I am trying to reproduce the PCA implemented in ENVI with IDL. I get the same covariance matrix but when I compute the eigenvectors (by using EIGENQL) I get some different results. The eigenvectors have the same shape but somethimes change sign so the final result is not the same. The same behaviour is obtained if I use the PCOM function. How does ENVI compute the eigenvectors of the covariance matrix?

Thanks,
John