Census Urbanized Areas and MPO/TMA Designation

The Use of ESRI Software in the Delineation of Urban Areas for the 2010 Census

Overview map of the U.S. showing urban areas.

For Presentation at the
ESRI International User Conference
July 12th, 2011

This presentation was created by the Census Bureau for the 2011 ESRI International User Conference, a technical audience of Geographic Information Systems (GIS) users. FHWA has modified it and added notes to make the presentation more accessible to a general transportation planning audience. Urban Areas for the 2010 Census

Urban Area Definitions

Image shows group of irregularly-shaped urban areas

Represent densely developed territory, encompassing residential, commercial, and other non-residential urban land uses in which social and economic interactions occur.
Represent the "Urban Footprint"
Structure has been explicitly defined through measures based primarily on population counts and residential population density
Criteria have also accounted for non- residential urban land uses that are functionally part of the urban landscape

After every decennial Census, the Census Bureau reviews and revises the criteria they use to define urban areas. Incorporated places may have very extensive boundaries that incorporate both rural and urban land, and some unincorporated areas may have development patterns with high population densities. Thus, the traditional method of using place boundaries (incorporated place or census designated place) as the basis for defining urban areas resulted in inconsistencies that were less than ideal. For the Census 2000, the Census Bureau took advantage of increased functionality of Geographic Information Systems (GIS), allowing for more consistent delineation processes. For the 2010 Census, the Census Bureau built upon this foundation to continue its efforts to produce an objective, comparable, and consistent urban area delineation.

Today's criteria applied to 2010 Census results use population counts, population density, and non-residential developments that are part of the urban landscape (e.g. industrial areas).

For the 2010 Census urban/rural delineation, the Census Bureau used ArcGIS 9.x software, employing Visual Basic for Applications (VBA) script and using ArcObjects to automate the delineation process for each major step:

Initial delineation of urban cores based on population density
Inclusion of impervious qualifying blocks
Inclusion of additional qualifying Blocks by hops and jumps
Inclusion of qualifying enclaves

Identification of Impervious Qualifying Blocks

The image shows a map of the edge of an urban area, overlaid with the 2006 NLCD impervious 30 meter raster dataset. The intensity of purple shading in each cell indicates the imperviousness score assigned to that cell.]

The incorporation of impervious surfaces into the criteria is new, and is meant to include commercial and industrial areas with low population density that are functionally part of the urban area, instead of relying only on residential densities to define urban areas. For this, the 2001 National Land Cover Database (NLCD), or NLCD 2006 update where available, impervious 30 meter raster dataset was used.

ArcGIS Spatial Analyst was used to identify the impervious qualifying Blocks, employing the following procedure:

Overlay Blocks on impervious raster and calculate the percentage of impervious area for each Block.
- If impervious area is ≥ 33.3%, the block is flagged.
Calculate the shape index for all Blocks as follows:
- I = 4πA / P² where I is the shape index, A is the area of the block, and P is the perimeter.
Blocks that are at least one-third impervious and are compact (I ≥ 0.185) are eligible to be added to urban cores.
Non-compact blocks with at least one-third impervious are eligible for inclusion if 40% of the perimeter of the block shares a boundary with a previously qualifying block

Impervious Qualifying Blocks

The image shows the same urban area as the previous slide, now with Census Blocks that qualify under the imperviousness criteria shaded in purple.]

Initial Delineation Demo

An animation demonstrates the GIS software performing the initial delineation process for an example geographic area that contains several urban areas.

The initial delineation process begins with identification of Census Tract-based cores:
- Start with tracts with population density of 1000 people per square mile (ppsm)
- Add adjacent tracts with population density of 500 ppsm
- Iteratively add blocks of 500 ppsm
Next Census Block-based cores are developed:
- Start with blocks with population density of 1000 ppsm
- Iteratively add adjacent blocks with population density of 500 ppsm
Iteratively add impervious blocks

Fill in holes (enclaves) less than 5 square miles that are completely surrounded by qualifying land

Image shows a qualifying urban core in dark purple, surrounded by several smaller qualifying areas of urban land identified during the initial delineation process. Three of the smaller areas are linked to the urban core via road connections that are highlighted in light purple or blue, indicating that they will be included in the urbanized area through either a "hop" or "jump".

Hops and Jumps

Use ArcGIS Network Analyst
Connect urban territory through road connections of less than 0.5 miles for hops and 2.5 miles for jumps
Evaluate blocks on each side of connection to form highest density corridor

The Census Bureau includes territory that is not contiguous with the urban core through "hops" and "jump" to allow for the inclusion of densely settled areas proximate to the urban core. Hops must connect along a road segment less than 0.5 miles in length (excluding exempted territory). Jumps can connect along a road segment up to 2.5 miles in length (excluding exempted territory). The 2010 Census criteria does not allow hops after a jump.

Areas qualify for inclusion via a hop or a jump if the area's total population is greater than 1,000 persons or if the population density of the area and the hop/jump corridor is greater than 500 ppsm.

Hops and Jumps

Once hops and jumps have been identified, Census Blocks adjacent to the connecting roadway are evaluated for population density. The hop or jump territory and the highest-density adjacent census blocks are included in the urban area if the total population of the destination is greater than 1,000 residents, or if the population density of the destination and the hop/jump corridor is greater than 500 persons per square mile.

***same image as previous slide but with the addition of the black-highlighted blocks.

Image on left shows a qualifying urban core in dark purple, surrounded by several smaller qualifying areas of urban land. Three of the smaller areas are linked to the urban core via road connections that are highlighted in light purple or blue, indicating that they may be included in the urbanized area through either a "hop" or "jump". The highest density census blocks that are adjacent to the road corridor connecting the urban core to a qualifying hop/jump area are highlighted in black.

Image on right shows all qualifying census blocks and linking road corridors highlighted in green. Qualifying areas that were not linked to the core in the previous images are no longer highlighted, indicated that they were not included in the Urbanized Area.

Post Hop/Jump Enclaves

Enclaves are small areas of non-qualifying territory that are surrounded by qualifying land and/or non-qualifying water territory. There are three types of enclaves:

Less than or equal to 5 square miles, surrounded by qualifying land only
Greater than 5 square miles, greater than 2.5 miles from the edge of the UA
Less than or equal to 5 square miles, surrounded by qualifying land and non-qualifying water territory

Image shows a post hop/jump enclave of non-qualifying land of 0.882 square miles that is surrounded by qualifying land. The enclave is highlighted in purple, indicating it would be included in the urbanized area.]

Less than or equal to 5 square miles,surrounded by qualifying land only

Post Hop/Jump Enclaves

Image shows four small areas of non-qualifying land that are all less than 1 square mile, and are surrounded by qualifying land and non-qualifying water territory. The small areas are highlighted in purple, indicating that they would be included in the urbanized area.

Less than or equal to 5 square miles, surrounded by qualifying land and non-qualifying water territory

Indentations

Use closure lines to close off areas that are indentations in the boundary.
The length of the closure line serves as the diameter of a circle. If four circles with the diameter fit into the indentation, we add territory to the UA.
If Ai ≥ 4π(½Li)2 and Ai ≤ 3.5 where Ai = area of indentation and Li = length of indentation closure line, then add the territory.

Following the inclusion of enclaves, indentations within the preliminary urbanized area shape are evaluated for inclusion in the final urbanized area. Indentations are areas of non-qualifying territory that are not completely surrounded by qualifying land and/or non-qualifying water (as enclaves are), but are substantially surrounded by qualifying land (according to the Census Bureau criteria).

Indentations

Described in previous image. Each image is two drawings.

Before Removal of Nonqualifying Cores

The image shows several preliminary urbanized areas. Only six of the areas are large, most are very small. --

Many small areas will be identified for potential inclusion in urban areas that are below the population threshold to qualify as an urban area.

After Removal of Nonqualifying Cores and Inclusion of Indentations

The image is of the same urbanized areas as the previous slide. Except, now all of the non-qualifying cores have been removed and indentations have been included. The final urbanized area boundaries are irregularly-shaped, often with jagged edges.

Removal of the non-qualifying cores and inclusion of indentations leaves only the high-density, compact areas as qualifying urbanized areas.

You can see by these examples that the edges can still be quite jagged. For the Census Bureau's purposes this is not important, but this is one reason why FHWA allows States/MPOs to create "Adjusted Urbanized Areas," adding additional territory to smooth out jagged edges.

Questions? Comments?

Vincent Osier, Kevin Hawley, Chris Henrie, Ted Sickley
Geographic Standards and Criteria Branch
Geography Division
U.S. Census Bureau
Vincent.osier@census.gov
(301) 763-9039