What is counted
The unit is a listing in the National Register of Historic Places: a building, district, site, structure or object that the National Park Service has listed, including the subset designated as National Historic Landmarks (NHLs). The roster is the NPS listed-properties spreadsheet (about 100,800 listings), refreshed when NPS republishes it.
Which city a listing belongs to
A listing counts for a city when its coordinate falls inside that city's legal boundary (Census TIGER/Line place polygons). Coordinates come from the NPS map service first, with Census address geocoding filling gaps. A listing within 250 m of a boundary whose Register record names that city is assigned to it; listings with withheld addresses are assigned by city name within their county; everything else counts for its county only. About a quarter of all listings sit outside any city limit (rural bridges, farms, districts), which is correct rather than a gap.
Size classes
Cities compete within the Census Bureau's standard size-of-place classes: 10,000–24,999; 25,000–49,999; 50,000–99,999; 100,000–249,999; 250,000–499,999; 500,000–999,999; 1,000,000 and over. Population is the ACS 5-year estimate (one vintage for every place). Census Designated Places are included and labelled. Places under 10,000 are mapped and profiled but not ranked.
Measures
- Per 10,000 residents
- Listings in the category ÷ population × 10,000.
- Per square mile
- Listings in the category ÷ land area in square miles (Census Gazetteer; water excluded).
- Historic buildings per 100 housing units
- Contributing buildings across all the city's listings (from the NPS map service) ÷ ACS housing units × 100. Contributing buildings include churches, schools and shops while housing units are residential only, so this is an index of how much of the built fabric is recognised, not a literal share. It also credits districts for every building they contain. Reported once per city, under "All listings".
- Composite percentile
- Within a size class, each measure becomes a percentile (0–100); the composite is their mean. Percentiles are class-relative, so when several classes are merged on screen each city's composite refers to its own class.
- Adjusted rate
- An empirical-Bayes estimate (Marshall 1991) that shrinks a city's per-capita rate toward its class mean in proportion to how little data it has. It is the default rate on every leaderboard, because a town with three listings should not top a table on luck; the raw rate is shown beside it on every row, and one click switches to it. Small counts shrink hard.
Ranking rules
A city appears in a category's ranking when it has at least 5 listings in that category. Below that the count is still shown, with the places behind it, but no rank. When only one city in a class qualifies it is shown as first of one, and percentiles are left blank.
Categories
- All listings
- Everything inside the city limits.
- Architecture
- The NPS Area of Significance includes Architecture, or (for landmarks) the NHL nomination states Criterion 4. NPS left the area blank on 109 landmarks designated 2000–2024; those were filled from their nomination documents and each correction cites its source.
- Criterion C
- Listed under National Register Criterion C (architecture/engineering), read from NRIS through NPGallery. Covers every listing uniformly; the cleanest "architecturally significant" tag.
- National significance
- National-level significance in NRIS, or an NHL.
- National Historic Landmarks
- Designated NHLs.
- Architecture landmarks
- NHLs that meet the Architecture definition above.
- Modern
- NRIS style Modern Movement, International Style, Moderne or Art Deco (official), or inferred from other evidence (see Tags).
- Mid-century modern
- Modern Movement or International Style with a period of significance touching 1950–1974 (official), or inferred: a modern style with a build year 1945–1975, or membership of a modernism Multiple Property Submission. Art Deco and Moderne are inter-war and excluded.
- Mid-century modern (strict)
- Only official or human-verified mid-century tags.
- Period 1925–2000
- Period of significance touches 1925–2000, whatever the style.
- Historic districts
- Listings whose resource type is District.
Styles
Architectural styles are a controlled vocabulary of about ninety terms arranged as a tree: family (the National
Register's own classification groups, such as Late Victorian or Modern Movement), then movement,
style and variant (for example Modern Movement → Mid-Century Modern → Ranch). Selecting any node counts every listing
tagged with that node or anything beneath it, so "Modern Movement" includes Art Deco, International Style and
Brutalism, while each is also selectable alone. Every term carries its typical date range in the United States and, where
one exists, its Wikidata identifier. The vocabulary is data/styles.csv in the repository.
A listing's style tags come, in order of trust, from the nomination form's own Architectural Classification
field, the NRIS style code and the NPS Atlas (all official); from Wikidata's structured style claim; and from
a model reading of the nomination's narrative description (and, where the nomination is silent, the
Wikipedia article), which may only answer with terms from the vocabulary and must quote the sentence that supports each one.
Model-read and Wikidata tags are inferred: high confidence when two independent sources agree, medium when
one source says it plainly, low when the support is a passing mention. Low-confidence tags are never counted. Inferred tags
are marked not yet verified by a human on every listing, with a link to verify or dispute them; a verification
becomes a row in data/verifications.csv and overrides the inference.
Tags: official, inferred, verified
Attributes that no single dataset records reliably (style, era) are kept as tags with a status and the evidence that produced them. Official tags come straight from an NPS or NRIS field. Inferred tags are synthesised from other evidence (Wikidata or Register Atlas style and build year, the Multiple Property group, Criterion C) and carry a confidence of high, medium or low. Verified and rejected tags have been checked by a person against a cited source and override everything else. Leaderboards count official, inferred and verified tags; the strict categories count only official and verified. Inferred tags are shown as such wherever they appear.
Sources and vintages
- National Park Service: National Register listed-properties and NHL spreadsheets; the NRHP map service (coordinates, contributing-resource counts); NPGallery (NRIS style, period of significance, criteria; nomination documents).
- U.S. Census Bureau: TIGER/Line place and county boundaries; Gazetteer land area; ACS 5-year population (B01003) and housing units (B25001); batch geocoder.
- Register Atlas open data (CC0): build year, style and architect where Wikidata records them.
Known limits
- The Register's 50-year rule under-counts recent architecture; a city's modern buildings enter it slowly. The index will change as they do, which is the point.
- "Architecture" is on three quarters of all listings, so the plain architecture ranking tracks the overall one; the Criterion C, landmark and era categories separate design-rich cities from merely old ones.
- Withheld (restricted) addresses are never located; those listings count by city name only.
- Style and period are missing for some listings in NRIS, and build years exist for about a quarter of listings in Wikidata. Coverage is reported with the results.
Missing or wrong?
Every city page lists the places behind its counts. If a listing is missing, misplaced or mis-tagged, the fix is a cited correction or verification in the data, applied on the next build. Use the "missing a place?" link on a city.