Thierry Ranchin

Synthesis of Multispectral Images to High Spatial Resolution: A Critical Review of Fusion Methods Based on Remote Sensing Physics

Our framework is the synthesis of multispectral images (MS) at higher spatial resolution, which should be as close as possible to those that would have been acquired by the corresponding sensors if they had this high resolution. This synthesis is performed with the help of a high spatial but low spectral resolution image: the panchromatic (Pan) image. The fusion of the Pan and MS images is classically referred as pan-sharpening. A fused product reaches good quality only if the characteristics and differences between input images are taken into account. Dissimilarities existing between these two data sets originate from two causes-different times and different spectral bands of acquisition. Remote sensing physics should be carefully considered while designing the fusion process. Because of the complexity of physics and the large number of unknowns, authors are led to make assumptions to drive their development. Weaknesses and strengths of each reported method are raised and confronted to these physical constraints. The conclusion of this critical survey of literature is that the choice in the assumptions for the development of a method is crucial, with the risk to drastically weaken fusion performance. It is also shown that the Amelioration de la Resolution Spatiale par Injection de Structures concept prevents from introducing spectral distortion into fused products and offers a reliable framework for further developments.

Image Fusion - The ARSIS concept and some successful implementation schemes

This article aims at explaining the ARSIS concept. By fusing two sets of images A and B, one with a high spatial resolution, the other with a low spatial resolution and different spectral bands, the ARSIS concept permits to synthesise the dataset B at the resolution of A that is as close as possible to reality. It is based on the assumption that the missing information is linked to the high frequencies in the sets A and B. It searches a relationship between the high frequencies in the multispectral set B and the set A and models this relationship. The general problem for the synthesis is presented first. The general properties of the fused product are given. Then, the ARSIS concept is discussed. The general scheme for the implementation of a method belonging to this concept is presented. Then, this article intends to help practitioners and researchers to better understand this concept through practical details about implementations. Two Multiscale Models are described as well as two Inter-Band Structure Models (IBSM). They are applied to an Ikonos image as an illustration case. The fused products are assessed by means of a known protocol comprising a series of qualitative and quantitative tests. The products are found of satisfactory quality. This case illustrates the differences existing between the various models, their advantages and limits. Tracks for future improvements are discussed.

The wavelet transform for the analysis of remotely sensed images

The wavelet transform is a mathematical tool allowing an image to be decomposed in terms of its structures and characteristic scales. This transform is reviewed briefly and applied to a remotely sensed image. Perspectives for the analysis and processing of remotely sensed images are presented.

Using Iterated Rational Filter Banks Within the ARSIS Concept for Producing 10 m Landsat Multispectral Images

The ARSIS concept is designed to increase the spatial resolution of an image without modification of its spectral contents, by merging structures extracted from a higher resolution image of the same scene, but in a different spectral band. It makes use of wavelet transforms and multiresolution analysis. It is currently applied in an operational way with dyadic wavelet transforms that limit the merging of images whose ratio of their resolution is a power of 2. Rational discrete wavelet transforms can be approximated numerically by rational filter banks which would enable a more general merging. Indeed, in theory, the ratio of the resolution of the images to merge is a power of a certain family of rational numbers. The aim of this paper is to examine whether the use of those approximations of rational wavelet transforms are efficient within the ARSIS concept. This work relies on a particular case: the merging of a 10 m SPOT Panchromatic image and a 30 m Landsat Thematic Mapper multispectral image to synthesize 10m multispectral image TM-HR.

Benefit of the future SPOT-5 and of data fusion to urban roads mapping

This article deals with the contribution of both the future SPOT-5 (which will produce images with the same bands as the existing SPOT 1-3 ones but with an improved spatial resolution) and a sensor fusion method to urban mapping. The ARSIS concept (in French: Amelioration de la Resolution Spatiale par Injection de Structures) is used for sensor fusion. It allows the improvement of spatial resolution of the multi-band images, while preserving spectral information, by use of the high frequencies of panchromatic images. A well-proven method for urban mapping is then applied to all multi-spectral images available in the context of the study. A photo-interpretation of the latter confirms the benefit of fine image resolutions to urban roads mapping, in the limit of the sensor studied here. Then, when comparing the roads surface at all resolutions with reference extracted from accurate maps of the city, we demonstrate quantitatively that the finer the resolution, the more accurate the cartography.

Combined extraction of high spatial resolution wind speed and wind direction from SAR images: A new approach using wavelet transform

Low spatial resolution wind extraction from scatterometer data has reached an operational status and the results are widely used in global-scale meteorological models. A new need is emerging concerning high spatial resolution wind fields extraction for the offshore industry. This need can be managed using high spatial resolution data, such as those from synthetic aperture radar (SAR) sensors. This paper presents a combined wind parameters extraction algorithm at high spatial resolution from a single SAR image. This algorithm makes use of advanced signal-processing tools. The results are compared with in situ data and future prospects are presented.

Detection and extraction of road networks from high resolution satellite images

This article addresses the problem of road extraction from new high resolution satellite images. The proposed algorithm is divided in two sequential modules : a topologically correct graph of the road network is first extracted, and roads are then extracted as surface elements. The graph of the network is extracted by a following algorithm which minimizes a cost function. The extraction algorithm makes use of specific active contours (snakes) combined with a multiresolution analysis (MRA) for minimizing the problem of geometric noise. This reconstruction phase is composed of two steps : the extraction of road segments and the extraction of road intersections. Results of the road network extraction are presented in order to illustrate the different steps of the method and future prospects are exposed.

COMMENT LIU "SMOOTHING FILTER-BASED INTENSITY MODULATION: A SPECTRAL PRESERVE IMAGE FUSION TECHNIQUE FOR IMPROVING SPATIAL DETAILS"

We discuss a critical point of the paper. It is shown on the one hand that the author of this paper has mistaken the change of spectral content with the change in spatial resolution, and on the other hand, that the protocol used to establish the advantages of his smoothing filter-based intensity modulation (SFIM) technique over other methods is not appropriate at all. Though the SFIM technique has its merits, the article does not demonstrate its expected qualities.We discuss a critical point of the paper. It is shown on the one hand that the author of this paper has mistaken the change of spectral content with the change in spatial resolution, and on the other hand, that the protocol used to establish the advantages of his smoothing filter-based intensity modulation (SFIM) technique over other methods is not appropriate at all. Though the SFIM technique has its merits, the article does not demonstrate its expected qualities.

The HelioClim-1 Database of Daily Solar Radiation at Earth Surface: An Example of the Benefits of GEOSS Data-CORE

The HelioClim-1 database contains daily values of the solar radiation reaching the ground. This GEOSS (Global Earth Observation System of Systems) Data Collection of Open Resources for Everyone (Data-CORE) covers Europe, Africa and the Atlantic Ocean, from 1985 to 2005. It is freely accessible at no cost through the SoDa Service (www.soda-is.com). Several assessments of the HelioClim-1 data against measurements made in meteorological networks reveal that the HelioClim-1 database offers a reliable and accurate knowledge of the solar radiation and its daily, seasonal and annual variations over recent years. The HelioClim-1 data may help in qualifying in situ measurements and may supplement them, thus offering 21 years of accurate daily means of surface solar irradiance. Several published works benefited from openness, availability and accuracy of the HelioClim-1 database in various domains: oceanography, climate, energy production, life cycle analysis, agriculture, forestry, architecture, health and air quality. This demonstration of the benefit of the HelioClim-1 database draws attention to resources open to everyone such as those labeled GEOSS Data-CORE.

Benefit of GEOSS Interoperability in Assessment of Environmental Impacts Illustrated by the Case of Photovoltaic Systems

Assessment of environmental impacts of a power system exploiting a renewable energy needs a large number of geographically-dependent data and of technological data. These data are located in various sources and available in various formats. To avoid the burden of data collection and reformatting, we exploit the interoperability capabilities set up in GEOSS and combine them with other GEOSS-compliant components proposed by projects funded by the European Commission. This is illustrated by the case of photovoltaic systems. A Web-based tool links the various sources of data and executes several models to offer various impacts factors in different areas: human health, climate change, primary energy, and ecosystems.