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.

This dataset contains rivers, streams, and water conveyance mechanisms (pipelines, aqueducts).

This dataset was downloaded, extracted and prepared from the National Hydrography Dataset. The NHD is a comprehensive effort by the US Geological Survey to manage and maintain a comprehensive, high quality, nationwide dataset for GIS mapping and analysis of water features and conveyance. While it is not perfect, and you may find a number of errors, LA County applauds this effort, and is determined to support the enhancement and maintenance of this valuable information since it supports our long-term goal of public, authoritative data. We are currently investigating the feasibility of stewarding the NHD data for the LA County region.

A description of the NHD from the USGS site:

The National Hydrography Dataset (NHD) is the surface water component of The National Map. The NHD is a digital vector dataset used by geographic information systems (GIS). It contains features such as lakes, ponds, streams, rivers, canals, dams and streamgages. These data are designed to be used in general mapping and in the analysis of surface-water systems.

This dataset contains lakes & reservoirs in LA County.

This dataset was downloaded, extracted and prepared from the National Hydrography Dataset. The NHD is a comprehensive effort by the US Geological Survey to manage and maintain a comprehensive, high quality, nationwide dataset for GIS mapping and analysis of water features and conveyance. While it is not perfect, and you may find a number of errors, LA County applauds this effort, and is determined to support the enhancement and maintenance of this valuable information since it supports our long-term goal of public, authoritative data. We are currently investigating the feasibility of stewarding the NHD data for the LA County region.

A description of the NHD from the USGS site:

The National Hydrography Dataset (NHD) is the surface water component of The National Map. The NHD is a digital vector dataset used by geographic information systems (GIS). It contains features such as lakes, ponds, streams, rivers, canals, dams and streamgages. These data are designed to be used in general mapping and in the analysis of surface-water systems.

This dataset contains hydologic points (gaging stations, gates, rock features, springs, water intakes, wells, waterfalls, reservoir points) in LA County.

This dataset was downloaded, extracted and prepared from the National Hydrography Dataset. The NHD is a comprehensive effort by the US Geological Survey to manage and maintain a comprehensive, high quality, nationwide dataset for GIS mapping and analysis of water features and conveyance. While it is not perfect, and you may find a number of errors, LA County applauds this effort, and is determined to support the enhancement and maintenance of this valuable information since it supports our long-term goal of public, authoritative data. We are currently investigating the feasibility of stewarding the NHD data for the LA County region.

A description of the NHD from the USGS site:

The National Hydrography Dataset (NHD) is the surface water component of The National Map. The NHD is a digital vector dataset used by geographic information systems (GIS). It contains features such as lakes, ponds, streams, rivers, canals, dams and streamgages. These data are designed to be used in general mapping and in the analysis of surface-water systems.

Countywide Statistical Area (formerly called Board Approved Statistical Areas) provide a common geographic boundary for reporting departmental statistics for unincorporated areas to the Board of Supervisors. They are not designed to perfectly represent communities, nor jurisdictional boundaries such as the Angeles National Forest. References below might still be for BASA – but they are now called ‘Countywide Statistical Areas’ (as of July 2016).

To learn more about this project, click on this link to go to the project page.

Download the data

The dataset below is the current final draft of the BASA boundaries. As of this December 2015 we are developing the language to have them approved by the Board of Supervisors.

BOS_COUNTYWIDE_STATISTICAL_AREAS_20170118 (zipped shapefile)

Purpose of the data

Communities are one of the most complex geographic issues in the County of Los Angeles, due to historical narratives, perceptions of value, intense public interest, and shifts over time. The County has a need to establish a consistent geographic reporting base that will enable statistics and information to reported to the Board, that attempts to represent all of the interactions but is primarily focused on reporting.

Historically, County departments provide statistics and reports to the Board for Unincorporated Areas of the Los Angeles County based upon inconsistent geographic boundaries and names. These disparities impacted the ability of the Board to work with communities and residents to communicate priorities, service levels, and effect positive change in the County.

Reporting of statistics by County departments to Board offices by consistent geographic areas will simplify reporting, improve internal and external communications, and support better decision making. Leveraging boundaries used by the United States Census Bureau will enable demographic information to be used as part of County reporting. To download the geographic building blocks for this dataset, which also provide city boundaries each year since 2010 along with relevant Census block group and tract boundaries, go to the Split 2010 Block Group/City – BASA page.

Background

The Geographic Information Officer and the LA County Enterprise GIS group worked with the Los Angeles County Board of Supervisors Unincorporated Area and Field Deputies to establish names that reflect as best as possible the general name preferences of residents and historical names of areas. A Board Motion will establish these area names as “Board Approved.” BASAs differ from the more informal “Community” geographies because:

They are primarily focused on broad statistics and reporting, not mapping of communities.
They represent board approved geographies comprised of Census block groups split by cities.
They must cover the entire unincorporated County
There can be no holes or overlapping areas
Naming Conventions

BASAs were be named according to the following recommended naming conventions:

All names will be assumed to begin with “Unincorporated” (e.g. Unincorporated El Camino Village). They will not be part of the Statistical Geography Name (so the name of the Statistical Area would be “El Camino Village”).
Names will not contain “Island” – beginning each name with “Unincorporated” will distinguish an area from any surrounding cities. There may be one or more exceptions for certain small areas (e.g. “Bandini Islands”)
A forward slash implies an undetermined boundary between two areas within a statistical geography (e.g. Westfield/Academy Hills or View Park/Windsor Hills)
Certain established names may include hyphens (e.g. Florence-Firestone)
Aliases may be defined in parentheses (e.g. Unincorporated Long Beach (Bonner/Carson Park))
Geographic Base

The BASAs were created using Census Block Groups split by cities (e.g. “Split Block Groups”) as a geographic building block. This allows users to leverage the rich information provided by the US Censu

This layer contains information for locating past and present legal city boundaries within Los Angeles County. The Los Angeles County Department of Public Works provides the most current shapefiles representing city annexations and city boundaries on the Los Angeles County GIS Data Portal. The Department also provides large format city annexation maps (pdf) on its FTP site. True, legal boundaries are only determined on the ground by surveyors licensed in the State of California. Numerous records are freely available at the Land Records Information website, hosted by the Department of Public Works.Principal attributes include:NO: corresponds with numbers on the tables displayed on City Annexation Maps.ANNEX_No: is a text version of the "NO" field listed above. Because this field is only used for the Long Beach and Los Angeles Annexation Maps, this value is null for all other cities.NAME: is the official name under which the annexation was filed.TYPE: is used to indicate which legal action occurred.A - represents an Annexation to that city.D - represents a Detachment from that city.V - is used to indicate the annexation was rendered Void or withdrawn before an effective date could be declared.33 - Some older city annexation maps indicate a city boundary declared 'as of February 8, 1933'.ANNEX_AREA: is the land area annexed or detached, in square miles, per the recorded legal description.TOTAL_AREA:is the cumulative total land area for each city, arranged chronologically.SHADE: is used by some of our cartographers to store the color used on printed maps.INDEXNO: is a matching field used for retriving documents from our department's document management system.STATE (Secretary of State): Date filed with the Secretary of State.COUNTY (County Recorder): Date filed with the County Recorder.EFFECTIVE (Effective Date):The effective date of the annexation or detachment.CITY: The city to which the annexation or detachment took place.FEAT_TYPE: contains the type of feature each polygon represents:Land - Use this value for your definition query if you want to see only land features on your map.Water - Polygons with this attribute value represent internal navigable waters. Examples of internal waters are found in the Long Beach Harbor.3NM Buffer - Per the Submerged Lands Act, the seaward boundaries of coastal cities and unincorporated county areas are three nautical miles (a nautical mile is 1852 meters) from the coastline.

The Us Congressional Districts layer contains polygons for US Congressional Districts. The source for the information in this layer is the Los Angeles Registrar/Recorder's Precinct Information File (PIF), which is designed to match the Thomas Brothers TRNL (road network) layer.
Where boundaries are defined by streets, water, city boundaries, or other features, those arc segments are copied to the districts layer, so that the boundaries and other layer features match perfectly.

Supervisorial Districts as of the 2001 redistricting. These boundaries were approved by the County Board of Supervisors in 2001, and represent the boundaries of their political jurisdictions and those areas that vote for them.

California State Assembly District Boundaries, established in 2011

2011 United States Congressional Districts clipped to the LA County Boundary.

This data was created by the California Citizen’s Redistricting Commission, and downloaded from their website. Click here to go to the site.

This data is based upon the US Census Bureau’s TIGER basemap, and are not the final version for the County. The TIGER file, while it represents physical features in the County accurately, does not have the same spatial accuracy as the County’s mapping layers. The Registrar/Recorder County Clerk is currently updating their precincts to include these new districts, and with the same precision as the County base.

However, the County has tested and found the Census Bureau’s data to substantially accurate for even large-scale mapping, so future changes will not dramatically impact mapping, but I do recommend coming back to this page to check for updates.