Gera Food Access Score — Methodology

What is the Gera Food Access Score?

The Gera Food Access Score (GFAS/100) is a proprietary composite index computed by Gera from official USDA data. It ranks each US county on a 0–100 scale measuring how well residents can physically reach a supermarket. A higher score means better food access — more residents live within supermarket distance and fewer are simultaneously low-income and food-access limited.

The GFAS is derived entirely from publicly available federal data (USDA Food Access Research Atlas, 2019) aggregated across 3140 US counties and county-equivalents from 72,531 census tracts. It is not investment advice and is not a prediction of future access conditions.

Data source

Dataset

USDA Food Access Research Atlas (2019)

Publisher

USDA Economic Research Service

Published

April 2021

Data year

2019 (census tract boundaries from 2010 Census)

Licence

US Government Work (Public Domain) — free to use, reproduce, and distribute.

API key

None required. Bulk CSV download, no authentication.

Coverage

3140 US counties and county-equivalents (all 50 states + DC)

Granularity

72,531 census tracts aggregated to county level

Fields used from source CSV

Formula

Step-by-step computation

1. 1

   Download USDA Food Access Research Atlas ZIP

   Fetch "food-access-research-atlas-data-download-2019.zip" from https://www.ers.usda.gov/data-products/food-access-research-atlas/download-the-data/. No API key required. Published April 2021. US Government Work (public domain).

2. 2

   Extract Pop2010, LAPOP1_10, LALOWI1_10 from 72,531 tract rows

   Open "Food Access Research Atlas.csv". Read Pop2010 (denominator), LAPOP1_10 (low-access population at 1mi/10mi), LALOWI1_10 (low-income/low-access at 1mi/10mi). Treat NULL as 0. Group by State + County columns.

3. 3

   Sum to county level

   For each county: totalPop = ΣPop2010, lowAccessPop = ΣLAPOP1_10, lowIncLowAccessPop = ΣLALOWI1_10. Exclude counties where totalPop < 100. Result: 3140 counties.

4. 4

   Compute percentage scores

   low_access_pct = lowAccessPop / totalPop × 100. low_inc_low_access_pct = lowIncLowAccessPop / totalPop × 100. raw_score = low_access_pct × 0.6 + low_inc_low_access_pct × 0.4.

5. 5

   Normalise and invert to GFAS

   norm = (raw_score − 0) / (83.99 − 0). GFAS = round((1 − norm) × 100, 1). Sort all counties by GFAS descending.

Weight rationale

Physical access distance (60%)

The primary structural determinant of food access. LAPOP1_10 captures all residents beyond the USDA threshold regardless of income — it measures the built environment's grocery store coverage. Weighted highest because a county with no nearby stores is a food desert for all income levels.

Low-income overlay (40%)

LALOWI1_10 captures the compound disadvantage: residents who are both far from a store AND low-income (least able to drive further or pay delivery costs). Weighted at 40% because this sub-population faces the greatest food access barrier; a county with many poor residents far from stores is more food-insecure than its raw distance figure alone suggests.

Limitations and caveats

• The atlas uses 2019 data with 2010 Census tract boundaries. Population figures and store locations may have changed since then.
• Supermarket proximity is measured as straight-line distance (1 mile / 10 miles), not drive or walk time. A store may be close by distance but inaccessible without a vehicle.
• The atlas defines "supermarket" by store format and size — smaller grocery stores, food co-ops, and farmers markets are generally excluded.
• GFAS is a county-level index. Within a county, access varies significantly by neighbourhood; the score reflects the average condition, not the worst-case tract.
• Low-income thresholds follow federal poverty guidelines at the time of the 2019 atlas publication.