s := zone.field.source
zone.field.source = s

Get/set where values are derived from when appropriate.

Certain values (like extra and pore-pressure) can be retrieved either from the zone structure or from the gridpoint structure. The values currently available are gridpoint and zone. The default value is gridpoint.

Returns:s - the structure from which values are derived
Accepts:s - the structure from which values are derived. Keyword matching rules apply. If no match is found, an error occurrs.