384 Table 2 continued Zone Developed land cover Environmental Management (2010) 46:378–390 Forest land cover Agriculture–grassland land cover Other land cover n PI SE NLCD Diff n PI SE NLCD Diff n PI SE NLCD Diff n PI SE NLCD Diff 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 94 31.9 127 26.0 69 17.4 107 29.0 130 33.1 71 8.5 130 24.6 66 21.2 199 43.2 59 25.4 77 26.0 109 39.4 138 44.9 91 17.6 143 28.7 72 27.8 75 34.7 182 39.6 38 26.3 4.8 3.9 4.6 4.4 4.1 3.3 3.8 5.0 3.5 5.7 5.0 4.7 4.2 4.0 3.8 5.3 5.5 3.6 7.1 19.5 28.3 21.2 25.3 21.1 21.7 20.7 17.5 26.4 13.1 19.5 19.8 33.3 18.9 21.7 20.5 22.2 34.6 21.1 12.4** 595 1.7 0.5 -2.3 151 0.7 0.7 -3.8 467 1.1 0.5 3.7 534 1.1 0.5 12.0** 91 2.2 1.5 -13.2** 747 0.8 0.3 4.0 609 1.3 0.5 3.7 627 1.6 0.5 16.8** 364 2.5 0.8 12.4* 748 1.9 0.5 6.5 549 2.4 0.6 19.6** 611 1.5 0.5 11.6** 439 1.1 0.5 -1.3 690 0.6 0.3 6.9 589 0.8 0.4 7.3 606 2.1 0.6 12.5* 716 1.8 0.5 5.0 686 3.1 0.7 5.2 885 1.8 0.4 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 1.7** 306 4.6 1.2 0.7 718 1.7 0.5 1.1* 431 2.1 0.7 1.1* 325 1.5 0.7 2.2 774 2.1 0.5 0.8* 172 5.2 1.7 1.3** 253 4.7 1.3 1.6** 267 3.0 1.0 2.4** 249 5.2 1.4 1.9** 193 4.7 1.5 2.4** 344 4.4 1.1 1.5** 280 8.6 1.7 1.1* 369 3.3 0.9 0.6* 207 3.9 1.3 0.8* 263 4.2 1.2 2.1** 312 2.6 0.9 1.8** 168 3.6 1.4 3.1** 114 7.9 2.5 1.8** 59 3.4 2.4 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 4.5** 20 10.0 6.7 0.6 1.7** 21 4.8 4.6 0.2 2.0** 32 0.0 3.1a 0.0 1.5* 34 5.9 4.0 0.0 2.1** 21 9.5 6.4 0.2 5.2** 29 10.3 5.7 0.0 4.7** 20 10.0 6.7 0.2 3.0** 40 2.5 2.5 0.0 5.2** 185 1.6 0.9 0.0 4.6** 20 0.0 4.9a 0.1 4.3** 30 0.0 3.3a 0.0 8.6** 20 5.0 4.9 0.0 3.2** 52 1.9 1.9 0.1 3.9** 26 7.7 5.2 1.6 4.2** 20 15.0 8.0 0.7 2.5** 23 0.0 4.3a 0.1 3.6* 27 0.0 3.6a 0.0 7.9** 21 9.5 6.4 0.0 3.4 27 7.4 5.0 0.0 9.4 4.6 0.0 5.9 9.3 10.3 9.8 2.5 1.6 -0.1 0.0 5.0 1.9 6.1 14.3 -0.1 0.0 9.5 7.4 n Number of photo interpreted points, PI photo-interpreted value, SE standard error of photo-interpretation estimate, NLCD NLCD derived cover value, Diff PI value minus NLCD value * Signiﬁcant difference at 95% conﬁdence level; ** Signiﬁcant difference at 99% conﬁdence level a Number of points hitting the cover type was 0, so SE = 0. Number of points hitting cover type was set to one to calculate displayed SE and that adjusted value was used in the signiﬁcance test to avoid testing using a zero standard error and conﬁdence interval of photo-interpreted tree cover (Spearman correlation coefﬁcient (rs) = 0.70). Differences between photo-inter- preted and NLCD impervious cover were also signiﬁcantly correlated with the amount of photo-interpreted impervious cover (rs = 0.89). Thus, differences between photo-inter- pretation and NLCD cover values tended to increase with increased amounts of tree canopy or impervious cover. Discussion Differences between cover estimates generated by com- puter-classiﬁed NLCD and photo-interpreted images are not surprising due to methodological differences (Dough- erty and others 2004) and the reported accuracy of the NLCD (MRLC 2009). However, the overall and variable underestimation of tree canopy cover by NLCD relative to photo-interpretation is important to understand because of the increasing use of NLCD products in environmental management and planning applications (e.g., use of NLCD canopy cover in evaluating habitat distribution and con- servation (Martinuzzi and others 2009) or hydrologic 123 modeling and monitoring using estimates of impervious surfaces (Journal of Hydrologic Engineering 2009). The average differences found in this analysis of all mapping zones were similar to differences projected from the sampling of geographically dispersed areas within varying population density classes in a preliminary analysis (Greenﬁeld and others 2009). The preliminary analysis estimated an average NLCD underestimation in tree can- opy cover of 9.7%, which is the same overall difference exhibited by the analysis of all 65 mapping zones. The preliminary analysis found an average underestimation in impervious cover of 5.7% within places (e.g., cities vil- lages) and 1.3% in counties. The analysis of all 65 mapping zones found an average difference of 1.4%, which is sim- ilar to the difference from the county analysis. The pre- liminary county estimates are more representative of the entire mapping zones because of the lower density of populations and impervious surfaces in the counties com- pared to places. The preliminary study underestimation of impervious surfaces in places (5.7%) is comparable the underestimation exhibited by developed lands (5.4%), as places contain signiﬁcant amounts of developed land.