Bidirectional Radiance Data Processing for Forest Canopy BRDF Model Studies
Wang Jindi,Li Xiaowern
Institute of Remote Sensing Application,
Chinese Academy of Sciences,
Beijing 100101, P.R. China
Alan H. Strahler
Certer for Remote Sensing , Boston University, Boston, MA 02215, USA
Abstract
The directional anisotropy of solar radiance reflected from terrestrial surfaces is by now almost as well recognized as spectral variation. Off-nadir measurements of a surface are becoming possible with the development of new satellite sensors ( e.g. HRV, MISR ) and airborne sensors ) e.g. ASAS). This paper presents an the study of modeling the Bidirectional Reflectance Distribution Function ( BRDF) of forest canopy.
With the Geometric – Optical BRDF model of Li and Strahler, biological and structural parameters of forest canopy can be deduced from the multiangle remote sensing data. The ASAS data were obtained from forest remote sensing experiment field in Maine, USA. The ASAS data set consists of 29 spectral bands and 14 viewing zenith angles for the same target. The field measurement data involve the structure parameters of crowns which were obtained at the same time. The data processing includes: 1) Data compression and color image displays for 14 different viewing angles. 2 ) Using the K – L Transformation, the first three principal components of 29 bands were extracted. By statistical analysis, the mean value and variance of the first three principal components on the BRDF principal plane were calculated and plotted. These plots show the “Hotspot” and “Bowl-Shape”, which can be explained by the overlaping function and the mutual shadowing of crowns in the Geometric – Optical model. 3) The data were processed to obtain the image sebset for which synchronous field measurement data were obtained. With the statistics data of these subset image and field measurement data, the model can be verified by running the Monte Carlo simulation program of Geometric-Optical model. As a result, the Geometric – Optical model fits measurement data very well.
Introduction
For remote sensing data application, many kinds of mathematics model are used to describe the relationship between multiangle and multispectrum remote sensing data and the feature of objects, and to estimate the geometric structure and biological parameters of objects.
The bidirectional reflectance distribution function (BRDF) describes the rate of change of the directional radiance of the surface as a function of illuminination zenith angle and azimuth, and the downwelling irradiance. BRDF models are used in the direct mode to simulate reflection and absorption of solar radiation as a function of canopy parameters, and in the inverse mode in order to estimate canopy parameters from measured reflectance data. In the Li-Strahler geometric – optical canopy model, the forest is treated as a collected on regular geometric shapes that cast shadows on a background and are viewed and illuminated from different directions. The directional radiance of the forest is a function of the sizes, shapes, orientations and placement of the objects ( i.e. individual tree crowns ) within the scene [1]. The model has been tested in woodland using Landsat Multispectral Scanner (MSS), Thematic Mapper (TM) , and SPOT High Resolution Visible (HRV) imagery [2]
The ASAS is an airborne, off-nadir-pointing imaging spectroradiometer used to acquire bidirectional radiance data for terrestrial targets. ASAS will permit measurement of radiance at a sufficient variety of angle to estimate the BRDF. ASAS also is being used to develop understanding of fine scale differences between directional reflectance and hemispherical reflectance
[3] As an example the ASAS data demonstrate combined effects of reflectance anisotropy and increased atmospheric path length on off-nadir observation, the result o these effects is a variation in vegetation indices as a function of view direction [4].
The objective of the study was to estimate the BRDF on principal plane from the processed ASAS images, and to test the geometric – optical canopy model.
ASAS data processing
ASAS employs detector array technology to acquire digital image data for visible and near – infrared spectral bands with a spectral resolution of 15nm. The spectrum range is 455nm—873nm. ASAS is able to track and image a target site through a sequence of fore –to-aft view angle which is up to 45 degree either fore or aft of the nadir view. And its spatial resolution at nadir was approximately 7.0 meter in the along-track, and 5.0 meter in the cross-track when the height of flight is 5,000 meter.
The ASAS data processed in this study were obtained from forest remote sensing experiment field in Maine, USA on Sept. 8, 1990. When the data acquiring, the solzr zenith angle is near +45 degree. The data consist of 7 images with different viewing direction on the principal plane and other 7 images on the cross plane. The viewing angle in each plan are from – 45 degree to + 45 degree with the incremence 15 degree. The original data were recorded in 12 bits, And after radiometric calibration the data were stored as a 16 bits interge in digital image files.
1 Data compression and color image display
In order to know the quality of the total 29 bands images, the 16 bits original data were compressed to 8 bits. The compression ratio depends on the statistical parameters of each band. Then the data can be displayed. By the display images, most bands of data are of high ratio of signal to noise and can be used to estimate the reflectance properties of the target.
Furthermore we take three bands of every image as a color combination image to display. The band No. 8(560nm), band No. 15(650nm) and band No.20(720nm) are coded as blue, green and red respectively to make a color combination image. In order to give a better color image and remain their different reflectance as well, we use the same compression ratio and enhancement method to process every color image. So the relative variation of brightness in these images can e seen clearly. When the viewing angle is equal to +45 degree, the image looks bright. As the viewing direction approach to the nadir, images look clearly and the average brightness of image decreases. The viewing zenith angle is equal to – 45 degree. These color images show that the reflectance of object is changing with the reflectance of object is changing with the viewing direction.
