That map is generated from self-reported data from ISPs filing form 477 (https://www.fcc.gov/economics-analytics/industry-analysis-division/form-477-resources h

That map is generated from self-reported data from ISPs filing form 477 (https://www.fcc.gov/economics-analytics/industry-analysis-division/form-477-resources https://www.fcc.gov/economics-analytics/industry-analysis-di...) The threshold for coverage is a single customer in a census block. The data is rarely validated except in extreme cases (https://www.telecompetitor.com/fcc-company-that-filed-false-form-477-data-still-seems-to-be-at-it/ https://www.telecompetitor.com/fcc-company-that-filed-false-...)