Without doubt, data from the 2010 Decennial Census will show changes in the size, composition and distribution of the population in New York State compared to those same indicators from the 2000 Census. These changes will occur at all levels—county, towns, cities and places, tracts, block groups and blocks. In addition to changes in counts of population and housing units, as well as age, sex and racial groupings, there will be changes to the geographies associated with those levels.
While it is common practice to examine the population and housing changes amongst the administrative units under our respective purviews, we are less familiar with examining the changes in the spatial extent and composition of those same administrative units and component geographies (i.e. tracts, block groups, blocks). Consequently, census geographies have been of less concern. Yet we all know that changes in counts may be a function of both population dynamics and changes in redrawn boundaries that enlarge, shrink or reshape the county, town, city, and so forth. Failure to consider that geographies have changed means that when we compare population and housing changes across time, we are assuming that the spatial units have remained constant. Frequently this is an erroneous assumption which can lead to bad analysis and interpretation.
Purpose of Analysis
The purpose of this analysis of boundary differences is to examine the extent to which the changes we observe when comparing data from the 2000 Decennial Census to the 2010 Decennial could be a function of changes in geographies as well as a function of natural increase/decrease and net migration. By examining these boundary changes we establish a firmer basis for asserting that the changes (1) are due entirely to the demographic forces of natural increase/decrease and net migration or (2) are partly due entirely to boundary changes as well, or (3) are due to both.
There are two main reasons for raising the importance of boundary changes at this time. One is that these changes affect not only the Decennial Census but the American Community Survey data products as well. The second reason is that recent advances in GIS technology easily permit such an examination and thereby remove the anxiety of not knowing whether the assumption of common geographies from time one to time two to time three are constant.
True enough that boundary changes only immediately impact comparisons between decennial censuses, but in the not too distant future, that same issue will arise for the American Community Survey (ACS) data as well. As you know, the already reported ACS data are mainly based on 2000 census geography but within two years we face the additional complication of the ACS being tagged to the 2010 census geography (i.e. changes in geographies). Even now some ACS data are based on the 2010 geographies while other ACS data are not. (For additional details, see http://www.census.gov/acs/www/data_documentation/geography_notes )
To increase our understanding of the size and structure of these boundary changes between 2000 and 2010 Census geography, the Cornell Program on Applied Demographics decided to examine the changes starting with changes in county, town and place boundaries. The maps displayed are the most significant changes, there are many more less significant ones.
With the hope that this investigation will prove of some use to the Affiliates, we share our findings. All analyses were conducted using ArcGIS 10. Details of procedures used can be provided to interested users by writing to Joe Francis