Environmental Management (2010) 46:378–390 Fig. 1 Differences in tree canopy cover estimates between photo-interpreted (PI) values and NLCD 2001 by mapping zone (PI minus NLCD value). Differences of 0% indicate no statistical difference Fig. 2 Differences in impervious cover estimates between photo-interpreted (PI) values and NLCD 2001 by mapping zone (PI minus NLCD value). Differences of 0% indicate no statistical difference underestimation may be reduced. The overall inﬂuence of potential changes in tree canopy cover due to the differing imagery dates is unknown, but believed to be minimal. If differing dates of imagery have a signiﬁcant effect, this would indicate signiﬁcant landscape change within the 2000 s, which would signify that the NLCD cover maps are currently obsolete due to rapid landscape change. Wickham and others (2010) found that time lags between reference and map image acquisition dates have little effect on agreement and discuss that land-cover change is rare. Landscape change is not likely the dominant factor in the differences exhibited between NLCD and photo-interpre- tation estimates. It is more likely that the NLCD cover maps are underestimating tree canopy cover and to a lesser extent, impervious cover. Another photo-interpretation limitation may be photo- interpreter error. As various quality control tests were conducted based on paired comparisons of image inter- pretation among photo interpreters, the photo-interpretation error is likely minimal. The issue of the horizontal posi- tional accuracy of Google Earth imagery is also not an issue as the mapping zones are large. Google Earth was 387 123