Hi, Previously I posted a thread asking help on how to best-fit (in the least squares sense) a straight line through a set of data points. Thanks a lot to all replying to it. I managed it in Matlab using a function 'fit_3D_data' (link: http://webscripts.softpedia.com/script/Scientific-Engineering-Ruby/Statistics-and-Probability/Orthogonal-Linear-Regression-in-3D-space-35532.html). But to add a 'simple' constraint to the problem, I am stuck. Here it is: I have two sets of data points, representing two separate straight lines (call A and R) that intersect at a point. I need to find the least-square fit to these lines, subject to their intersection constraint. So, the important part is that the two best-fit lines must intersect at the 'best' intersection point as determined by the data points. Is there any way to do this in R (I am also trying in Matlab, but yet to get any solution)? Here is what it looks like in Matlab: load pointsA.txt xdataA = pointsA(:,1); ydataA = pointsA(:,2); zdataA = pointsA(:,3); load pointsR.txt xdataR = pointsR(:,1); ydataR = pointsR(:,2); zdataR = pointsR(:,3); fit_3D_data(xdataA,ydataA,zdataA,'line','on','on'); fit_3D_data(xdataR,ydataR,zdataR,'line','on','on'); Thanks a lot for your time. -- View this message in context: http://www.nabble.com/Regression-with-Intersection-Constraint-tp19064048p19064048.html Sent from the R help mailing list archive at Nabble.com.