The CT00 layer contains polygons for census areas. The source for the information in this layer is the U.S. Bureau of the Census and appropriate county government agencies.
This file is extracted from the Los Angeles County Registrar/Recorder's Precinct Information File, which is designed to match the Thomas Brothers TRNL (road network) layer.
Where census boundaries are defined by streets, water, city boundaries, or other features, those arc segments are copied to the CT00 layer, so that census tracts and other layer features match perfectly.

2010 Census Blocks clipped to the County Boundary. A new field “COMMNAME” was added and populated with city and community names based on 2011 redistricting unit community names in the census block data.

2010 Census Tracts clipped to the County Boundary. Two new fields were added to this census tract data – area in square feet and area in square miles.