I'm building a model in which several of my covariates live on a "circle", in the sense that they take values in the interval [0,1), and 0=1. I'm wondering about techniques for dealing with this situation. One idea is to represent a circular variable theta as a pair of variables ( sin(theta), cos(theta) ). Any thoughts on this approach or better approaches?
I'm specifically using the mgcv package GAMs. Is there a way to tell the model that certain additive pieces should have the same values at the endpoints? Another package?