Also I'd recommend to add navigation to such huge forms -- the operator should have the possibility to access necessary group as fast as possible. We use in our hospital medical system this solution for patient registry entry:
download bmml source – Wireframes created with Balsamiq Mockups
Left menu allows to quickly scroll to necessary group of fields (in our case it is also possible with keyboard shortcuts).
When user scrolls the form itself (not only with mouse wheel, but also when he moves from field to field by Tab), left menu shows current location in form -- similar to behavior of modern one page websites.
This solutions allows to fill-in the whole form and to quickly fill-in or edit only necessary groups.
Also we move labels always on the left of fields to make visual search of necessary groups and fields easier. Some fields are made 100% width, other -- of necessary for data size. Such irregularity also helps to identify current location on the webform better.