Assessing the Accuracy of Small Area Forecasts Ground truth project validates housing data with ArcGIS By Douglas A. Skuta, Economist; Jim Herries, product Engineer; and Earl A. nordstrand, Manager, Esri Data Development Team Esri undertook an innovative project that produced valuable insights into its modeling process and validated the accuracy of its housing unit estimates at a small area level (block group) using ground truth information gained by manually comparing data sources with aerial imagery. Although the decennial census provides block-level counts of population and housing units along with basic demographic traits every 10 years, data users prefer more current information. Unfortunately, current data is not reported for small census areas such as census blocks or census block groups. [A census block is the lowest level of tabulated census geography. Census blocks are assembled into block groups.] This data is provided for postal areas (carrier routes or nine-digit ZIP Codes), larger areas such as places or counties, or the smallest unit possible—addresses. Calculating a segue from areas with current data to the smallest census areas has always been challenging because the boundaries for these areas do not correspond. Why not use the addresses as a direct measure of the population distribution? Because address lists are flawed: they include duplicate listings, incomplete address information, group quarters, nonresidential address listings, and a lot of post office boxes. Postal databases also include errors. Even census counts require review and correction. While it is incumbent on data providers to reconcile the differences among the many data sources that enable current information, that is easier said than done. In 2005, Esri developed a method called Address Based Allocation (ABA) to assign current data from postal areas to census blocks using addresses. ABA uses the actual locations of mail deliveries and solves the complex challenge of converting delivery counts from carrier routes to block groups. [For more information on ABA, see "The Secret Life of Polygons: Understanding the relative accuracy of user-defined areas" by Douglas Skuta and Lynn Wombold in the summer 2008 issue of 14 ArcUser Fall 2010 www.esri.com