  In the past many papers have appeared which simulated surface growth with
different growth models. The results showed that, if models differed only
slightly in their `growth' rules, the resulting surfaces may belong to
different universality classes, i.e. they are described by different
differential equations. In the present paper we describe a mapping of ``growth
rules'' to differential operators and give plausibility arguments for this
mapping. We illustrate the validity of our theory by applying it to published
results.
