NHGIS Data Source Reference

IPUMS NHGIS — census geography crosswalks and demographic data for education research. Via the Education Data Portal: geographic crosswalk tables linking K-12 schools (ncessch) and colleges (unitid) to census tracts, block groups, CBSAs, and regions. The 1990-2020 here denotes the census-geography vintages the crosswalks resolve to; the mirror files themselves span 1980-2023 file-years (the education/institution linkage years). See datasets-reference.md for per-file year ranges. Census demographic variables (income, poverty, race, educational attainment) are NOT in the Portal. Browse those variables publicly in the NHGIS Data Finder; sign in with a free IPUMS NHGIS account to submit, manage, or download extracts, and use an API key for API extract requests. Use when linking school or institutional data to census geography for contextual analysis.

Census geography and demographic data source for education research. NHGIS provides the foundation for linking schools to community characteristics via census tracts, block groups, and school district boundaries.

CRITICAL: Value Encoding

When accessing NHGIS data through the Education Data Portal (not NHGIS directly), categorical variables use integer encodings, not string labels. Always verify the exact codes in the mirror codebook.

Variable	Integer Code	Meaning
census_region	1	Northeast
census_region	2	Midwest
census_region	3	South
census_region	4	West
cbsa_type	1	Metropolitan
cbsa_type	2	Micropolitan
geocode_accuracy	4	Did not geocode

See ./references/variable-catalog.md for complete encoding tables.

CRITICAL: Portal Data Scope

The Education Data Portal provides ONLY geographic crosswalk tables that link schools and colleges to census geography (tracts, block groups, regions, CBSAs). These contain geographic identifiers and assignment columns — approximately 35-47 columns per file.

The Portal does NOT provide census demographic data (population, income, poverty, race, education attainment, housing, language, etc.). For demographic variables, browse the public NHGIS Data Finder, then use a free IPUMS NHGIS account to submit, manage, or download an extract. API extract requests additionally require an API key. See ./references/data-access.md for direct access methods.

This skill documents both contexts: Portal crosswalk data (with integer encodings above) and direct NHGIS census variables (in ./references/variable-catalog.md). Direct NHGIS products remain subject to separate IPUMS terms, citation requirements, and redistribution restrictions; the Portal's ODC-By terms do not automatically apply to them.

What is NHGIS?

NHGIS (from IPUMS, University of Minnesota) provides free access to census geography and demographic data.

• Collector: IPUMS, University of Minnesota
• Coverage: US census data from 1790-present (decennial census + ACS)
• Content: Summary tables, GIS boundary files, time series tables, geographic crosswalks
• Frequency: Decennial census (every 10 years) + ACS (annual, 5-year rolling)
• Available years: 1790-2020 (decennial), 2005-2023 (ACS 5-year)
• Primary identifiers: GISJOIN (NHGIS internal), GEOID (Census Bureau standard)
• Education relevance: Links school locations to community demographics via census tracts, block groups, and school district boundaries
• Available through Education Data Portal: Geographic crosswalk tables only (school-to-census and college-to-census links for census 1990, 2000, 2010, 2020). Census demographic data requires direct NHGIS access.

Reference File Structure

File	Purpose	When to Read
geographic-units.md	Census geography hierarchy (tracts, blocks, districts)	Understanding census geography
school-geography-links.md	Linking schools to census areas	Connecting school data to demographics
time-series.md	Historical data, harmonization methods	Longitudinal analysis
variable-catalog.md	Key demographic variables, codes, special values	Selecting census variables or interpreting encodings
boundary-changes.md	How boundaries change between censuses	Handling geographic inconsistencies
data-access.md	Direct NHGIS access methods (registration, Data Finder, ipumspy)	Custom census analysis beyond Portal

Decision Trees

What geographic level should I use?

Research question about...
├─ Individual schools
│   ├─ School's immediate neighborhood → Census tract or block group
│   ├─ School attendance zone → SABINS (limited years) or block-to-school crosswalk
│   └─ School district overall → School district boundaries
├─ School districts
│   ├─ District-level demographics → School district geographic level
│   ├─ Within-district variation → Census tracts within district
│   └─ District poverty estimates → SAIPE (via Education Data Portal)
├─ Regional patterns
│   ├─ County-level → County boundaries
│   ├─ Metro area → CBSA (Core Based Statistical Area)
│   └─ State-level → State boundaries
└─ Historical analysis
    ├─ Consistent boundaries needed → Geographically standardized tables
    └─ Original boundaries OK → Nominally integrated tables

How do I link schools to census data?

Linking schools to census demographics?
├─ Have school coordinates (lat/lon)
│   ├─ Point-in-polygon → Spatial join to tract/block group boundaries
│   └─ Need tract ID only → Geocoding service or FCC API
├─ Have school NCES ID only
│   ├─ Use NCES EDGE files → School District Geographic Relationship Files
│   └─ Use Education Data Portal → NHGIS source provides tract links
├─ Need school attendance zones
│   ├─ 2009-2012 data → SABINS school areas
│   └─ Current data → Contact school district (no national source)
└─ See ./references/school-geography-links.md for details

What time period data do I need?

Time period?
├─ Single recent year
│   ├─ Tract/block group level → ACS 5-year (most recent)
│   ├─ Larger areas (65K+ pop) → ACS 1-year
│   └─ Full census count → 2020 Decennial Census
├─ Historical comparison
│   ├─ Same boundaries across time → Geographically standardized tables (to 2010)
│   ├─ Original boundaries → Nominally integrated time series
│   └─ Custom standardization → Use geographic crosswalks
├─ Long time series (1970+)
│   └─ See ./references/time-series.md
└─ Pre-1970
    └─ Limited tract coverage; county/state more complete

Quick Reference: Geographic Levels and Variables

Geographic Levels

Level	Typical Size	Education Use	NHGIS Coverage
Block	~40 people	Point locations	1990-2020
Block Group	~1,500 people	School neighborhoods	1990-2020
Census Tract	~4,000 people	Community context	1910-2020
County Subdivision	Varies	Rural areas	1980-2020
Place	City/town	Urban context	1980-2020
School District	Varies	District analysis	2000-2020
County	~100,000 people	Regional patterns	1790-2020
State	Varies	Policy analysis	1790-2020

Key Identifiers

ID	Format	Level	Example	Notes
ncessch	Int64	School	10000201704	NCES school ID (schools Portal data)
unitid	Int64	College	100654	IPEDS institution ID (colleges Portal data)
GISJOIN	String with prefix	Any	G0600010	NHGIS internal ID; use for direct NHGIS joins (not in Portal data)
GEOID	Numeric string	Any	06001402100	Census Bureau standard; use for non-NHGIS joins (not in Portal data)
tract	Int64	Tract	402100	Census tract number (in Portal data)
block_group	Int64	Block Group	1	Block group within tract (1-9; 0=unassigned)
geoid_block	Int64	Block	60014021001001	Full block FIPS code (in Portal data — stored as Int64, not String)
cbsa	Int64	Metro area	41860	Core Based Statistical Area code (2000+ census files only)

Key Education Variables

Topic	Example Variables	Source
Child population	Under 18, 5-17 school-age	Decennial, ACS
Race/ethnicity	Hispanic, White, Black, Asian, etc.	Decennial, ACS
Poverty	Persons below poverty, SNAP receipt	ACS (sample)
Education attainment	HS diploma, BA+ (adults)	ACS (sample)
Language	English proficiency, language at home	ACS (sample)
Housing	Owner/renter, median value, crowding	Decennial, ACS
Family structure	Single-parent, grandparent households	ACS (sample)
Immigration	Foreign-born, recent immigrants	ACS (sample)

Data Sources by Type

Source	Years	Geographic Detail	Content
Decennial Census	1790-2020	Block (1990+)	100% count: age, sex, race, housing
ACS 5-Year	2005-2023	Block group	Sample: income, education, language
ACS 1-Year	2010-2023	Areas 65K+ pop	Sample: same as 5-year
Time Series	1790-2020	Varies	Harmonized across years
Geographic Crosswalks	1990-2020	Block+	Interpolation weights

Portal Variables (Schools NHGIS)

Key geographic and identifying columns in the schools NHGIS datasets. Census 2020 files have 47 columns; earlier census years have fewer (e.g., 1990 has 35 columns — no CBSA or legislative district fields).

Variable	Description	Type
ncessch	NCES school ID	Int64
leaid	NCES district ID	Int64
tract	Census tract number	Int64
block_group	Block group number (1-9; 0 = unassigned)	Int64
geoid_block	Full block FIPS identifier	Int64
census_region	Census Bureau region (1-4, 9)	Int64
census_division	Census Bureau division (1-9)	Int64
cbsa	CBSA code (2000+ census files only)	Int64
cbsa_type	Metropolitan (1) or Micropolitan (2)	Int64
cbsa_city	Principal city indicator (0=No, 1=Yes; 2000+ only). See note below.	Int64
geocode_accuracy	Geocode confidence (1=High, 2=Medium, 3=Low, 4=Did not geocode, -2=N/A)	Float64
geocode_accuracy_detailed	Geocode match type (1-12)	Int64
class_code	FIPS place class code	Int64
lower_chamber_type	State legislative district lower chamber type (1-8; census 2010 only). See variable-catalog.md for code mapping.	Int64
geo_latitude / geo_longitude	Geocoded coordinates	Float64
latitude / longitude	CCD-reported coordinates (many nulls in early years)	Float64
fips	State FIPS code	Int64
puma	Public Use Microdata Area (2000+ census files only)	Int64

Portal Variables (Colleges NHGIS)

Colleges NHGIS datasets have 38 columns (2020 census). Different identifier set from schools.

Variable	Description	Type
unitid	IPEDS institution ID	Int64
opeid	Office of Postsecondary Education ID	String
tract	Census tract number	Int64
block_group	Block group number (1-9)	Int64
geoid_block	Full block FIPS identifier	Int64
census_region	Census Bureau region (1-4, 9)	Int64
census_division	Census Bureau division (1-9)	Int64
cbsa	CBSA code	Int64
cbsa_type	Metropolitan (1) or Micropolitan (2)	Int64
cbsa_city	Principal city indicator (0=No, 1=Yes; 2000+ only)	Int64
geocode_accuracy	Geocode match score (Int64 in colleges, Float64 in schools)	Int64
county_fips	County FIPS code	Int64
county_name	County name	String
state_abbr	State abbreviation	String

Missing Data Codes

Code	Meaning	When Used
-2	Not geocoded	geocode_accuracy field in Portal data
-1	Missing/not reported	General missing data indicator (e.g., latitude, county_code)
0	Unassigned	block_group (rare, ~4 rows in schools)
null	Not available	Variable not applicable to this record; many columns heavily null in early years

Schema Difference: Schools NHGIS 2020 files (47 columns) have a different schema than colleges NHGIS 2020 files (38 columns). Schools data includes school-specific identifiers (ncessch, leaid, school_name, mailing/location address fields) while colleges data includes institution-specific identifiers (unitid, opeid, inst_name, county_name). Both entity types have block-level geographic precision. Earlier census years have fewer columns (e.g., Schools 1990 has 35 columns — no CBSA or legislative district fields). Do not assume identical column structures when working across entities or census years.

Data Access

Datasets for NHGIS are available via the mirror system. See datasets-reference.md for canonical paths, mirrors.yaml for mirror configuration, and fetch-patterns.md for fetch code patterns.

Dataset	Type	Years	Path	Codebook
Schools Census 1990	Single	1986-2023	nhgis/schools_nhgis_geog_1990	nhgis/codebook_schools_nhgis_census1990
Schools Census 2000	Single	1986-2023	nhgis/schools_nhgis_geog_2000	nhgis/codebook_schools_nhgis_census2000
Schools Census 2010	Single	1986-2023	nhgis/schools_nhgis_geog_2010	nhgis/codebook_schools_nhgis_census2010
Schools Census 2020	Single	1986-2023	nhgis/schools_nhgis_geog_2020	nhgis/codebook_schools_nhgis_census2020
Colleges Census 1990	Single	1980-2023	nhgis/colleges_nhgis_geog_1990	nhgis/codebook_colleges_nhgis_census1990
Colleges Census 2000	Single	1980-2023	nhgis/colleges_nhgis_geog_2000	nhgis/codebook_colleges_nhgis_census2000
Colleges Census 2010	Single	1980-2023	nhgis/colleges_nhgis_geog_2010	nhgis/codebook_colleges_nhgis_census2010
Colleges Census 2020	Single	1980-2023	nhgis/colleges_nhgis_geog_2020	nhgis/codebook_colleges_nhgis_census2020

Codebooks are .xls files co-located with data in all mirrors. Use get_codebook_url() from fetch-patterns.md to construct download URLs.

Truth Hierarchy: When interpreting variable values, apply this priority:

1. Actual data file (what you observe in the parquet/CSV) — this IS the truth
2. Live codebook (.xls in mirror) — authoritative documentation, may lag
3. This skill documentation — convenient summary, may drift from codebook

If this documentation contradicts the codebook, trust the codebook. If the codebook contradicts observed data, trust the data and investigate.

Filtering

import polars as pl

# Filter to a specific school
school_census = df.filter(pl.col("ncessch") == 10000201704)

# Filter to metropolitan areas only (cbsa_type only in 2000+ census files)
metro = df.filter(pl.col("cbsa_type") == 1)

# Filter to a specific census region (South)
south = df.filter(pl.col("census_region") == 3)

# Filter to a specific year
recent = df.filter(pl.col("year") == 2023)

library(dplyr)

# Filter to a specific school
school_census <- df |> filter(ncessch == 10000201704)

# Filter to metropolitan areas only (cbsa_type only in 2000+ census files)
metro <- df |> filter(cbsa_type == 1)

# Filter to a specific census region (South)
south <- df |> filter(census_region == 3)

# Filter to a specific year
recent <- df |> filter(year == 2023)

Direct-access note: The Portal provides pre-processed school/college-to-census-geography links. For custom census analysis (tract-level demographics, time series, boundary files), browse the NHGIS Data Finder without signing in; a free IPUMS NHGIS account and login are required to submit, manage, or download extracts, and API requests additionally need an API key. See ./references/data-access.md. Direct NHGIS data is governed by separate IPUMS terms and citation requirements.

Common Pitfalls

Pitfall	Issue	Solution
Boundary changes	Tracts split/merged between censuses break longitudinal analysis	Use crosswalks or geographically standardized tables
ACS margins of error	Small-area estimates have high uncertainty	Check MOE; aggregate areas if needed
Block data limitations	Only 100% count variables available (no income/poverty)	Use block groups for sample data (ACS)
GISJOIN vs GEOID	Different ID formats cause join failures	Use GISJOIN for NHGIS joins, GEOID for Census Bureau joins
2020 Census noise	Differential privacy added noise to small-area counts	Check for negative values; prefer ACS for detailed characteristics
Schools vs colleges schema	Different column counts (47 vs 38 for 2020) and identifier sets	Check schema before joining; do not assume identical structures
Census year schema drift	Earlier census files have fewer columns (e.g., 1990 lacks CBSA/legislative fields)	Check available columns per census year before relying on them
geocode_accuracy type	Float64 in schools, Int64 in colleges	Cast to consistent type before cross-entity comparison
Using string codes	Portal data uses integer encodings, not string labels	Always verify codes against codebook (see encoding warning above)

Related Data Sources

Source	Relationship	When to Use
education-data-source-ccd	School identifiers for linking	Join school data to census geography via ncessch
education-data-source-saipe	District-level poverty	Use SAIPE for district poverty; NHGIS for tract/block group poverty
education-data-source-meps	School-level poverty	MEPS provides school-level poverty estimates; NHGIS provides community context
education-data-source-ipeds	College identifiers for linking	Join college data to census geography via unitid
education-data-explorer	Parent discovery skill	Routing questions to mirror dataset files
education-data-query	Data fetching	Downloading parquet/CSV files

Topic Index

Topic	Reference File
Census tract definition	./references/geographic-units.md
Block group definition	./references/geographic-units.md
School district boundaries	./references/geographic-units.md
School-to-tract linking	./references/school-geography-links.md
SABINS attendance areas	./references/school-geography-links.md
NCES EDGE files	./references/school-geography-links.md
Time series tables	./references/time-series.md
Geographic standardization	./references/time-series.md
Geographic crosswalks	./references/time-series.md
Population variables	./references/variable-catalog.md
Income/poverty variables	./references/variable-catalog.md
Education variables	./references/variable-catalog.md
Tract boundary changes	./references/boundary-changes.md
2022 Connecticut changes	./references/boundary-changes.md
TIGER/Line versions	./references/boundary-changes.md
Direct NHGIS access	./references/data-access.md
ipumspy Python package	./references/data-access.md
Data Finder workflow	./references/data-access.md