# Retrieval of Biophysical Crop Variables from Multi-Angular Canopy Spectroscopy
## 4 Discussion  
For the interpretation of directional angular effects on spectral reflectance, the anisotropy factor (ANIF) can be consulted: due to self-shading effects, forward scatter images appear darker than nadir or backscatter images. In the latter case, a greater fraction of incident sunlight is reflected back to the direction of its origin and is consequently missing on the opposite viewing direction. This so-called hot spot effect leads to a spectral saturation and superimposes parts of the signal of leaf constituents. Moreover, ANIF is highly correlated with the magnitude of reflectance itself. If the canopy reflectance is higher, discrepancies increase between nadir and forward scatter but decrease between nadir and backscatter. On the other hand, if more radiation is absorbed or transmitted by the canopy, anisotropy decreases for forward scatter and increases for backscatter. Spectrally, high anisotropy occurs for blue and red portions of the solar spectrum from which leaf chlorophyll mainly absorbs radiation to photosynthesize. This was also found by [52] for both directions, but in our study this could only be confirmed for backscatter mechanisms. This phenomenon may be the reason why it was more difficult for the PROSAIL model to reproduce the measured spectra from this direction, leading to a weaker estimation of LCC from backscatter in comparison to forward scatter spectra. \begin{table}
\begin{tabular}{c c c c c c c c c} \hline \hline \multirow{2}{*}{**Number of Best Fits**} & \multicolumn{4}{c}{**RMSE Cost Function**} & \multicolumn{4}{c}{**MAE Cost Function**} \\ \cline{2-9}  & \multicolumn{2}{c}{**LAI**} & \multicolumn{2}{c}{**LCC**} & \multicolumn{2}{c}{**LAI**} & \multicolumn{2}{c}{**LCC**} \\ \cline{2-9}  & **Slope** & **rRMSE** & **Slope** & **rRMSE** & **Slope** & **rRMSE** & **Slope** & **rRMSE** \\ \hline
1 & 0.88 & 0.28 & 1.61 & 0.44 & 0.88 & 0.28 & 1.55 & 0.42 \\
50 & 0.94 & 0.19 & 1.44 & 0.32 & 0.93 & 0.18 & 1.36 & 0.32 \\
100 & 0.89 & 0.19 & 1.30 & 0.29 & 0.92 & 0.18 & 1.18 & 0.28 \\
200 & 0.90 & 0.2 Our main study objective was to assess the effect of off-nadir observations on the prediction accuracy for leaf and canopy variables, namely LAI and leaf chlorophyll content, as it will have major implications for the user community of future EnMAP data. Generally, for both off-nadir observations, accuracy decreased when estimating LAI: rRMSE = 18% at nadir vs. rRMSE = 25% (backscatter) and rRMSE = 24% (forward scatter). For LCC and CCC, the off-nadir mode with forward scatter yielded highest accuracies with rRMSE = 20% and rRMSE = 33% respectively. Once again, the complex structure of the canopy plays an important role for the output of PROSAIL. Turbid medium assumptions are best met for homogeneous crops with least possible complexity in plant structural traits. Winter wheat is thought to be particularly well suited for a representation through RTMs [19]. Nevertheless, the leaf surface of wheat exhibits anisotropic reflectance that is mathematically described by the BRDF [40]. Backscattering leads to glare effects and thus complicates the retrieval of LAI. In the opposite direction, i.e., forward scatter OZAs, the canopy appears darker which seems slightly better suited for the retrieval of LAI. If EnMAP data is only available for backscatter observations, an inversion will still be successful, but the user will encounter larger uncertainties. Leaf glint generally leads to a reduced accuracy for the inversion of LCC. Senescent plant material absorbs less of the incident radiation and is more subjected to hot spot effects [53]. This could be confirmed (see Figure 5), as LCC \(>\) 40 \(\upmu\)g cm\({}^{-2}\) was poorly estimated from backscatter spectral images, but well inverted from forward scatter observations. The findings suggest that LCC is best retrieved, the greater the difference of the zenith angle between sun and observer becomes. CCC acts like a linear combination of LAI and LCC in any statistical analysis.