inout() in splancs working properly?
Rainer:
On Sat, 2 Jun 2001, Rainer Hurling wrote:
I have a problem with function 'inout()' in package 'splancs' on CRAN-R Version 1.2.3 under FreeBSD4.3-STABLE. The following script produces and draws points and a polygon-surrounded area. Repeating the same script many times shows, that points on the polygon-line often, but not always, are outside of the polygon-area. If points lay on the polygon-line, inout() tells sometimes they are inside (black points), sometimes outside (red points) of the surrounded (here ligthblue) area. Is my understanding of function inout() correct? I am looking for a function, that gives all points inside the area, including points laying on the polygon-line, as 'TRUE' or 'inside'.
Yes and no. Computational geometry is a black art, and point-in-polygon algorithms usually return the answer to whether the point is inside the polygon or not (inside meaning inside the boundary). Some algorithms will also tell you whether the point is on the (vanishingly thin) boundary itself. In your trial case, the vertices of the convex hull are both in the set of points being tested and are actual boundary vertices, a very untypical situation in empirical settings. If you "jittered" them so that they didn't coincide exactly in their binary representation, the algorithm would answer happily either yes or no - as it is, it says that boundary points may be either in or not, depending (somewhat) on where your vertex is around the boundary. There is no general acceptance for points on boundaries being either inside or outside, but in the next splancs release, the documentation will be changed to reflect this issue, and I may try some alternative algorithms to give three replies, and the user the choice of including points on boundaries. Hope this helps, Roger
Roger Bivand Economic Geography Section, Department of Economics, Norwegian School of Economics and Business Administration, Breiviksveien 40, N-5045 Bergen, Norway. voice: +47 55 95 93 55; fax +47 55 95 93 93 e-mail: Roger.Bivand at nhh.no and: Department of Geography and Regional Development, University of Gdansk, al. Mar. J. Pilsudskiego 46, PL-81 378 Gdynia, Poland. -.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.- r-help mailing list -- Read http://www.ci.tuwien.ac.at/~hornik/R/R-FAQ.html Send "info", "help", or "[un]subscribe" (in the "body", not the subject !) To: r-help-request at stat.math.ethz.ch _._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._