Environmental Management (2010) 46:378–390 Forest land cover Agriculture–grassland land cover Other land cover 382 Table 1 continued Zone Developed land cover n PI SE NLCD Diff n PI SE NLCD Diff n PI SE NLCD Diff n PI SE NLCD Diff 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 69 39.1 107 38.3 130 10.0 71 54.9 130 40.0 66 47.0 199 26.1 59 35.6 77 37.7 109 45.9 138 29.7 91 45.1 143 44.1 72 34.7 75 38.7 182 42.9 38 42.1 5.9 12.6 4.7 14.5 2.6 6.1 5.9 32.9 4.3 30.6 6.1 28.1 3.1 8.0 6.2 29.3 5.5 16.2 4.8 17.5 3.9 10.8 5.2 24.0 4.2 17.0 5.6 17.8 5.6 33.6 3.7 29.3 8.0 7.4 26.5** 467 83.1 1.7 72.6 23.8** 534 88.2 1.4 70.2 3.9 91 89.0 3.3 76.8 22.0** 747 94.1 0.9 85.2 9.4* 609 89.7 1.2 81.0 18.9** 627 89.3 1.2 78.7 18.2** 364 69.2 2.4 55.1 6.3 748 93.7 0.9 86.8 21.5** 549 84.7 1.5 66.8 28.4** 611 89.5 1.2 82.3 19.0** 439 85.9 1.7 80.0 21.0** 690 92.9 1.0 74.7 27.1** 589 87.4 1.4 82.1 16.9** 606 87.3 1.4 78.3 5.1 716 90.5 1.1 87.1 13.5** 686 88.9 1.2 85.9 34.7** 885 91.5 0.9 78.0 10.5** 431 10.7 1.5 18.0** 325 16.3 2.0 12.2** 774 7.1 0.9 8.9** 172 30.8 3.5 8.7** 253 35.2 3.0 10.6** 267 25.1 2.7 14.1** 249 17.3 2.4 6.9** 193 14.5 2.5 17.9** 344 23.8 2.3 7.2** 280 27.5 2.7 5.8** 369 22.5 2.2 18.1** 207 17.4 2.6 5.4** 263 19.0 2.4 9.0** 312 18.3 2.2 3.5** 168 21.4 3.2 3.0* 114 22.8 3.9 13.5** 59 28.8 5.9 0.9 4.7 1.3 9.0 1.9 2.0 2.4 1.3 2.4 1.6 0.4 4.2 3.4 9.5 0.7 0.4 4.2 9.8** 32 18.8 6.9 5.5 11.6** 34 50.0 8.6 19.9 5.8** 21 14.3 7.6 26.6 21.8** 29 20.7 7.5 2.2 33.3** 20 30.0 10.2 0.3 23.1** 40 15.0 5.6 2.8 14.9** 185 31.4 3.4 3.8 13.2** 20 35.0 10.7 0.2 21.4** 30 13.3 6.2 3.1 25.9** 20 55.0 11.1 1.1 22.1** 52 26.9 6.2 1.2 13.2** 26 7.7 5.2 6.0 15.6** 20 20.0 8.9 6.6 8.8** 23 17.4 7.9 29.8 20.8** 27 29.6 8.8 1.6 22.4** 21 9.5 6.4 0.6 24.6** 27 33.3 9.1 14.1 13.3 30.1** -12.3 18.5* 29.7** 12.2* 27.5** 34.8** 10.2 53.9** 25.7** 1.7 13.4 -12.4 28.0** 8.9 19.2* 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 differences between photo-interpretation and NLCD values were correlated with the amount of photo-interpreted cover. Results Comparisons of photo-interpreted and NLCD-derived val- ues reveal that NLCD underestimates tree canopy cover by a national average of 9.7% (standard error [SE] = 1.0%) and underestimates impervious cover by 1.4% (SE = 0.4%). Results varied by mapping zone with a maximum underes- timation of tree canopy cover of 28.4% (zone 3) and a maximum underestimation of impervious cover by 5.7% (zone 56) (Tables 3, 4; Figs. 1, 2). Overall, NLCD signiﬁ- cantly underestimated tree canopy cover in 64 of the 65 zones (98%) and impervious cover in 44 zones (68%) compared to photo-interpreted cover values. Based on photo-interpretation, tree canopy cover varied by mapping zone from a low of 1.6% in zone 33 to a high of 84.7% in zone 66 (Table 3). Impervious cover varied by mapping zone from a low of 0.1% in zone 21 to a high of 11.3% in zone 65 (Table 3). Within developed land, NLCD signiﬁcantly underesti- mated tree canopy cover in 31 mapping zones, over- estimated tree cover in one zone, and had an overall 123 underestimation of 13.7% (SE = 4.6%). NLCD estimates in developed land also signiﬁcantly underestimated impervious cover in 14 mapping zones, overestimated impervious cover in three zones, and had an overall impervious cover underestimation of 5.2% (SE = 4.8%) (Tables 1, 2). Within forest lands, NLCD signiﬁcantly underestimated tree canopy cover in 57 mapping zones with an overall underestimation of 11.7% (SE = 1.4%). It also signiﬁ- cantly underestimated impervious cover in 30 mapping zones with overall impervious cover underestimation of 0.9% (SE = 0.4%) (Tables 1, 2). Within agricultural and grass lands, NLCD signiﬁcantly underestimated tree canopy cover in 55 mapping zones with an overall underestimation of 6.7% (SE = 1.0%). It also signiﬁcantly underestimated impervious cover in 38 mapping zones with overall impervious cover underesti- mation of 1.5% (SE = 0.5%) (Tables 1, 2). Within other lands, NLCD signiﬁcantly underestimated tree canopy cover in 19 mapping zones with an overall underestimation of 8.0% (SE = 3.6%). It did not signiﬁ- cantly underestimate impervious cover in any mapping zones; overall impervious cover in other lands was underestimated by 1.3% (SE = 1.4%) (Tables 1, 2). Differences between photo-interpreted and NLCD tree canopy cover were signiﬁcantly correlated with the amount