Key Points

Case Study: Food Deserts

A food desert combines two conditions: low income and low physical access to supermarkets, which the USDA formalizes as LILA tracts
The Food Access Research Atlas publishes several distance based measures rather than one official definition, and the choice of measure changes the map
The Atlas data supports policy analysis, site selection for new grocery stores, and spatial analysis of transportation networks
In July 2026 the Atlas became two maps: the Large Retailer Access Map (2019 data) and the new SNAP-Authorized Retailer Access Map (2025 data, with driving distances); they measure different things
The Beginner notebook visualizes the published 2019 Atlas for any state; the Expert notebook reproduces the methodology from open data, validates it against the Atlas, and extends it to other years

A clear scenario and a consistent schema make field data analyzable
Prepare the project and form in QGIS before going out, and package it with QFieldSync
Place points by a fixed rule, record GPS accuracy, and use “Unknown” instead of guessing
Check the data in QGIS as soon as possible after collection

Grocery store points become far more useful for access questions when combined with a road network
Reproject layers and set the project CRS to a projected system so that costs are in meters
Shortest path tools compare routes; service area tools show which streets are within reach
Network distance is longer than straight-line distance, sometimes by much more than expected
Clipping a network to an irregular boundary can break routes; check for unreachable destinations