SANDAG performs an annual land use and housing unit inventory in the interest of maintaining a robust and accurate catalog of the existing conditions for any given year. This catalog of snapshots are the land use data on which the regional forecast is based every four years. Prior to performing our annual inventory, the polygon geometry for the regionwide layer is updated with a snapshot of parcels from SanGIS representative of January first of the given year. The land use information has been updated continuously since 2000 using aerial photography, the County Assessor Master Property Records file, and other ancillary information. To make the Landuse2017 feature class, adjacent parcel polygons with the same land use have been aggregated (dissolved) into a single feature.