Hi guys Just to follow up on Ted's comments.
See http://erich.realtimerendering.com/ptinpoly/ for other points in polygon algorithms. Eric Haines has made some performance evaluation of the methods. I quote here Eric: The worst algorithm in the world for testing points is the angle summation method. It's simple to describe: sum the signed angles formed at the point by each edge's endpoints. If the sum is near zero, the point is outside; if not, it's inside (Figure 2). The point.in.polygon function in package sp is referencing: References: Uses the C function InPoly(). InPoly is Copyright (c) 1998 by Joseph O'Rourke. It may be freely redistributed in its entirety provided that this copyright notice is not removed. (see e.g. ftp://ftp.ecs.umass.edu/pub/fshill/ece660/triangulate/inpoly.c) It seems that this corresponds to the edge/ray crossing method in Eric's wording. The performance is between the best as shown by Eric. I know there are other implementations in R in other packages but I do not have the full overview of all methods. Yours sincerely / Med venlig hilsen Frede Aakmann Tøgersen Specialist, M.Sc., Ph.D. Plant Performance & Modeling Technology & Service Solutions T +45 9730 5135 M +45 2547 6050 fr...@vestas.com http://www.vestas.com Company reg. name: Vestas Wind Systems A/S This e-mail is subject to our e-mail disclaimer statement. Please refer to www.vestas.com/legal/notice If you have received this e-mail in error please contact the sender. > -----Original Message----- > From: r-help-boun...@r-project.org [mailto:r-help-boun...@r-project.org] > On Behalf Of Ted Harding > Sent: 24. marts 2014 09:06 > To: r-help@r-project.org > Subject: Re: [R] points with-in boundaries of a map > > Apologies (red-faced)!! In my reply below I wrote "pi" and "-pi" where > it should have been "2*pi" and "-2*pi". See at [***] below. > Sorry for the noise, > Ted. > > On 23-Mar-2014 22:50:50 Jim Lemon wrote: > > On 03/23/2014 10:29 PM, eliza botto wrote: > >> Thankyou very much jim. it worked! but regarding second part of my > >> question, isn't there a way to read the coordinates of intersecting > >> lines with the premises of the map? > > > > Hi Eliza, > > I think you want the "locator" function, which will return the > > coordinates to the accuracy that you can click on a point in the plot. > > > > Jim > > There is a possible analytical approach to this. If the boundary of > a simple closed region is given as a series of straight lines which > join successive points as one proceeds continuously round the boundary > of theregion, and one has the data of the (x,y) coordinates of the points > in that order (with the last point the same as the first), then one > can proceed as follows: > > Let the points be P1=(x1,y1), P2=(x2,y2), ... , PN=(xN,yN)=P1 > in the order specified above. > > Since the grid lines have been drawn at specified intervals, you > can work through every point (x0,y0) which is an intersection. > > Let (x0,y0) be a point of intersection of the grid of lines. Now you > know the coordinates of (x0,y0) already, so the question is whether > this point is inside or outside the region. > > Consider a line jpoining (x0,y0) to (x1,y1). Now work out the angle > through which the line rotates when it is moved so that it joins > (x0,y0) to (x2,y2); say this is phi2 (clockwise positive, anticlockise > negative). Similarly work out phi3, the angle through which the line > next rotates when it is further moved so that it joins (x0,y0) to (x2,y2); > and so on. > > Now add up phi2 + phi3 + ... + phiN > > If the point (x0,y0) is inside the region, and you have proceeded > consecutively along the boundary, then this sum of angles will be > [***]pi or -pi. [***Should be] 2*pi or -2*pi. > If (x0,y0) is outside the region, then this sum will be zero. > > So you can work through all the intersections of the grid lines, > and for each intersection you can determine whether it is inside > or outside; and, if it is inside, you already know its coordinates. > > It can get more complicated if the region is not simply-connected. > E.g. if you do not want to count points (x0,y0) which are within > the boundary coastline but also are within the boundary of an > inland lake; or, further, you do want to count points which are > on an island in the lake; and so on. > > The essential for following a procedure of this kind is that you > can extract from the data from which the map is drawn the coordinates > of a series of points which are in consecutive order as one proceeds > along the boundar of a region. Not all geographic data have the > coordinates in such an order -- one can find datasets such that the > boundary is drawn as a set of separate partial boundaries which > are in no particular order as a whole; and in some datasets the > different separate parts of the boundary do not exactly match up > at the points where they should exactly join. > > Hoping this helps, > Ted. > > ------------------------------------------------- > E-Mail: (Ted Harding) <ted.hard...@wlandres.net> > Date: 23-Mar-2014 Time: 23:47:15 > This message was sent by XFMail > > ______________________________________________ > R-help@r-project.org mailing list > https://stat.ethz.ch/mailman/listinfo/r-help > PLEASE do read the posting guide http://www.R-project.org/posting- > guide.html > and provide commented, minimal, self-contained, reproducible code. > > ------------------------------------------------- > E-Mail: (Ted Harding) <ted.hard...@wlandres.net> > Date: 24-Mar-2014 Time: 08:06:23 > This message was sent by XFMail > > ______________________________________________ > R-help@r-project.org mailing list > https://stat.ethz.ch/mailman/listinfo/r-help > PLEASE do read the posting guide http://www.R-project.org/posting- > guide.html > and provide commented, minimal, self-contained, reproducible code. ______________________________________________ R-help@r-project.org mailing list https://stat.ethz.ch/mailman/listinfo/r-help PLEASE do read the posting guide http://www.R-project.org/posting-guide.html and provide commented, minimal, self-contained, reproducible code.
