J.-P. Muller

Sampling the surface bidirectional reflectance distribution function (BRDF): 1. Evaluation of current and future satellite sensors

Abstract The Bidirectional Reflectance Distribution Function (BRDF) of earth surface materials contains information relating to their physical structure and composition that cannot be inferred from their spectral properties alone. Knowledge of the BRDF is also critical to the accurate retrieval of earth surface albedo, since the BRDF describes the angular distribution of reflected radiation under given illumination conditions. Although the BRDF cannot be measured directly, it can be estimated using models of surface scattering in conjunction with reflectance data acquired at different viewing and illumination angles. The ability of a satellite sensor to characterise the BRDF of any point on the earths surface is therefore dependent on (i) the range of view angles over which it is able to acquire data, (ii) the orbital characteristics of the satellite on which it is mounted, and (iii) the time period over which the data are recorded. This paper explores the BRDF sampling capabilities of several satellite ...

Topographic effects in AVHRR NDVI data

An investigation is made of the errors induced in the normalized difference vegetation index (NDVI) by topographic variation. The Advanced RAdiometric RAy Tracer (ARARAT) is used to simulate AVHRR imagery for various viewing and illumination conditions at 1.1-km and 50-m resolution. Topographic error is calculated as the difference between the variable terrain simulation and a flat plane simulation. Correlation between this error and various topographic factors such as shadowing and adjacement hill illumination are examined. The importance of sky occlusion as a topographic factor is highlighted. While topographic errors of 13.5% are found in the 50-m resolution simulations, these errors are reduced markedly in the 1.1-km dataset to approximately 3%.

Estimating land surface albedo in the HAPEX-Sahel southern super-site: inversion of two BRDF models against multiple angle ASAS images

Abstract The provision of spatially and temporally disaggregated values of albedo is critical to an improved understanding of energy interactions at the Earth surface. Remotely sensed estimates of land surface albedo can best be obtained by inverting models of the Bidirectional Reflectance Distribution Function (BRDF) against bidirectional reflectance factor measurements sampled at different sensor view angles and solar illumination angles. This paper describes the preliminary results obtained using such an approach over the HAPEX-Sahel (Hydrological and Atmospheric Pilot Experiment) southern super-site in Niger. Two BRDF models, one empirical (modified-Walthall) and one semi-empirical (a linear kernel-driven model, employing isotropic, geometric and volume scattering kernels), are inverted analytically against each pixel in a set of co-registered multispectral images acquired by NASAs Advanced Solid-state Array Spectroradiometer (ASAS). The paper describes the methods used to register these images to sub-pixel accuracy, to perform radiometric and first-order atmospheric correction of the data, and to invert the BRDF model against the pre-processed image data to yield spatially referenced estimates of the model parameters and angularly integrated terms related to albedo. Spatial patterns, closely related to variations in land cover type, are clearly evident in these data. It is shown that, in this instance, the simple empirical model provides a better fit to the measured data, particularly at red and near-infrared wavelengths. The poorer performance of the semi-empirical model at this particular study site is discussed in terms of the assumptions that the model makes about energy interaction with the land surface. The impacts of changes in the projected instantaneous field-of-view as a function of sensor view angle and of residual image-to-image mis-registration on the derived BRDF model parameters and estimated albedo values are also examined.

Automatic Seed Point Generation For Stereo Matching And Multi-image Registration

Three interest-operator-based methods are presented for automatic generation of seed points for SPOT satellite imagery, aerial photographs digitized on a kern DSR-11 analytical plotter, and any other imagery with stereo-overlap. Results for the three methods and a comparison of interest operators are also presented. In all cases and in virtually every example the results of the working versions of the seed-generation programs were promising. It is shown that interest-operator-based automatic seed-point generation for virtually any form of imagery is not only feasible, but can also be achievable with some degree of reliability.

Application Of Ray-tracing to Satellite Image Understanding

A Monte Carlo ray-tracing scheme has been developed for the simulation of image formation processes in satellite images. It consists of an object-oriented ray-tracer which includes satellite-specific models for exterior and interior orientation, empirical models of sky radiance and several simple analytical models and functional approximations for Bi-Directional Reflectance Distribution Functions (BRDFs). Preliminary results from this ray-tracer simulation system will be described including a comparisgn with simple shading schemes; a study of the effects of sky radiance and an investigation of the potential role of terrain variations for BRDF measurements using simple specular-diffuse surface reflectance models. The future role of this system for modelling different surface cover types including vegetation canopies at visible, thermal and microwave wavelengths in the Eos era will be discussed.

Real-time Stereo Matching Spot Using Transputer Arrays

The Alvey MMI-137 project is concerned with the development of real-time 2.5D vision systems using transputer arrays. Over the last two years, we have developed an accurate (rms 50m) directly from level 1A data without geocoding or resampling. Results illustrating the accuracy, reliability and sampling density of our stereo matcher will be shown using statistical and dynamic visualisation techniques. A 22 (T414) transputer array, installed at RSRE Malvern and Kilostream-networked to UCL has been used to test architecture issues of real-time performance. Timings on transputer arrays and distributed Sun-3 workstations will be shown. The complete system is currently being implemented on a 32 (T800-4MB) PARSYS supercomputer at UCL.

A new quality validation of global digital elevation models freely available in China

Global Digital Elevation Models (DEMs) are widely used in the study of natural hazards and environmental change on a global scale. This paper focuses on validation of the most widely used and newly released global DEMs (SRTM v4.1, NASA SRTM v3, SRTMX and ASTER GDEM v2) in China. Authors use independent and precise ground GPS observations to assess their absolute accuracies. The SRTMX DEM performs best with a height RMSE of 9.7 m, while the RMSE of the ASTER GDEM2 is slightly better than that of the SRTM v4.1, and SRTM v3 in these regions have the largest RMSE of 16.6 m. However, systematic negative bias still exists in all the global DEMs. Results of the raster-based comparisons between the DEMs are dependent on the knowledge of vegetation type, density and structure to a large extent as well as accurate co-registration. Slope comparisons exhibit a hierarchical slope difference of about 2° between the SRTMX DEM, ASTER GDEM2 and SRTM C-band DEM (v4.1 and v3). This paper provides the first direct evidence and measurement of the product quality of SRTMX and SRTM v3 DEM in China and also offers a benchmark for the future evaluation of following global DEM products.

The ESA globAlbedo project: Algorithm

This paper describes the algorithm underlying the ESA DUE globAlbedo product. The purpose of the project is to produce a global 8-day land surface albedo product with associated uncertainty on a 1 km grid with continuous spatial coverage using data from European sensors. The product covers the period 1999-2011.

Measurement Of Surface Microtopography Using Helicopter-mounted Stereo Film Cameras And Two Stereo Matching Techniques

A common problem in acquiring ground-truth data for use in microwave interaction modeling is the capture of surface roughness data that are both sampled at distances comparable to a fraction of the wavelength and extensive enough to represent the surface statistics in at least one resolution cell of the microwave remote sensor used. A technique has been developed for acquiring the necessary photogrammetric data using twin 70-mm film cameras mounted on a helicopter boom. The apparatus is described, and the accuracy with which ground surface roughness can be characterized using this device is estimated. Both standard and cross-correlation methods were used for data reduction. Stereogrammetry is compared with a completely automated image-matching technique. Dense disparity images were generated from the helicopter stereo pairs. Using interior orientation parameters supplied by the camera manufacturers, and assuming that exterior orientation parameters remain constant between control target and test field photography, an extremely dense DEM (digital elevation model) for a test field has been derived. Results are compared, and accuracy estimates are presented.

An application of stereomatching to the problem of geo-referencing historical air-photos

This paper describes the application of a stereomatching algorithm to the problem of geo-rectifying a set of declassified military air photos over Cat Tien National Park, Vietnam. A disparity model, related to topographic features, is first built and then co-registered to a geo-referenced elevation model to provide the transformation required to correct the air-photos. Accuracy of results is assessed through correlation coefficients between retrieved disparities and known elevations.