Columbia subject headings:
Airports--Delaware River Valley (N.Y.-Del. and N.J.)
Delaware River Valley (N.Y.-Del. and N.J.)--Commerce--Statistics.
Harbors--Delaware River Valley (N.Y.-Del. and N.J.)
Harvard subject headings:
Airports--Delaware River Valley (N.Y.-Del. and N.J.)
Airports--United States--Delaware Valley.
Harbors--Delaware River Valley (N.Y.-Del. and N.J.)
Harbors--United States--Delaware Valley.
Penjerdel (Corporation)
Regional planning--Pennsylvania--Philadelphia Metropolitan Area.
Princeton subject headings: