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SKILL.md
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 (census 1990-2020). Census demographic variables (income, poverty, race, educational attainment) are NOT in the Portal — access directly from NHGIS via free IPUMS registration. 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, you must access NHGIS directly via IPUMS (free registration required). 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, clearly marked as requiring direct NHGIS access).
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
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:
Actual data file (what you observe in the parquet/CSV) — this IS the truth
Live codebook (.xls in mirror) — authoritative documentation, may lag
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
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
