Topographic Height (m) Topographic Height (m) optimum inversion performance across a wide(r) range of forest heights and a better stability against non-volumetric decorrelation contributions [132]. An example of Pol-InSAR forest height estimation is shown in Fig. 18. In Figure 18(a), an L-band SAR image of the Traunstein forest site, located in southern Ger50 many is shown. The Traunstein 40 forest is characterized by a large variety of forest stand conditions 30 in the presence of locally variable 20 topography. In the middle and on the right of Fig. 18 forest height 10 [m] maps derived from Pol-InSAR data 0 acquired at L-band in 2003 (b) (a) (b) (c) and 2008 (c) are shown. Comparing the two forest height maps a FIGURE 18. (a) L-band HV intensity image of the Traunstein test site. Forest height map comnumber of changes within the forputed from Pol-InSAR data in (b) 2003 and (c) 2008. est become visible: The logging of individual tall trees as a result of development of accurate and robust allometric biomass a change in forest management between 2003 and 2008 estimators. The initial proposed forest height-to-biomass (marked by the green box); the damage caused in January allometry, even if robust and unsaturated, depends criti2007 by the hurricane Kyrill which blew down large parts cally on forest stand density. This reduces the estimation of the forest (marked by the orange box); and finally foraccuracy in heterogeneous forest conditions [136], [137]. est growth on the order of 3 to 5 m over young stands as A way forward is the establishment of a more general alloseen within the area marked by the white circle. metric relationship between forest biomass and vertical The availability of multiple Pol-InSAR interferograms at forest structure. This generalized allometry may allow lower frequencies makes it possible to determine not only robust and accurate biomass estimation independent forest height and ground topography [133]-[135], but also the lower frequency components of the vertical scattering structure according to (46). Fig. 19 shows 690 two such L-band profiles across the Traunstein test site derived using up to third-order Legendre polynomials by means of (46). The coefficients have been derived using the cross-polarized coherence meaMixed Forest Stand sured at only two spatial baselines 570 Slant Range while the ground topography and forest height are both estimated 690 Mature Spruce Stand from the inversion of Pol-InSAR interferograms. The structure information allows to distinguish clearly between the younger mixed forest stands characterized by a rather heterogeneous vertical structure distribution from mature spruce stands 570 Slant Range with their more homogeneous structure profiles defined by their elevated canopies. FIGURE 19. Vertical scattering profiles obtained from the inversion of dual-baseline Pol-InSAR Forest height and vertical for- data at L-band over the Traunstein test site. The profiles allow younger mixed (orange) and est structure are essential for the mature spruce stands (yellow) to be distinguished. march 2013 ieee Geoscience and remote sensing magazine 29