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. 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.
