NHGIS Data Source Reference

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.

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

Reference File Structure

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

Key Identifiers

Key Education Variables

Data Sources by Type

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).

Portal Variables (Colleges NHGIS)

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

Missing Data Codes

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.

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)

Note: The Portal provides pre-processed school/college-to-census-geography links. For custom census analysis (tract-level demographics, time series, boundary files), use NHGIS directly via methods in ./references/data-access.md (requires free IPUMS registration).

Common Pitfalls

Related Data Sources

Topic Index