This repo contains spatial files which combine Mapping Inequality polygons and Census tracts from the National Historic Geographic Information System to create a data crosswalk. The files are useful for joining historically redlined areas to demographic data. This data is licensed under a CC-BY-NC license.
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area_id(integer) is a unique identifier for each area created by the Digital Scholarship Lab. -
city(string) is the name of the city, town, county, etc. -
state(string) is the 2-letter U.S. Postal Service abbreviation for the state. -
city_survey(boolean, true=1, false=0) denotes whether the map was created as part of the HOLC City Survey Program or not. -
cat(string) is the assigned category from a redlining map. On standard HOLC City Survey Program maps, the category values are “Best”, “Still Desirable”, “Declining”, or “Hazardous”. -
grade(string) is the letter grade used to grade the area. For non-residential areas and most cities that were not part of the City Survey, the value is null. -
label(string) is the label from a redlining map. For most HOLC City Survey Program maps, this value is a letter and number, which often corresponds to an area description viewable on the Mapping Inequality website. -
com(boolean, true=1, false=0) denotes whether or not an area is labeled explicitly as commercial or inferred to be commercial from a redlining map. -
ind(boolean, true=1, false=0) denotes whether or not an area is labeled explicitly as industrial, or inferred to be industrial. -
res(boolean, true=1, false=0) denotes whether or not an area is labeled explicitly as residential or inferred to be residential. -
fill(string) is a hexadecimal color code for symbology. The value is typically an approximation of the color shown on a redlining map. -
GEOID(string) is a numeric code that uniquely identifies all administrative/legal and statistical geographic areas for which the Census Bureau tabulates data. -
GISJOIN(string) is a numeric code for joining ables from NHGIS to spatial data. -
calc_area(decimal) the area of the feature in meters. -
pct_tract(decimal) the features's areal percentage of a census tract.
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On the census tract layer, Select by Location features that intersect with Mapping Inequality polygons.
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Invert Feature Selection.
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Begin an Edit session on the census tract layer. Delete the selection. Save and stop editing. This removes tracts that don’t intersect with Mapping Inequality Polygons and speeds up processing.
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Run the Union geoprocess, using Mapping Inequality polygons and the input layer and Census tracts as the overlay layer. This spits the Mapping Inequality polygons into parts around Census tract boundaries
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On the newly generated Union layer, Select by Expression where
area_id IS NULL. -
Begin an Edit session on the Union layer. Delete the selection. This removes the parts of Census tracts that don’t overlap with Mapping Inequality polygons.
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Open Attribute Table on the Union layer, then Open Field Calculator.
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Create New Field with Output field name:
calc_area, Output type:decimal number real, and Expression:$area. Save. This calculates the area of each feature in meters. -
Create New Field again, this time with Output field name:
pct_tract, Output type:decimal number real, and Expression:'calc_area'/'Shape_area'. Save. This calculates the area percentage that Union feature comprises of the original Census tract. -
Open Properties on the Union layer. Open the Fields tab and select fields from the Census layer that are no longer relevant such as
Shape_areaandShape_len. Do not removeGEOIDandGISJOiN. -
Save and end edit session.
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If you created a temporary scratch layer for the Union layer, make sure to make the layer permanent before exiting QGIS.
