RE: [mitab] How to know if a point object is Within a polygon

Tue, 11 Apr 2006 08:19:49 -0700

Dear Andrew
Thank you for your help !. This routine, works propertly with any coordinate system ?. I thouth mitab could have a method to obteing this but at any rate this is very usefull for me.
 
King Regards
Alejandro


"Canfield, Andrew" <[EMAIL PROTECTED]> escribió:
Is this what you are wanting to know?

int IsPtInPoly(Point dPtIn, std::vector< double > Xpts, std::vector< double > Ypts)
{
      double Ax = dPtIn.x;
      double Ay = dPtIn.y;
      double Xl;
      double Yl;
      double Xr;
      double Yr;
      const double Fx = Xpts[0];
      const double Fy = Ypts[0];
      double Yc;

      long lLoopCtrl = 0;
      long lCrossings = 0;

      double Xtemp;
      double Ytemp;
     
      //int intRes;
      const int iNumEntry = Xpts.size();

      for(lLoopCtrl = 0; lLoopCtrl < iNumEntry; )
      {
            Xl = Xpts[lLoopCtrl];
            Yl= Ypts[lLoopCtrl];

            if(!(lLoopCtrl + 1 > iNumEntry - 1))
            {
                  Xr = Xpts[lLoopCtrl + 1];
                  Yr = Ypts[lLoopCtrl + 1];
            }
            else
            {
                  Xr = Fx;
                  Yr = Fy;
            }

            if((Xl < Ax) && (Xr < Ax))
            {
                  lLoopCtrl=lLoopCtrl + 1;
                  continue;
            }
            if((Xl > Ax) && (Xr > Ax))
            {
                  lLoopCtrl=lLoopCtrl + 1;
                  continue;
            }
            if((Yl < Ay) && (Yr < Ay))
            {
                  lLoopCtrl=lLoopCtrl + 1;
                  continue;
            }

           
            if(((Yl > Ay) && (Yr > Ay)) && ((Xl > Ax) || (Xr > Ax)))
            {
                  lCrossings = lCrossings + 1;
                  lLoopCtrl=lLoopCtrl + 1;
                  continue;
            }
            if(Xl > Xr)
            {
                  Xtemp = Xl;
                  Ytemp = Yl;
                  Xl = Xr;
                  Yl = Yr;
                  Xr = Xtemp;
                  Yr = Ytemp;
            }

            Yc = Yl + (((Yr - Yl) / (Xr - Xl)) * (Ax - Xl));
            if(Yc > Ay)
            {
                  lCrossings = lCrossings + 1;
            }

            lLoopCtrl=lLoopCtrl + 1;
      }
     
return lCrossings & 1;
}

-----Original Message-----
From: [email protected] [mailto:[EMAIL PROTECTED] Behalf Of
sagent100
Sent: Friday, April 07, 2006 1:04 PM
To: [email protected]
Subject: [mitab] How to know if a point object is Within a polygon


Hi all!
Is it possible to know if a point object (x-y coordinates) is within a
polygon object ?

I will appreciate a lot an example using VB or in other case C.

Thanks !!
Alejandro











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