> -----Original Message----- > From: r-help-boun...@r-project.org [mailto:r-help-boun...@r- > project.org] On Behalf Of Peter Langfelder > Sent: Tuesday, November 09, 2010 3:49 PM > To: Barry Rowlingson > Cc: r-help@r-project.org; Ab Hu > Subject: Re: [R] Centre of gravity of a mountain > > On Tue, Nov 9, 2010 at 3:40 PM, Barry Rowlingson > <b.rowling...@lancaster.ac.uk> wrote: > > On Mon, Nov 8, 2010 at 5:15 PM, Peter Langfelder > > <peter.langfel...@gmail.com> wrote: > > > >> If you also need the z coordinate, it simply the mean of the matrix > Z. > >> > >> zCenter = mean(Z) > > > > How can that be right? Suppose your mountain is very flat, so that > > your mountain is effectively a cube. The Z values are all the same, > > and so their mean is the same. However the centre of mass is, by > > symmetry, clearly at height/2. > > > > Similarly suppose your mountain matrix is one large cell value and > > all the rest are near zero - the mean Z will be close to zero but the > > centre of mass will be almost half way up the single cell value, > > because all the near-zeros contribute nothing to the centre of mass > > position. > > Yup, the z coordinate is wrong. Only the x and y are right. > > Peter >
I believe that should have been mean(z)/2 Dan Daniel J. Nordlund Washington State Department of Social and Health Services Planning, Performance, and Accountability Research and Data Analysis Division Olympia, WA 98504-5204 ______________________________________________ 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.