# Understanding Forest Health with Remote Sensing-Part II--A Review of Approaches and Data Models
## 2 Trends in Close-Range RS Approaches for Assessing FH
### Radar  
Synthetic Aperture RADAR (SAR) uses an antenna to transmit microwave pulses in a specific waveband (or frequency) at an oblique (incident) angle to the target area [147]. RADAR data can be acquired in a variety of modes, including standard polarizations (e.g., HH, VV, HV, VH), polarimetric (phase information is preserved [148]), and interferometric (InSAR; phase processing/analysis of two signals at slightly different incident angles [149]). These data can be analyzed for: (1) backscatter intensity response, which increases with surface moisture and roughness, but also varies with incident angle (steeper angle signals penetrate further into the canopy, with associated increases in volume and perhaps surface scattering), wavelength [150] (longer wavelengths penetrate deeper), polarization (cross polarizations (HV; VH) penetrate deeper), and topography (slopes perpendicular to the incident signal generally produce the highest backscatter [151]); (2) polarimetric response, including phase related variables, degree of depolarization, and decomposition analysis [152; 153; 154; 155] to determine relative contributions of scattering mechanisms (e.g., surface, dihedral, volume scattering); and (3) InSAR phase differences, which are strongly related to the vertical position of the height of the scattering phase center (h\({}_{\text{spc}}\)) in vegetation or to surface elevation. For the latter, in dense forest and for shorter wavelengths (X, C-bands), h\({}_{\text{spc}}\) is closer to the top of the canopy but it is also dependent on system parameters (e.g., wavelength, baseline length and attitude between the two signals, polarization, incident angle, phase noise [156] and slope [157]). An InSAR generated DSM can be differenced from an accurate DTM (e.g., from long wavelength InSAR [158; 159] or LiDAR) to estimate tree height [157; 160; 161] and potentially height/cover loss due to disturbance. Integration of polarimetric response with InSAR (i.e., PolInSAR) has also been successful [159]. In addition, the coherence or correlation between the \begin{table}
\begin{tabular}{p{113.8 pt} p{113.8 pt} p{113.8 pt}} \hline \hline
**Application** & **Example Studies** & **Main Findings** \\ \hline Terrain determination (DTM) & [140; 141; 142; 143] & Best remote sensing technique to produce high accuracy DTM’s under dense forest canopy with an RMSE of 0.15–0.35 m. \\ \hline Forest height measurement (DCM) & [93; 94; 95] & Best remote sensing technique to produce high accuracy forest height measurements. \\ \hline Area based approach & [100; 101; 102; 103; 104; 105; 106; 107] & Statistical approach where measurements of field plots were set in relation to LiDAR-metrics. Precision of \(\sim\)6\% for height, \(\sim\)10\% for mean diameter, \(\sim\)10\% for basal area, \(\sim\)20\% for stem density and \(\sim\)12\% for AGB estimations. \\ \hline Individual tree approach & [112; 144; 145; 146; 147] & In a first step, single trees were automatically delineated from the point cloud and in a second step tree parameters were estimated. With these methods about 80\% to 97\% of the trees in the upper canopy can be detected and height, crown parameters, DBH, volume, species and health conditions estimated. \\ \hline Coarse woody debris & [96; 113; 114; 96] & Detection of standing and laying coarse woody debris on a single tree basis from LiDAR and in combination with optical data \\ \hline Leaf area index, canopy cover & [108; 119; 120] & For high quality results, calibration with hemispherical photography or LAI-2000 measurements are needed. But even without calibration, fairly reliable results can be obtained for fractional cover. \\ \hline Vertical forest structure & [125; 126; 127; 128; 129; 144] & LiDAR data is widely used to represent vertical forest canopy complexity and forest regeneration. These variables have great potential to predict species diversity. \\ \hline \hline \end{tabular}
\end{table} 
Table 6: Example studies illustrating the use of LiDAR in common FH applications.