Every photograph in the collection is part of a 'flight,' or mission.  During the 20th century, producers of aerial photography adopted a convention by which each corporation would prefix their flight codes with a corporate identifier.  The USDA altered this practice by designating unique three letter codes for counties for which it produced photographs (we have a map of California counties and their codes).  As a result, we are able to easily group photographs together, both intellectually and on the shelf.

Flight Code Description
AMI Aerial Map Industries.
Flight codes typically followed by a county and a 2-digit year, ie: AMI-SBD-86
AN Mark Hurd.
Typically followed by a 2-letter accession code 
ASCS The USDA's Agricultural Stabilization and Conservation Service.
These flights are typically older than USDA flights with 3-letter county codes
C Fairchild Aerial Surveys California office.  
C- flights are chronological, with C-1 flown in 1927 and C-25000 in 1965.
CAS Cartwright Aerial Surveys
CH2M CH2M Hill Engineering.  Primarily photos of rice-growing regions of California
EAG Eagle Aerial Imaging
GS Golden State Aerial Surveys
GS US Geological Survey
HA, HB, HO Mark Hurd.
Followed by a 2-letter sequential code
NY Fairchild Aerial Surveys New York office.
PAI Pacific Aerial Industries
PW Pacific Western Aerial Surveys 
TA Mark Hurd.
Typically followed by a 2-letter accession code 
TG Teledyne Geotronics
USDA Large flights created by the United States Department of Agriculture. Primarily smaller scale and of a later vintage than USDA county-coded flights
WAS Watson Aerial Services
YY-### As in 'year.' Many NASA flights have numeric flight codes beginning with a two-digit year followed by a 2 digit accession number.  Actual flight codes are much longer, ie: 73-055-01079