Abstract. A new wave retrieval method for the ERS synthetic aperture radar(SAR)wave mode is presented. The new algorithm, named semiparametric retrieval algorithm (SPRA), uses the full nonlinear mapping relations as proposed by Hasselmann and Hasselmann [1991]. It differs from previous retrieval algorithms in that it does not require a priori information on the sea state. Instead, it combines the observed SAR spectrum with the collocated wind vector from the ERS scatterometer to make an estimate of the wind sea spectrum. The residual signal in the SAR spectrum is interpreted as swell. The method has been validated by collocating over 5 years of SAR wave mode observations with spectral buoy measurements at 11 locations on the open ocean. For wave components longer than 225 m, the standard deviation between the retrieved spectra and buoy observations is 0.41 m, which corresponds to a relative RMS error of 29%. About 10% of the observed SAR spectra were rejected, in particular in light wind conditions when nonwave features such as those caused by slicks dominated the imagette. The bias and scatter in the results obtained under light wind conditions could be reduced by introducing a wind-dependent tilt modulation. This wind-dependent tilt formulation is derived from the empirical CMOD4 relation between the wind vector, the incidence angle, and the radar backscatter for the ERS scatterometer. BackgroundIn this paper we present a method that extracts spectral wave data from ERS synthetic aperture radar (SAR) observations. The ERS-1 and ERS-2 satellites (launched in 1991 and 1995, respectively) have gathered a few million SAR spectra over the open ocean. The retrieval algorithm described here was developed to exploit this unique data set, and use it to augment the description of the wave climate in remote areas.The way in which an ocean wave spectrum is mapped onto a SAR spectrum is reasonably well understood. The method described in this paper does not require a priori knowledge of the sea state to retrieve an ocean wave spectrum from a SAR observation. Instead, the fact that on the ERS satellites the scatterometer is operated simultaneously with the SAR wave mode is ex- Hasselmann and

<p><strong>Abstract.</strong> Representation of scenes on the Earth surface by using voxels is gaining attention because of its suitability for integrating heterogeneous data sources in simulations and quantitative models. Computation of shadows in such models is needed, for example, to obtain crop suitability of agricultural fields in the presence of trees and buildings, or to analyze urban heat island causes and effects. We present an efficient algorithm to compute which of the voxels in a dataset receive direct sunlight, given the solar azimuth and elevation angles. The algorithm can work with multiple (sparse and dense) voxel storage strategies.</p>

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Introduction Under the appropriate conditions, satellite or aircraft radar images overshallow seas show features of the seabed. These conditions are a reasonablystrong current, a moderate wind speed, a stable atmosphere and a well-mixedwater column. The underlying mechanism is that depth variations inducevariations in the tidal current, which affect the surface waves and thereforeradar backscatter. Use of radar images to map the seabed topography is nottrivial, as the image brightness pattern depends on current direction, wind-wave direction and the orientation of the topographic features to becharted. ARGOSS has developed techniques for mapping of the seabed that use radar imagesas the primary data source and a limited number of sounding data forcalibration. The most recent version of this mapping software, BAS2D, incorporates a comprehensive model of the physical mechanisms involved. Potential advantages of the technique are that fewer sounding data are neededto obtain charts with the same degree of detail and accuracy, and possibly alsothat charts can be updated more frequently without requiring more frequentship-borne surveys. The BAS2D software is currently being tested on both satellite imagery andairborne imagery in differnt radar wavelength bands. Tests of the method on L-and C-band aircraft imagery planned for last year were postponed due toproblems with data acquisition, but they will be completed in the course ofthis year. This paper discusses case studies carried out using C-band radar imagery fromthe ERS-2 satellite at two sites in the North Sea. One is the Plaatgat area near a tidal inlet between the islands of Ameland andSchiermonnikoog in the north of the Netherlands, and the other in the FlemishBanks of the coast of Belgium. The morphology and the hydraulic regime of thesesites are very different. The dominant seabed features in the Plaatgat area aredeep curved tidal channels and shoals. BAS2D has been used to produce maps of a3 km × 3 km area with a resolution of 50 m × 50 m which were compared withsoundings along tracks at 200 m intervals from Rijkswaterstaat. The FlemishBanks test area is characterised by large banks almost parallel to thecoastline with on top of these short steep sand waves with wavelengths about200 m. A small area of 1.5 km by 1.5 km of the Kwintebank was charted with aresolution of 25 m × 25 m using a SAR image as well as sounding data from WWK(Administration of Water Infrastructure and Sea Affairs) in Belgium. Methods Under favourable meteorological and hydrodynamic conditions (moderate winds of3 to 10 m/s, a neutrally stable atmosphere and moderate tidal currents of about0.5 m/s), Synthetic Aperture Radar (SAR) imagery shows features of the bottomtopography of shallow seas (Alpers and Hennings 1984).

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