Census Collection District (CD) CDs are designed for use in census years for the collection and dissemination of Population Census data In non-census years, CDs are undefined In aggregate, CDs cover the whole of Australia without gaps or overlaps The CD is the smallest spatial unit in the ASGC CDs aggregate to form the larger spatial units of SLAs in the Main, Statistical Region, Statistical District and LGA Structures, SOS in the SOS Structure, Urban Centres and Localities in the UC/L Structure and Remoteness Areas in the Remoteness Structure Aggregation of SLAs in turn forms the remaining spatial units in the ASGC Therefore, in census years, the CD is the common denominator which integrates all classification structures in the ASGC The traditional concept of a CD is that it defines an area that one census collector can cover, delivering and collecting census forms, in about a ten-day period However, in the interests of comparability between censuses, this criterion is no longer strictly observed In the 2001 edition, many urban CDs are of a size such that census collectors may be allocated more than one CD In urban areas CDs average about 220 dwellings In rural areas the number of dwellings per CD reduces as population densities decrease By design, CD boundaries not cross SLA (and thus LGA) boundaries Therefore, an aggregation of CDs covers the administrative area of a local government