Environmental Management (2010) 46:378–390 regression application, it may have overcompensated and produced this underestimation. While the NLCD tree canopy cover and impervious surface data are a free and easily accessible data set created with consistent methodology that may be used effectively in comparisons across the United States, users of the NLCD tree canopy cover maps should be aware of the overall and variable underestimation of tree canopy and impervious cover. Utilizing NLCD cover estimates for secondary analysis (e.g., tree biomass, rainfall interception) can lead to regional to national underestimation of these cover-depen- dent secondary estimates. Future research should investi- gate the apparent widespread underestimation to help improve national cover mapping. A formal accuracy assessment would help in this regard (e.g., Stehman and others 2008; Stehman and Czaplewski 1998, 2003). Conclusion NLCD 2001 cover estimates appear to be underestimating tree canopy and impervious cover across the conterminous United States to varying degrees. The absolute underesti- mation of tree canopy cover (9.7%) is much higher than that exhibited for impervious cover (1.4%). These results indicate that underestimation of tree canopy and impervi- ous cover was related to the amount of tree and impervious cover, with developed lands exhibiting the greatest underestimation of both tree canopy and impervious cover. A better understanding of the differences between NLCD and photo-interpreted cover values can be used to produce more accurate cover maps across the United States. Acknowledgments Funding for this project was provided, in part, by the U.S. Forest Service’s RPA Assessment Staff and State & Private Forestry’s Urban and Community Forestry Program. The use of trade, ﬁrm, or corporation names in this article is for the infor- mation and convenience of the reader. 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