TIA also is estimated from population density data by estimating the population in an area of interest and using regression equations to calculate the associated TIA. Population-density data are used because nationally consistent census-block data are available in GIS formats for the entire United States. Population-density methods also can be used for predicting potential effects of future development. Although there may be substantial variation in relations between population density and TIA the accuracy of such estimates tend to improve with increasing drainage area as local variations are averaged out. Granato (2010) provides a table with 8 population-density relations from the literature and a new equation developed by using data from 6,255 stream basins in the USGS GAGESII dataset. Granato (2010) also provides four equations to estimate TIA from housing density, which is related to population density.