Tim J. Malthus

High-spectral resolution data for determining leaf water content

Laboratory measurements of the spectral reflectance of leaves from a variety of species were made over the 400-2500 nm range using a spectroradiomctcr. The relation between spectral reflectance and leaf water content was investigated and the effect of differences in leaf structure were found to be important. The first derivative of the reflectance spectrum, at wavelengths corresponding to the slopes on the edges of the water absorption bands, was found to be highly correlated with leaf water content and insensitive to differences in leaf structure.

Remote sensing of the coastal zone: an overview and priorities for future research

This paper uses the Special Issue of the International Journal of Remote Sensing on Remote Sensing of the Coastal Marine Environment to highlight recent advances in knowledge of remote sensing of the coastal zone and to define a series of priorities where future research into the application should be addressed. Advances were identified in the benefit of high spatial and spectral resolution data and complementary remote sensing techniques (e.g. optical and acoustic, optical and Synthetic Aperture Radar (SAR)). Further benefits are identified in rapid and more frequent data acquisition, faster and more automated processing and a greater sampling intensity over conventional field-based techniques. Issues associated with adoption of remotely sensed data for management are discussed. Research priorities include the need for improved understanding and description of biotope classes and the functional interpretation of biotope maps and continued developments in understanding the radiative transfer properties of coastal environments. New knowledge is required on spatial and temporal variations of water column optical properties and its constituents. Methods for the best approaches to processing hyperspectral data require further investigation, as does the need for further testing of hyperspectral sensors for bottom type discrimination using data obtained at space-borne altitudes. Areas of value which continue to remain poorly investigated include the improvements to be gained from synergistic use of multi-wavelength remote sensing approaches, change detection techniques and multi-temporal comparisons and knowledge-based approaches to improve classification. The importance of specifically dedicated coastal zone sensors is discussed, as is alternative means of deployment (e.g. International Space Station (ISS) and Un-inhabited Aerial Vehicles (UAVs)). The potential role of airborne digital photography for marine mapping is highlighted. The lack of accurate near-shore bathymetric data is identified as a key limitation in the application of geospatial data to coastal environments.

The empirical line method for the atmospheric correction of IKONOS imagery

The empirical line method is an atmospheric correction technique that provides an alternative to radiative transfer modelling approaches. It offers a relatively simple means of surface reflectance calibration, providing that a series of invariant-in-time calibration target measurements are available. This technique has been applied with variable success to both airborne data and coarser spatial resolution satellite sensor data. However, with the advent of higher spatial resolution space-borne sensors there is a need to re-evaluate its potential. The empirical line method was tested for correcting multispectral IKONOS imagery acquired over the tropical island of San Andres, Colombia. The high spatial resolution (4 m) of the data made it possible to identify a number of homogeneous targets with a range of reflectances that were used for the calibration. Coefficients of determination of the prediction equations observed were large, ranging from 0.96-0.99 for each of the four wavebands. An accuracy assessment was performed using a set of independent targets. It demonstrated that the empirical line method can be applied to correct such imagery with accurate results.

Quantitative modeling of inland water quality for high-resolution MSS systems

For a lake system in The Netherlands, Secchi disk transparency, seston dry weight, vertical attenuation coefficients, chlorophyll-a, and phaeopigments were determined. Inherent optical properties were estimated from measured apparent optical properties. Subsurface spectroradiometric measurements and airborne (non)-imaging spectrometric measurements were performed. This made it possible to model the performance of high-resolution multispectral scanning systems. Results of multivariate statistical modeling of water quality parameters versus simulated spectral band ratios for a water quality spectral bandset of the programmable multispectral imager (PMI), also known as the fluorescence line imager (FLI), are presented. >

A Multispectral Canopy LiDAR Demonstrator Project

The first demonstration of a multispectral light detection and ranging (LiDAR) optimized for detailed structure and physiology measurements in forest ecosystems is described. The basic principle is to utilize, in a single instrument, both the capacity of multispectral sensing to measure plant physiology [through normalized difference vegetation index (NDVI) and photochemical reflectance index (PRI)] with the ability of LiDAR to measure vertical structure information and generate “hot spot” (specular) reflectance data independent of solar illumination. A tunable laser operated at four wavelengths (531, 550, 660, and 780 nm) was used to measure profiles of the NDVI and the PRI. Laboratory-based measurements were conducted for live trees, demonstrating that realistic values of the indexes can be measured. A model-based analysis demonstrates that the LiDAR waveforms cannot only capture the tree height information but also picks up the seasonal and vertical variation of NDVI inside the tree canopy.

Airborne remote sensing of macrophytes in Cefni Reservoir, Anglesey, UK

Abstract This paper evaluates the use of Daedalus Airborne Thematic Mapper (ATM) remotely sensed imagery for monitoring the distribution of aquatic macrophyte species in Cefni Reservoir on the Isle of Anglesey. Images acquired by the ATM were compared to spectroradiometric measurements of the reflectance characteristics of the water and selected stands of submersed floating-leaved and emergent macrophytes. The information content of individual and combinations of ATM spectral bands were investigated for mapping macrophyte distributions. Discriminant analysis performed on both data sets indicated that good identification of macrophytes could be achieved by a combination of green, red and near infrared wavebands. A minimum distance supervised classifier using ATM bands 3, 7 and 8 showed separation of the species surveyed. The results indicate that airborne remotely sensed data have good potential for monitoring freshwater macrophyte species.

Hyperspectral discrimination of coral reef benthic communities in the western Caribbean

Determining a subset of wavelengths that best discriminates reef benthic habitats and their associated communities is essential for the development of remote sensing techniques to monitor them. This study measured spectral reflectance from 17 species of western Caribbean reef biota including coral, algae, seagrasses, and sediments, as well as healthy and diseased coral. It sought to extend the spectral library of reef-associated species found in the literature and to test the spectral discrimination of a hierarchy of habitats, community groups, and species. We compared results from hyperspectral reflectance and derivative datasets to those simulated for the three visible multispectral wavebands of the IKONOS sensor. The best discriminating subset of wavelengths was identified by multivariate stepwise selection procedure (discriminant function analysis). Best discrimination at all levels was obtained using the derivative dataset based on 6–15 non-contiguous wavebands depending on the level of the classification, followed by the hyperspectral reflectance dataset which was based on as few as 2–4 non-contiguous wavebands. IKONOS wavebands performed worst. The best discriminating subset of wavelengths in the three classification resolutions, and particularly those of the medium resolution, was in agreement with those identified by Hochberg and Atkinson (2003) and Hochberg et al. (2003) for reef communities worldwide. At all levels of classification, reflectance wavebands selected by the analysis were similar to those reported in recent studies carried out elsewhere, confirming their applicability in different biogeographical regions. However the greater accuracies achieved using the derivative datasets suggests that hyperspectral data is required for the most accurate classification of reef biotic systems.

The effect of spectral bandwidth and positioning on the spectral signature analysis of inland waters

Abstract Airborne and satellite remote sensing systems (PMI/FLI, Landsat-Thematic Mapper, and SPOT-HRV) were used to assess their potential for inland water quality detection and monitoring. Airborne imaging spectrometry was acquired. Simultaneous (sub)surface spectroradiometric and laboratory-based spectrophotometric measurements allowed the description of the underwater lightfield through determination of the inherent and apparent optical properties. Spectral signature simulation of various systems increased understanding of the performances and led to the development of the CAESAR Inland Water Mode spectral bandset. These results apply to the operational use of high resolution airborne systems (CASI) and are relevant for future satellite systems such as MERIS, MODIS, and HIRIS, especially in the area of data reduction through selection of spectral bands and the development of algorithms.

Evaluation of an improved version of SAIL model for simulating bidirectional reflectance of sugar beet canopies

Abstract The processing of remote-sensing data requires simple but accurate models of directional reflectance of the vegetation canopy. In this study, a reflectance model for a homogeneous canopy is evaluated over an extensive set of radiometric measurements performed on sugar beet canopies. The model corresponds to the Scattering by Arbitrary Inclined Leaves (SAIL) model (Verhoef, 1984) in which the term for first order scattering is corrected for hot-spot and leaf specular reflectance. Leaf optical properties are calculated using the PROSPECT model (Jacquemond and Baret, 1990). Experimental data correspond to a two-year experiment and express a large variability of leaf area index, chlorophyll concentration and soil background optical properties. In the first data set, reflectance was measured about midday under vertical viewing in five optical Thematic Mapper bands. In the second data set, both vertical and oblique measurements (zenith angle 45°, four azimuth angles) were performed from sunrise to sunset in the three SPOT bands. Except for leaf cuticle reflectance, structure and optical variables were measured in the field or adjusted to field measurement, independently of reflectance calculations. Although the structure of sugar beet canopies departs strongly from a turbid medium, a good agreement with measurements was obtained in the case of vertical, north and south view directions. However, the model underestimated the measurements close to the hot-spot direction. In the near infrared, there was also some underestimation of canopy reflectance in the opposite direction to the hot-spot. Possible reasons for these differences are discussed.

Macroinvertebrates associated with various aquatic macrophytes in the backwaters and lakes of the upper Clutha Valley, New Zealand

Abstract The invertebrate communities associated with 3 native aquatic macrophytes (Potamogeton cheesemanii, Myriophyllum propinquum and Characeae) and 3 adventive species (Elodea canadensis, Lagarosiphon major, and Ranunculus fluitans) in Lakes Wanaka and Roxburgh and several backwaters of the Upper Clutha River were studied in February‐March 1980. Underwater divers obtained a total of 24 × 1 m2 macrophyte samples, from which the invertebrates were removed. 26 invertebrate taxa were identified, several of which had not previously been recorded from macrophyte communities in New Zealand. The communities were all dominated by the gastropod Potamopyrgus antipodarum, with Physa sp. (Mollusca) or Chironomidae (Diptera) being sub‐dominant. Overall, the mean abundance of invertebrates was 1585 m‐2 , and the mean biomass of invertebrates was 0.994 g m‐2. The Characeae had the highest mean abundance and biomass of invertebrates, and P. cheesemanii the lowest. Significant differences in invertebrate abundance and ...