Table 4 Difference between photo-interpreted and NLCD 2001 derived impervious cover values by mapping zone Zone n Overall percent impervious cover Table 4 continued Zone n Photo NLCD Difference Environmental Management (2010) 46:378–390 Overall percent impervious cover NLCD Difference 386 1 2 3 4 5 6 7 8 9 10 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 123 992 996 1017 1000 1000 995 999 998 930 1023 1013 1002 1000 1031 1003 1000 1007 1009 1016 1012 1011 1013 1015 999 1018 1017 1004 1016 1010 1008 1011 1024 1013 1008 1000 998 1018 1008 997 996 1017 1014 1020 1016 1013 1017 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 1015 1017 999 1000 1016 1019 1012 1000 997 1020 1000 1020 998 1014 1015 1013 986 1003 1009 Photo 5.6 4.4 2.6 4.5 5.9 2.3 5.0 3.5 10.5 4.0 4.7 7.8 7.8 2.8 5.8 4.1 4.6 11.3 3.0 SE 0.7 0.6 0.5 0.6 0.7 0.5 0.6 0.6 0.8 0.6 0.6 0.8 0.7 0.5 0.7 0.6 0.6 0.9 0.5 1.9 3.4 1.5 2.8 2.6 1.8 2.5 1.3 4.8 0.9 1.5 2.3 4.4 1.7 3.1 1.5 1.7 7.2 0.9 n Number of photo interpreted points, Photo photo-interpreted value, SE standard error of photo-interpretation estimate, NLCD NLCD derived cover value, Difference Photo value minus NLCD value * Signiﬁcant difference at 95% conﬁdence level; ** Signiﬁcant dif- ference at 99% conﬁdence level imagery. NLCD cover maps were based on circa 2001 imagery, while Google Earth imagery tended to be from the mid 2000s. When image interpretation began, dates of Google Earth imagery were not available. When dates became available, imagery dates ranged from the early to late 2000s, with the plurality of image dates tending to be around 2005–2006. Thus, the Google Earth images are from dates subsequent to NLCD images, and with varying temporal differences. Later imagery dates would tend to lead to increased impervious cover due to urban develop- ment, which would tend to enhance underestimation by NLCD estimates. The same development factors that increase impervious cover would potentially decrease tree canopy cover over time as trees may be cleared to make space for impervious surfaces. However, increases in tree canopy cover could also be occurring through time through tree planting or natural regeneration. In the northeastern United States, forest land has increased by nearly 7% since 1953, mainly due to agricultural lands that have reverted to forests (Smith and others 2009). Thus in some areas, underesti- mation of NLCD tree canopy cover may be exacerbated by tree growth between the dates of imagery; in other areas the 3.1 3.4 0.7 8.1 3.8 0.9 0.6 1.8 0.3 1.1 0.3 1.5 3.2 0.4 0.6 1.6 1.8 0.7 0.7 0.1 0.7 0.6 0.7 1.8 0.8 1.0 1.5 0.5 0.8 1.0 3.6 2.5 1.6 2.2 2.6 2.5 2.3 2.2 1.4 3.3 2.7 3.0 3.3 3.4 3.4 3.4 SE 0.5 0.5 0.3 0.7 0.6 0.3 0.2 0.4 0.2 0.3 0.1 0.4 0.5 0.2 0.2 0.3 0.4 0.3 0.2 0.0 0.3 0.2 0.2 0.3 0.3 0.3 0.3 0.2 0.3 0.3 0.5 0.4 0.4 0.4 0.5 0.5 0.5 0.4 0.4 0.5 0.5 0.5 0.5 0.5 0.5 0.6 2.3 2.4 0.7 6.2 2.9 0.6 0.3 0.7 0.2 0.4 0.2 0.8 1.4 0.3 0.2 0.8 0.5 0.4 0.3 0.1 0.4 0.3 0.3 0.6 0.2 0.3 0.3 0.3 0.3 0.4 2.0 0.7 0.4 0.9 2.2 1.9 0.9 0.5 0.5 1.1 1.1 1.4 1.1 0.6 1.2 1.4 0.8 1.0* 0.0 1.9* 0.9 0.3 0.3 1.0** 0.0 0.7* 0.1 0.7* 1.8** 0.2 0.4 0.8* 1.3** 0.3 0.4 0.0 0.3 0.3 0.4 1.2** 0.6* 0.7* 1.2** 0.2 0.5 0.7* 1.5** 1.9** 1.2** 1.3** 0.4 0.6 1.4** 1.6** 0.9* 2.1** 1.6** 1.5** 2.1** 2.8** 2.2** 2.0** 3.7** 1.0 1.1* 1.6** 3.3** 0.5 2.5** 2.2** 5.7** 3.1** 3.2** 5.5** 3.3** 1.1* 2.7** 2.6** 2.9** 4.1** 2.1**