The main code is odr.pro (Orthogonal Distance Regression)

This code calls lsqfitgm.pro, which conducts a simple 
geometric mean regression and fitexy.pro (see Numerical
Recipes for details), which in turn calls several other
procedures (which are contained within fitexy.pro).

Please see Miller and Tans, Tellus 55B,2 for details and
Miller et al, Tellus 55B for an application.