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Census Research Guide: Defining/Identifying a Geography


Defining/Identifying a Small Area Geography

The most difficult part of defining census geography for a small area is figuring out how census designated geographies overlay/compare with the streets and neighborhoods most of us are more familiar with.

To find data for a specific community/neighborhood, you will first need to determine what census tracts, block groups and/or blocks best match up with your community. The boundaries may not match up exactly with the official boundaries of your community, but street maps and tract/block maps can help you define an area that will work best for your research.

Census tracts were first used in the 1940 census, but not all cities had yet designated tracts. Below are some tools to help determine community boundaries in San Diego and beyond, as well as tract boundaries. Note that census tract numbers & boundaries may change over time; the further back in time you go, the larger your tracts are likely to be.

To identify community (neighborhood) boundaries:

The City of San Diego's community plans website will help identify the generally accepted boundaries for San Diego neighborhoods.

For detailed street maps, check out the San Diego County, including portions of Imperial County, street guide and directory in the library or use a web based mapping tool such as Google Maps. With Google Maps, you can search for a neighborhood name and the resulting map will show boundaries. You may have to drill down and reposition the map until street names appear.

To identify census tracts:

Social Explorer has a mapping function that's great for identifying tracts back to 1950. While it doesn't identify blocks or block groups, you can drill down to see street labels and other identifiers. This can help you compare differences in tract boundaries over time.

SANDAG's Parcel Lookup Tool is great for identifying census tracts, ZIP code areas, community planning areas, and more. also includes a mapping application that helps identify tracts and other boundaries. Click the "boundaries" tool just above the map area, then use the options on the right to select the desired boundary area.

Tract maps are also available from these sources:

In the first image below, you can see that I've used Google Maps to identify the boundaries for the University City area of San Diego. The second image is from the SANDAG Parcel Lookup Tool, showing that same area with census tract numbers I've framed in red. Note that tract number 83.05 includes a large section outside of the University City boundary, with about half of the tract on either side of the I-5 freeway. This is an example of when you would need to decide how you want to approach statistics for that tract, if your goal is to get/show data for the University City neighborhood. For example, do you want to use data for the entire tract and include a footnote/narrative explaining that much of the tract is outside the University City neighborhood? Or, do you want to use block data for just those blocks inside the University City neighborhood instead of using tract data?