FD-2: Vegetation Structure from Lidar and SAR
Presented by: Ralph Dubayah, Jacqueline Rosette, Juan Suarez-Minguez and Kostas Papathanassiou
1,2,3 University of Maryland, USA
4 German Aerospace Center (DLR), Oberpfaffenhofen, Germany
Part 1 - Deriving Vegetation Structure from Lidar Remote Sensing:
In this tutorial participants will learn the basic theory behind lidar remote sensing, and its applied use for deriving vegetation structure such as canopy height, vertical canopy profiles and biomass. The session will include a general introduction to lidar principles and systems, as well as background on active laser interactions with vegetation. We will then cover the various types of commonly used lidar, included discrete return small footprint and waveform lidar from aircraft and space. We will give examples of the use of lidar for forest inventory applications, carbon modeling and habitat assessment, among others. We will demonstrate the use of common processing tools for the derivation of lidar metrics. After a short break will give examples of applications for forest inventory, biomass, and biodiversity studies. We will then cover the basic principles of terrestrial laser scanning. The session will end with a discussion of emerging lidar technologies, such as single photon counting and a look ahead at the forthcoming ICESAT-2 mission.
Part 2 - Deriving Vegetation Structure from SAR:
In the second part of the tutorial the estimation of forest vertical structure by means of SAR is discussed. The unique ability of low frequency (i.e. long wavelength) SAR to penetrate into and through even dense vegetation and thus to interact with the different vegetation layers provides sensitivity to vegetation structure. Two approaches - both inherently related to each other - have been established in the last years to estimate vegetation structure from SAR meassurements:
- SAR Tomography is based on "real" 3D imaging of the complex reflectivity function of the scatterer by building up through multiple acquisitions an aperture in elevation.
- (Polarimetric) SAR Interferometry that relies on the fact that the (volume) interferometric coherence is directly related to the vertical (complex) reflectivity function.
Conventional SAR tomography has demonstrated the potential to "image" vertical structure by means of multiple acquisitions. However, the lack of multi-static space-borne SAR configurations able to perform these acquisitions, combined with the (temporal) scene decorrelation limit the maximum number of suitable (i.e. coherent) acquisitions possible for a realistic space-borne scenario. Therefore, the application of conventional tomographic imaging is rather limited at least in terms of the actual state-of-art in space-borne SAR missions. Given the availability of only a limited number of acquisitions, alternative approaches have to be used in order to assess vertical structure information by means of SAR. The proposed techniques can be distinguished into:
- (Polarimetric ) Interferometric approaches reconstruct the vertical reflectivity function (i.e. the vertical distribution of scatterers) from interferometric (volume) coherence meassurements at different polarisations. This information can be either extracted by model based inversion or by approximating the structure function through a weighted sum of a series of (orthogonal) basis functions.
- (Complex) Reflectivity approaches separate the location of the phase centers associated to the individual vertical structure components. This is then used to reconstruct the vertical structure of forest scatterers.
Common in both approaches is the necessity to parameterize the vertical structure function using a limited number of parameters, a challenging step when accounting the complexity of forest structures. The individual parameterization has then to be inverted using a (limited) number of interferometric or reflectivity measurements at the same or different polarizations.
- The tutorial starts with a short introduction in SAR, SAR-Interferometrty and SAR-Tomography in order to provide the background needed to follow the course.
- The tomographic reconstruction of vegetation vertical structure is introduced and discussed.
- The theoretical framework, principles and concepts of Polarimetric SAR Interferometry (Pol-InSAR) including physical interpretation, 3d modelling approaches, signal processing techniques and inversion models are introduced and discussed.
- Model based tomographic reconstruction of vegetation vertical structure is introduced and discussed.
- The role of polarisation in forest structure reconstruction is discussed.
- The choice of frequency and its implications in terms of forest structure reconstruction is addressed.
All points above are illustrated by using examples from relevant air- and space-borne campaigns. Finally, as a prominent example of vegetation structure application the estimation of (above ground) forest biomass is introduced. The pros and cons of the different vertical structure estimation approaches by means of SAR are discussed.
Ralph Dubayah: Ralph Dubayah is Professor of Geographical Sciences at the University of Maryland, College Park. His main areas of interest are ecosystem characterization for carbon modeling, habitat and biodiversity studies. A common goal of his research is to develop and apply emerging technologies of spatial data acquisition and analysis to address environmental issues at policy-relevant scales. He has over 15 years of experience in the development and application of lidar remote sensing for terrestrial ecology studies. He was also principal investigator for the Vegetation Canopy Lidar (VCL), a NASA mission to measure the three-dimensional structure of the Earth's forests. He currently serves as an Associate Editor for the Journal of Geophysical Research (Biogeosciences), and is on the editorial board of Remote Sensing of Environment. He has served as the Ecosystem Structure Science Lead for for NASA's DESDynI mission for the past three years.
Jacqueline Rosette: Dr. Rosette is an Assistant Research Scientist at NASA Goddard Space Flight Center, US and a Postdoctoral Researcher at Swansea University, UK in partnership with the Forest Research Northern Research Station, Research Agency of the UK government Forestry Commission. Dr. Rosette received her Ph.D from the University of Wales and has research interests in the use of large and small footprint LiDAR, as well as single-photon counting sensors for forest parameter assessment and inventory. She also has strong interests in the use of radiative transfer modeling for understanding the interactions of lidar sensor design with landscape canopy structure.
Juan Suarez-Minguez: Dr. Suarez-Minguez is Project Manager for the Remote Sensing Applications Programme for the Forest Research Northern Research Station, Research Agency of the UK government Forestry Commission. He received his Ph.D from Sheffield University, UK. He has research interests in the use of airborne LiDAR and terrestrial laser scanning for forestry applications including forest inventory, ecology, carbon sequestration and the impact of pest infestations. He is also active in the development of algorithms to extract critical forest parameters as inputs to models of timber quality and stability.
Konstantinos Panagiotis Papathanassiou received the Dipl. Ing degree (Honors) in 1994 and the Dr. degree (Honors) in 1999 from the Technical University of Graz, Austria. From 1992 to 1994 he was with the Institute for Digital Image Processing (DIBAG) of Joanneum Research, in Graz, Austria. Between 1995 and 1999 he worked at the Microwaves and Radar Institute (HR) of the German Aerospace Center (DLR), in Oberpfaffenhofen, Germany. From 1999 to 2000 he was an EU post-doctoral fellow with Applied Electromagnetics (AEL) in St. Andrews, Scotland. Since October 2000 he is again with the Microwaves and Radar Institute (HR) of the German Aerospace Center (DLR). Actually he is a senior scientist leading the Information Retrieval research group at DLR-HR.
His main research interests are in polarimetric and interferometric processing and calibration techniques, polarimetric SAR interferometry, and the quantitative parameter estimation from SAR data, as well as in SAR mission design and SAR mission performance analysis. He has more than 100 publications in international journals, conferences and workshops. He was awarded with the IEEE GRSS IGARSS Symposium Prize Paper Award in 1998, the Best Paper Award of the European SAR Conference (EUSAR) in 2002 and the DLR science award in 2002. In 2011 he was awarded with DLR's Senior Scientist Award.