  We present a statistical study of the correlations and dimensionality of
emission lines carried out on a sample of over 40,000 Sloan Digital Sky Survey
(SDSS) galaxies. Using principal component analysis, we found that the
equivalent widths of the 11 strongest lines can be well represented using three
parameters. We also explore correlations of the emission pattern with the
eigenspace representation of the continuum spectrum. The observed relations are
used to provide an empirical prescription for expectation values and variances
of emission-line strengths as a function of spectral shape. We show that this
estimation of emission lines has a sufficient accuracy to make it suitable for
photometric applications. The method has already proved useful in SDSS
photometric redshift estimation.
