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Table 3 Final sets of image predictors (left side) and model coefficients for respective total fuel density (TFD; kg m−3) and canopy bulk density models (CBD; kg m−3) calibrated using dependent ground data collected in 2015 and 2016 in the Superior National Forest, Minnesota, USA. Model subscript notations 1, 12, and 123 reference the zenith angle range used for canopy gap fraction (CGF) measurements (0 to 7°, 0 to 23°, and 0 to 38°, respectively), while FC represents FuelCalc-derived field estimates of CBD. Dashes (–) indicate image predictors removed during model calibration. Image predictors include synthetic aperture radar (SAR) backscatter amplitudes and ratios, optical sensor bands and ratios, and previously derived estimates of forest basal area (BA) by species or group: Abies balsamea (ABBA), conifer (CON), broadleaf (BRD), Larix laricina (LALA), Picea glauca (PIGL), Picea mariana (PIMA), Pinus resinosa (PIRE), Pinus strobus (PIST), and total basal area (Total). Where necessary, image month is indicated. Combinations of SAR send and receive polarization are indicated with H (horizontal) and V (vertical). Spatially filtered (low-pass) versions of original resolution SAR variables have notations 3 × 3, 5 × 5, and 7 × 7. Optical sensor indices include the normalized difference vegetation index (NDVI), shortwave infrared to near-infrared ratio (SWIR:NIR), and SWIR to visible ratio (SVR). Sensors include Phased Array L-band Synthetic Aperture Radar (PALSAR-1); Sentinel-1 C-band SAR; Sentinel-2 optical MultiSpectral Instrument (MSI); Satellite Pour l’Observation de la Terre (SPOT-5) multispectral (XS) optical sensor; and Landsat-8’s optical Operational Land Imager (OLI)

From: Modeling sub-boreal forest canopy bulk density in Minnesota, USA, using synthetic aperture radar and optical satellite sensor data

  TFD1 TFD12 TFD123 CBD1 CBD12 CBD123 CBDFC
Sentinel-2A (MSI)
 MSI 2 0.0014 0.0104 0.0070 0.0085
 MSI 3 −0.0058 −0.0083 −0.0041
 MSI 4 −0.0019 −0.0109 −0.0044
 MSI 8 0.0005 −0.0021 −0.0018
 MSI 11 0.0000 0.0055
 MSI 12 0.0000 0.0068 0.0118 0.0084
 MSI NDVI −0.0247 0.0585
 MSI SVR −0.0419 −5.2946 −3.1050 −5.5613 −7.2481
 MSI SWIR:NIR 2.9540
Palsar-1 (P1)
 HH, 5 × 5, Dec 17.7236 25.2843 −12.0514
 HH, 7 × 7, Dec −12.5204 −8.0620 −33.3317 −47.0995
 HH, 3 × 3, Nov −2.8668
 HH, 5 × 5, Nov −0.7608 −12.8692 −24.8212
 HH, 7 × 7, Nov 3.5551 14.6509 27.6294
 HV, Nov −2.5963
 HV, 5 × 5, Nov −19.4630 −48.3685
 HV, 7 × 7, Nov 11.7498 15.6451 44.7208 80.4010
Landsat-8 (OLI) Feb
 OLI 2 −0.0002
 OLI 3 0.0002 −0.0032
 OLI 5 −0.0001 0.0004 0.0013 0.0030
 OLI 6 0.0010 −0.0007 −0.0023 −0.0033
 OLI NDVI −0.0265 −0.0223
 OLI SVR −5.9211 0.0121 0.0053 −0.0314
 OLI SWIR:NIR 24.7883 50.4194
Sentinel-1 (S1)
 VH 0.0066 0.0141 0.0332 0.0259
 VH, 3 × 3 0.0630 0.0625
 VH, 5 × 5 −0.1781 −0.1152 −0.0349 −0.1061 −0.2172 −0.1708
 VH, 7 × 7 0.0642 0.0291 0.0645 0.0961
 VV −0.0106 −0.0251 −0.0103 −0.0061 −0.0142 −0.0095
 VV, 3 × 3 0.0205 0.0147
 VV, 5 × 5 0.0452 0.0488 0.1091 0.0981
 VV, 7 × 7 0.0488 −0.0182 −0.0350
 VH:VV −0.0034 −0.0030 −0.0005 −0.0013 −0.0028
 VH:VV, 5 × 5 0.0146 0.0027 0.0108 0.0278 0.0319
 VH:VV, 7 × 7 0.0038 0.0185
S1:P1 (Nov) ratios
 VH:HV −0.0051 −0.1553
 VH:HV, 3 × 3 0.3947
 VH:HV, 5× 5 −0.0051 −0.4542 −0.0950 −0.1553
 VH:HV, 7 × 7 −0.0025 −0.0706 0.0770 0.4612
 VV:HH, 5 × 5 1.2505 −0.8196 −1.6456 −3.7624
 VV:HH, 7 × 7 1.9131 0.9522 2.1599 4.6099
SPOT-5 (XS)
 XS 1 −0.0341 −0.0851 −0.1435
 XS 2 0.1409 0.0368 0.0488 0.1022 0.1278
 XS 3 −0.0172 −0.1854 −0.0781
 XS 4 −0.0028 −0.0201 0.0618 −0.1030
 XS NDVI 0.0367 0.0770 0.0228
 XS SVR −1.2025 −2.5112 −10.5606 −17.6685 4.6225
 SWIR:NIR 2.1590 6.5851 9.2176
BA estimates
 ABBA −0.6006 1.8371 2.3587
 CON 0.3828 −0.1695 −0.3521
 BRD −0.3770 −0.5943
 LALA 2.8078 1.6604
 PIGL 0.4334 1.2487 1.6620 1.9919 2.2904 2.8725 −2.5577
 PIMA −0.5171 0.2985
 PIRE 0.4379 1.2880 1.3795 −0.9524
 PIST 1.3065 4.5840 −2.8129 −4.6389 −1.1636
 Total 0.2282 1.7470