Tracking Residential Growth after the 2000 Census:

By Ken Needham

Microgrid_Article Valencia, California, like thousands of similar suburban communities in the United States, underwent major growth in the '90s, and the pace of its expansion has not slowed in the first decade of 2000.New housing continues to come online in quantity and on large tracts of undeveloped land.In order to account for this new residential development in its household counts, to place the households on the streets where they live and to attach to them a demographic profile, Scan/US developed a National Household Growth Tracking process.These new consumers are now tracked on the national landscape using Scan/US, Inc's MicroGrid cartography.MicroGrids - grid squares as small as 1/16 of a square mile - allow the market analyst to directly estimate the impact of the updated consumer base on nearby retail activity.

To explain how Scan/US identifies household change at the street level and then distributes these households to the MicroGrid cartography, let's look at one particular census block group, BG (6 37) 9201.07-1, in Valencia, as it undergoes its update process.Although this block group is one of tens of thousands of neighborhoods in the United States that has experienced household growth over the past four years, its story is interesting enough to make the strengths of this update methodology clear.

Block group 9201.07-1 is typical of a Census 2000 block group encompassing undeveloped land on the edge of suburban sprawl and ripe for development in this decade (see Map 1).The block group is large, covering five square miles.While the block group contains an office park development, it had no residents at the time of the 2000 Census.On the southwest the block group is bordered by a major interstate, I-5, and extends 3.5 miles north to Farmer John Lat in the hills overlooking Valencia.

Map 1: All 5 square miles of block group (6 37) 9201.07-1 in Valencia, CA.Click image for larger view.

By the time of the 2004 update cycle BG 9201.07-1 had acquired 445 households with an estimated $20.57 million in aggregate annual disposable dollars to contribute to the local economy.The existence of these new households had been confirmed by two independent data sources: the geocoded ADVO ZIP+4 active residential deliveries and the block group-
allocated USPS carrier route delivery counts.These household counts were distributed to the MicroGrids corresponding to their block face location, with their demographic profile assumed to be analogous to recent in-migrants in the surrounding neighborhoods. The much finer geographic resolution of microgrids compared to block groups allows a more focused market evaluation of the impact of these new consumers.

Assigning a Demographic Profile to new households
When household growth occurs in block groups with a significant base population as of Census 2000, these new households are assumed to have a demographic profile similar to other households in the block group.However, with no reference 2000 households to rely on, associating a demographic profile to the new households in BG 9201.07-1 requires a different strategy.Like thousands of other block groups with none or only a few households in 2000, it is necessary to recruit the demographic profile for these new households from the households in surrounding block groups.This is done by screening nearby block groups to identify those that had experienced rapid growth of a similar housing type in the five years prior to the 2000 Census.The ADVO residential delivery counts, classified into apartment and non-apartment deliveries, provide an important clue to the type of housing being built.The demographic profile of the most similar block group is applied to the new households.

Map 2: Block Face Points (BFP) in study area with BG 9201.07-1 highlighted.Click image for larger view.

Street-level assignment of households
The basic ingredients for allocating the nation's households to streets that they live on are:

  • the TIGER Line 2003 database;
  • the April 2004 USPS ZIP+4 inventory; and
  • the corresponding ADVO ZIP+4 database of active residential deliveries.
These data are geocoded and processed into a database of 27.9 million Block Face Points (BFP) which link postal delivery data by address to the street segments that border or are internal to Census blocks.While the eight million 2000 Census blocks will remain static throughout this decade, each year the BFPs continue to grow in number as the street network expands to accommodate residential growth.Map 2 shows the distribution of the BFPs along the street network in block group 9201.07-1 and the surrounding community.

Importance of the MicroGrid cartography
The MicroGrid cartography was designed to capture the distribution and density of residential and business development throughout the United States at the level of detail suitable for location-specific analysis.The MicroGrid cartography is annually updated to accurately reflect change in the distribution.MicroGrids make up a latticework of 1.9 million grid squares, pinned to latitude and longitude, which serve as containers for summary data on population, households and businesses.Where there is any locatable residential or commercial activity you will find grids of three sizes.A minute grid approximates 1 square mile, a 1⁄4-minute grid approximates .25 square mile and a 1/16-minute grid approximates .0625 square mile.The density of development determines the size of the grid: the denser the development the smaller the grid. Where there is no development there are no grids.Block group 9201.07-1 at five square miles in size covers five times the area of the largest grid and 80 times the area of the smallest grid.When the BFPs in block group 9201.07-1 are summarized by MicroGrids, the spatial representation of new households on the landscape becomes dramatically more precise.

Map 3: Scan/US MicroGrid geography overlaid on BG 9201.07-1 and the BFPs.Click image for larger view.

Precision with MicroGrid Cartography
So, where are the new households in block group 9201.07-1 really located? Map 3 shows there are 14 grids from the 2004 MicroGrid cartography that are entirely or partially within block group 9201.07-1.The Northern half of the block group is undeveloped and therefore no microgrids are present.Only five of the 14 grids contain households in 2004 and only three of these had any households according to Census 2000 (Table 1).The other nine grids contain businesses only. Of the five grids with households, two in the extreme north pick up their households from development in adjacent block groups.Grid 2534.0, a 1-minute grid overlaying the southwest corner of the block group, picks up about eight of its '04 household total of 227 from block group 9201.07-1.Grid 2535.23 has all of its 439 new households coming from block group 9201.07-1. Most of the new households detected in BG 9201.07-1 are actually concentrated in this 1/16-minute grid (1) at the Southern most tip of this large block group.Grid 2525.23 is 1⁄2 mile from the intersection of Magic Mountain Parkway and McBean Parkway, a major retail hub in Valencia.

Table 1: Key data on the 14 MicroGrids associated with BG 9201.07-1. Click image for larger view.

Published Wednesday, September 15th, 2004

Written by Ken Needham

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