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International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences, Volume XXXIX-B7, 2012 
XXII ISPRS Congress, 25 August — 01 September 2012, Melbourne, Australia 
AIRBORNE X-HH INCIDENCE ANGLE IMPACT ON CANOPY HEIGHT 
RETREIVAL: IMPLICATIONS FOR SPACEBORNE X-HH TANDEM-X GLOBAL 
CANOPY HEIGHT MODEL 
M. Lorraine Tighe“”, Doug King”, Heiko Balzter‘’, Abderrazak Bannari”, Heather McNairn® 
@ Intermap Technologies, Inc., 8310 South Valley Highway, Suite 400, Englewood, CO, 80112-5809 USA 
® Carleton University, Dept. of Geography and Environmental Studies, 1125 Colonel By Dr. Ottawa, Ontario K1S 5B6, Canada 
© University of Leicester, Centre for Landscape and Climate Research, Department of Geography, Leicester LE1 7RH, UK 
« Department of Geography, University of Ottawa, 60 University, Simard 029, Ottawa, ON, Canada KIN 6N5 
© Agriculture and Agri-Food Canada, 960 Carling Avenue, Ottawa Ontario, K1A 0C6 Canada 
Commission VII WG2: SAR Interferometry 
KEY WORDS: Canopy Height, InSAR, X-band, DTM, DSM, DEM 
ABSTRACT: 
To support international climate change mitigation efforts, the United Nations REDD+ initiative (Reducing Emissions from 
Deforestation and Degradation) seeks to reduce land use induced greenhouse gas emissions to the atmosphere. It requires 
independent monitoring of forest cover and forest biomass information in a spatially explicit form. It is widely recognised that 
remote sensing is required to deliver this information. Synthetic Aperture Radar interferometry (InSAR) techniques have gained 
traction in the last decade as a viable technology from which vegetation canopy height and bare earth elevations can be derived. The 
viewing geometry of a SAR sensor is side-looking where the radar pulse is transmitted out to one side of the aircraft or satellite, 
defining an incidence angle (0) range. The incidence angle will change from near-range (NR) to far-range (FR) across of the track of 
the SAR platform. InSAR uses image pairs and thus, contain two set of incidence angles. Changes in the InSAR incidence angles can 
alter the relative contributions from the vegetation canopy and the ground surface and thus, affect the retrieved vegetation canopy 
height. Incidence angle change is less pronounced in spaceborne data than in airborne data and mitigated somewhat when multiple 
InSAR-data takes are combined. This study uses NEXTMap® single- and multi-pass X-band HH polarized InSAR to derive 
vegetation canopy height from the scattering phase centre height (h,,.). Comparisons with in situ vegetation canopy height over three 
test sites (Arizona-1, Minnesota-2); the effect of incidence angle changes across swath on the X-HH InSAR h,,, was examined. 
Results indicate at steep incidence angles (6 = 35), more exposure of lower vegetation canopy structure (e.g. tree trunks) led to 
greater lower canopy double bounce, increased ground scattering, and decreased volume scattering. This resulted in a lower 
scattering phase centre height (h,,.) or a greater underestimation of vegetation canopy height given by the single-pass X-HH InSAR 
data. The opposite effect occurs in the far range (0 = 55 ), an increase in volume scattering resulted in more accurate vegetation 
canopy heights when compared to in situ measurements. These findings indicate that incidence angle corrections should be applied 
to airborne X-HH single-pass InSAR. In contrast, NEXTMap X-HH (multi-pass data) h,,. data experienced little or no effect of 
incidence angle, possibly because NEXTMap is an aggregation of multi-pass flight line strips, which averages data over several 
incidence angles. These results may aid in the understanding of potential incidence angle effects in Astrium spaceborne Tandem-X 
data, which will have global digital surface elevation coverage by 2015. 
1. INTRODUCTION interferometric Synthetic Aperture Radar (cited as IFSAR or 
InSAR in the literature) exhibit frequency-dependent sensitivity 
Mapped estimates of vegetation canopy height of forests are 
relevant to understanding carbon storage and cycling, 
susceptibility to wildfire, changes in vegetation structure from 
disturbance (e.g. insect outbreaks, wildfire, storms, forest 
management practices such as thinning and logging), and 
assessment of biodiversity and wildlife habitat. Furthermore, 
vegetation canopy height is useful in obtaining more accurate 
estimates of aboveground woody biomass and is a key indicator 
of succession status (Balzter et al. 2007a). Knowledge of 
vegetation canopy structure is required for modelling processes 
such as photosynthesis, energy transfer, evapotranspiration, and 
climate change at both local and global scales. Furthermore, 
vegetation canopy height is of great value in many types of 
regional- to global-scale modelling and is an essential precursor 
to many techniques for extracting physical, topographic, and 
cultural data for a plethora of applications. Examples of 
applications include floodplain modelling, geological hazard 
assessment, landslide analysis, urban planning, topographic and 
geologic mapping, biomass studies, and land-fire initiatives, 
amongst others. Digital surface models (DSMs) derived from 
to the height of vegetation canopy elements (e.g. leaves, twigs, 
branches, and tree trunks), and a number of investigators have 
had success in retrieving estimates of canopy height from 
interferometric measurements (Treuhaft and Siqueira, 2000; 
Kellndorfer et al., 2004; Balzter et al., 2007a; 2007b; Walker et 
al., 2007; Sexton et al., 2009). Methods to estimate vegetation 
canopy height from InSAR techniques vary. One approach, 
applied in this study, is to subtract an independent elevation 
measurement of the bare ground surface (e.g. digital terrain 
model — DTM) from the interferometric surface height (e.g. 
digital surface model — DSM) to estimate the scattering phase 
centre height (h,,.; Kellndorfer et al., 2004; Simard et al., 2006; 
Andersen et al., 2008) to yield an estimate of vegetation canopy 
height. While a significant amount of research has been 
published on the application of InSAR for vegetation parameter 
extraction—in particular for vegetation canopy height— 
additional research is needed to gain further understanding of 
the effect of incidence angle changes on InSAR derived 
vegetation canopy height given by h,, and methods must be