenMap Users] shapefiles - getting
> the outerRing
> >> To: "C S" <usmsci_at_yahoo.com>
> >> Cc: openmap-users_at_bbn.com
> >> Date: Tuesday, November 23, 2010, 9:22 AM
> >> Hi,
> >>
> >> I believe the outer ring is the first one in the
> array, and
> >> that
> >> coordinates are always clockwise with that outer
> ring (its
> >> in the
> >> shapefile specification), and change direction for
> the next
> >> ring, and
> >> then back and forth for subsequent rings.
> >>
> >> Hope this helps,
> >>
> >> - Don
> >>
> >>
> >> On Thu, Nov 18, 2010 at 10:26 AM, C S <usmsci_at_yahoo.com>
> >> wrote:
> >> > Hi again all,
> >> >
> >> > I was wondering if there was a way to
> retrieve
> >> information about the outer ring of each
> shapeRecord of a
> >> shapefile. It doesnt look like ESRIRecord or any
> of its
> >> subclasses offer any way to look into each record
> of a
> >> shapefile and give you info about clockwise or
> >> counter-clockwise orientation of rings. below is a
> small
> >> snippet of code for reference. I have done a
> shpdump on the
> >
well and there doesnt seem to be an
> indicator
> >> of which was would be the outer ring.
> >> >
> >> > shapeFile = new ShapeFile(shpInputfile);
> >> > shapeCount = dbf.getRowCount();
> >> >
> >> > for (int i = 1; i <= shapeCount; i++) {
> >> > ESRIPolygonRecord shapeRecord = null;
> >> >
> >> > shapeRecord = (ESRIPolygonRecord)
> >> shapeFile.getNextRecord();
> >> >
> >> > for (int polyIndex = 0; polyIndex <
> >> shapeRecord.polygons.length; polyIndex++) {
> >> > ESRIPoly.ESRIFloatPoly poly =
> >> (ESRIPoly.ESRIFloatPoly)
> shapeRecord.polygons[polyIndex];
> >> >
> >> > .......
> >> >
> >> > }
> >> >
> >> >
> >> >
> >> >
> >> >
> >> >
> >> > --
> >> > [To unsubscribe to this list send an email to
> "majdart_at_bbn.com"
> >> > with the following text in the BODY of the
> message
> >> "unsubscribe openmap-users"]
> >> >
> >>
> >
> >
> >
> >
>
> --
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Received on Thu Dec 09 2010 - 11:57:26 EST