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You may be wondering how we’ve gathered so many different political district boundaries across the state—covering everything from entire counties to smaller subdistricts like city council wards or board of supervisor districts. This page explains the main sources we use and why you might notice a few quirks in certain boundaries.

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What is a Shapefile?

A shapefile is a standard file format used in GIS (Geographic Information Systems) to store the boundaries—or “shapes”—of districts and other regions. These files are crucial not only for visually displaying district maps on our site, but also for analyzing and aggregating underlying population and election data. In many ways, shapefiles are the backbone of how we link data to specific areas.

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Official Districts (At-Large & Legislative)

For entire counties, entire cities, entire school districts (i.e. at-large), and state legislative districts, we typically download shapefiles directly from official sources. Because these boundaries are widely recognized and consistently maintained, obtaining them is usually straightforward. Examples include:

• County-level shapefiles from county GIS portals
• City boundary shapefiles from municipal open data sites
• State legislative or full school district maps from state agencies

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Subdistricts & Smaller Boundaries

Things get more complicated when it comes to subdistricts, such as individual city council wards, school board trustee areas, or county supervisor districts. Some local governments make these subdistrict shapefiles available, and we can simply import them. However, in many cases:

• We Reconstruct from Precincts: When no official shapefile exists, we piece together the subdistrict boundary using precinct maps and documentation that spells out which precincts fall under each subdistrict.
• We Digitize from Offline Sources: In a handful of counties that don’t provide digital files, we create a shapefile by tracing PDF maps or other offline resources. This process is more labor-intensive and can introduce minor inaccuracies, but it’s often the only way to represent those smaller boundaries.

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Why Some Maps Might Look “Off”

• Population vs. Physical Geography: Precincts usually emphasize where people live, while a district line may extend over uninhabited land or water. Consequently, if that land is outside any precinct, it may appear missing in our map (though it doesn’t affect represented population).
• Stitching Multiple Sources: Subdistrict lines built from multiple precinct shapefiles can sometimes produce noticeable “seams.” These rarely affect the real boundaries used for representation or voting; they’re just visual quirks in combining different datasets.
• Ocean & Shoreline Differences: Some county shapefiles extend far offshore, but subdistrict data might exclude those areas as irrelevant to voting. This can result in boundary lines that don’t match up perfectly at the coastline.

Overall, these quirks mostly appear in remote or uninhabited spots. The core districts—where people actually live—remain aligned as closely as possible with the official boundaries.

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If you find a subdistrict boundary that looks unusual, it’s likely due to either the limited shapefile data we received, or the fact that we reconstructed it from precincts. We work hard to ensure all maps are accurate for real-world representation, but these small discrepancies are the nature of how local agencies maintain (or sometimes don’t maintain) their digital boundaries.