Environmental Management (2010) 46:378–390 DOI 10.1007/s00267-010-9536-9 Evaluating The National Land Cover Database Tree Canopy and Impervious Cover Estimates Across the Conterminous United States: A Comparison with Photo-Interpreted Estimates David J. Nowak • Eric J. Greenﬁeld Received: 18 June 2009 / Accepted: 4 July 2010 / Published online: 30 July 2010 The Author(s) 2010. This article is published with open access at Springerlink.com Abstract The 2001 National Land Cover Database (NLCD) provides 30-m resolution estimates of percentage tree canopy and percentage impervious cover for the con- terminous United States. Previous estimates that compared NLCD tree canopy and impervious cover estimates with photo-interpreted cover estimates within selected counties and places revealed that NLCD underestimates tree and impervious cover. Based on these previous results, a wall- to-wall comprehensive national analysis was conducted to determine if and how NLCD derived estimates of tree and impervious cover varies from photo-interpreted values across the conterminous United States. Results of this analysis reveal that NLCD signiﬁcantly underestimates tree cover in 64 of the 65 zones used to create the NCLD cover maps, with a national average underestimation of 9.7% (standard error (SE) = 1.0%) and a maximum underesti- mation of 28.4% in mapping zone 3. Impervious cover was also underestimated in 44 zones with an average underes- timation of 1.4% (SE = 0.4%) and a maximum underes- timation of 5.7% in mapping zone 56. Understanding the degree of underestimation by mapping zone can lead to better estimates of tree and impervious cover and a better understanding of the potential limitations associated with NLCD cover estimates. Keywords National cover maps Photo-interpretation Tree canopy cover Impervious cover D. J. Nowak (&) E. J. Greenﬁeld USDA Forest Service, Northern Research Station, 5 Moon Library, SUNY-ESF, Syracuse, NY 13210, USA e-mail: dnowak@fs.fed.us 123 Introduction Data on tree canopy and impervious cover provide important information on the extent and variation of these characteristics across a region. Measurements of tree can- opy cover provide basic structural data used to model tree services, such as air pollution mitigation and carbon dioxide sequestration (Nowak and Crane 2002; Nowak and others 2006), while impervious surface data are important for assessing development impacts on urban temperatures, precipitation runoff, and water quality (Heisler and others 2007; Theobald and others 2009). Tree canopy and impervious cover data provide essential information related to natural resources and development planning and policies at the local to national scale. The 2001 National Land Cover Database (NLCD) pro- vides free, easily accessible, 30-m resolution percentage tree canopy and percentage impervious cover values for the conterminous United States created from a consistent peer- reviewed methodology (MRLC 2009). Several studies have used NLCD data for assessing the urban tree canopy cover (Bridges 2008; Nowak and Greenﬁeld 2008), urban tem- perature modeling (Heisler and others 2007), estimates of canopy height (Walker and others 2007), distribution of constructed manmade surfaces (Elvidge and others 2007), non-point source nitrogen export into water systems (Shields and others 2008), and wildlife habitat distribution (Martinuzzi and others 2009). While a formal accuracy assessment of NLCD land cover estimates has been con- ducted (Wickham and others 2010), a formal accuracy assessment of NLCD tree canopy and impervious cover data has yet to be completed (Stehman and others 2008; US EPA 2010). In 2007, the 2001 NLCD was made publicly available by the Multi-Resolution Land Characteristics Consortium