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Understanding Our Geographic Data
Geographic data is the backbone of The Ballot Book. It allows us to draw district maps, link election results to specific locations, and perform demographic analysis. This page explains our sourcing process and provides important context for how to interpret our maps.
Our Data Sourcing Process
To provide a comprehensive view of California politics, we gather geographic data (known as "shapefiles") from a wide variety of sources. Our method depends on the type of district.
Official Boundaries
For larger, "at-large" jurisdictions (like entire counties, cities, and school districts) and for state legislative districts, we download shapefiles directly from official government sources, such as state open data portals and county GIS departments. These boundaries are standardized and consistently maintained.
Reconstructed Boundaries
Smaller "sub-districts" (like city council wards or school board trustee areas) are more complex. When official shapefiles for these areas are not provided, we must manually reconstruct them by piecing together individual precinct maps or by digitizing boundaries from PDF maps. This intensive process is often the only way to map these smaller but critical jurisdictions.
Understanding Our Maps: Key Concepts & Caveats
While we strive for perfect accuracy, the process of combining dozens of different data sources can sometimes produce visual quirks. Understanding these is key to interpreting our maps correctly.
What is a Shapefile?
A shapefile is a standard file format used to store the boundaries—or “shapes”—of geographic regions. These files are crucial not only for displaying maps on our site but also for the underlying analysis that connects population and election data to specific areas.
Common Visual Quirks & Why They Occur
- Population vs. Physical Geography: Precincts are drawn based on where people live. If a district boundary extends over unpopulated areas like water or empty land, those areas might be excluded from the precinct-based map, making the district appear incomplete even though no voters are missing.
- Stitching Artifacts: When a sub-district map is built by "stitching" together multiple precinct files, you may sometimes see visual seams. These are cosmetic artifacts of the process and do not affect the accuracy of the underlying data.
- Coastline Differences: An official county boundary might extend far out into the ocean, but the precincts within it will stop at the shore. This can create a mismatch at the coastline between at-large and sub-district views.
How This Affects Data Analysis
It is important to remember that these visual quirks almost always occur in unpopulated areas and do not impact the accuracy of our electoral or demographic data. Our analysis is focused on where people actually live and vote. While a map may look slightly "off" in a remote corner, you can be confident that the data presented for the populated areas of the district is as accurate as possible.