Evaluating the spatial transferability and temporal repeatability of remote sensing-based lake water quality retrieval algorithms at the European scale:a meta-analysis approach

Many studies have shown the considerable potential for the application of remote-sensing-based methods for deriving estimates of lake water quality. However, the reliable application of these methods across time and space is complicated by the diversity of lake types, sensor configuration, and the multitude of different algorithms proposed. This study tested one operational and 46 empirical algorithms sourced from the peer-reviewed literature that have individually shown potential for estimating lake water quality properties in the form of chlorophyll-a (algal biomass) and Secchi disc depth (SDD) (water transparency) in independent studies. Nearly half (19) of the algorithms were unsuitable for use with the remote-sensing data available for this study. The remaining 28 were assessed using the Terra/Aqua satellite archive to identify the best performing algorithms in terms of accuracy and transferability within the period 2001–2004 in four test lakes, namely Vänern, Vättern, Geneva, and Balaton. These lakes represent the broad continuum of large European lake types, varying in terms of eco-region (latitude/longitude and altitude), morphology, mixing regime, and trophic status. All algorithms were tested for each lake separately and combined to assess the degree of their applicability in ecologically different sites. None of the algorithms assessed in this study exhibited promise when all four lakes were combined into a single data set and most algorithms performed poorly even for specific lake types. A chlorophyll-a retrieval algorithm originally developed for eutrophic lakes showed the most promising results (R2 = 0.59) in oligotrophic lakes. Two SDD retrieval algorithms, one originally developed for turbid lakes and the other for lakes with various characteristics, exhibited promising results in relatively less turbid lakes (R2 = 0.62 and 0.76, respectively). The results presented here highlight the complexity associated with remotely sensed lake water quality estimates and the high degree of uncertainty due to various limitations, including the lake water optical properties and the choice of methods

Perceptual Versus Gaussian Smoothing

This paper brings to the fore a set of masks that are used in combination to simulate the pattern-colour separability function of the human visual system. These kernels give a perceptual degree of smoothing which correspond to measurements estimated from human psychophysical experiments. Wehave found that they are a better approachtoimage smoothing for colour images than the traditional Gaussian masks which are widely practised

Using a Fuzzy Object-Relational Database

The paper presents a fuzzy database management system, and a fuzzy method for dominant colour description of images, on which an image retrieval system is built. The paper shows the suitability of the fuzzy database management system for this kind of applications when the images are characterized by fuzzy data. The synergy of these two introduced components, improves traditional image retrieval systems in three aspects: natural and automatic image description, a natural and easy query language, and high performance in query resolution

A Hierarchical Approach to Fuzzy Segmentation of

In this paper we introduce a methodology for the segmentation of colour images by means of a nested hierarchy of fuzzy partitions. Colour image segmentation attempts to divide the pixels of an image in several homogeneously-coloured and topologically connected groups, called regions. Our methodology deals with the different (but related) aspects of imprecision that are present in this process. First, the concept of homogeneity in a colour space is imprecise, so a measure of distance/similarity between colours is introduced. As a direct consequence, boundaries between regions are imprecise in general, so it is convenient to define regions as fuzzy subsets of items. The proposed distance in a perceptual colour space is employed to calculate fuzzy regions and membership degrees. In addition, fuzzy segmentation can be different depending on the precision level we consider when looking for homogeneity. Starting from an initial fuzzy segmentation, a hierarchical approach, based on a similarity relation between regions, is employed to obtain a nested hierarchy of regions at different precision levels

COLOUR CONSTANCY AND RELATIONAL COLOUR

Four issues concerning colour constancy and relational colour constancy are briefly considered: (1) the equivalence of colour constancy and relational colour constancy; (2) the dependence of relational colour constancy on ratios of cone excitations due to light from different reflecting surfaces, and the association of such ratios with von Kries ’ coefficient rule; (3) the contribution of chromatic edges to colour constancy and relational colour constancy; and (4) the effects of instruction and observer training. It is suggested that cognitive factors affect colour constancy more than relational colour constancy, which may be an inherently more robust phenomenon. *C 1997 Elsevier Science Ltd. All rights reserved. Chromatic induction Colour constancy Cone-excitation ratios von Kries scaling Relational colour constancy In a previous article in this journal, Cornelissen and Brenner (1995) attributed to what has been called “relational colour constancy ” certain properties that may seem at variance with earlier proposals by Craven and Foster (1992), Foster et al. (1992), and Foster and Nascimento (1994). Subsequent discussions between the two groups of authors concentrated on four issues: 1. the equivalence of colour constancy and relational colour constancy; 2. the dependence of relational colour constancy on ratios of cone excitations due to light from different reflecting surfaces, and the association of such ratios with von Kries ’ coefficient rule; 3. the contribution of chromatic edges to colour constancy and relational colour constancy; and 4. the effects of instruction and observer training. The results of these discussions, which have also involved several other contributors, are summarized here since they may be of more general interest