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Research

Satellite Imagery in the Study and Forecast of Malaria

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Box 1: Satellite Sensors for Monitoring Diseases

Satellite sensor designs are rarely ideal for epidemiological studies because of trade-offs between spectral, spatial and temporal resolution, determined by constraints of the Earth's atmosphere, or the original requirements of commissioning agencies. Passive satellite sensor data (that is, reflections or emissions arising ultimately from the Sun) have been used most commonly for epidemiological studies, and are discussed here, but there is increasing interest in radar satellites with active sensors that can produce images even under cloudy conditions.

Spectral Resolution

Satellite sensors detect reflected sunlight or infrared radiation emitted by all bodies above absolute zero. Data are most readily available in three to seven wavebands or channels in the humanvisible and near-to-thermal infrared part of the electromagnetic spectrum (0.3-14-µm wavelengths).

Spatial Resolution

Earth-observing satellites produce data with spatial resolutions of 1-4 m (Ikonos-2), 10-20 m (Satellite pour l'Observation de la Terre; SPOT), 30-120 m (Landsats 1-5) or 15-60 m (Landsat 7). Images, made up of picture elements or 'pixels' of these sizes, have swath widths of ˜11 km (Ikonos), ˜60 km (SPOT) and 185 km (Landsat). The 'vegetation instrument' on SPOT-4 has a spatial resolution of 1 km and a 2,250-km swath width.

Meteorological satellites have lower spatial resolutions, with pixel sizes down to 1.1 km (National Oceanographic and Atmospheric Administration Advanced Very High Resolution Radiometer; NOAAAVHRR), and a correspondingly wider swath width of ˜2,400 km. Geostationary satellites maintain a constant position relative to the Earth, giving spatial resolutions of 1-8 km (Geostationary Operational Environmental Satellite for the Americas) or 2.5-5 km (Meteosat 4-6 for Europe/Africa) and images of the entire Earth halfdisk.

Temporal Resolution

Satellites with a higher spatial resolution have a repeat frequency of 11 (Ikonos), 16 (Landsat) or 26 (SPOT) days. Orbiting meteorological satellites produce two images per day of the entire Earth's surface, whereas geostationary satellites produce two images per hour to monitor weather systems.

New Satellites and Sensors

New systems promise greater spectral and spatial resolutions, and greater signal stability over time. These include the Moderate Resolution Imaging Spectrometer on the Terra spacecraft (http://terra.nasa.gov), and the Spinning Enhanced Visible and Infrared Imager on the geostationary Meteosat Second Generation (http://www.esa.int).

Images for Epidemiology

Imagery is adversely affected by atmospheric contamination such as clouds and other aerosols. The low repeat frequency of the higher spatial resolution satellites prevents the recording of important seasonal determinants of pathogen transmission rates. In contrast, frequent images from NOAA-AVHRR and Meteosat sensors can be combined to produce relatively cloud-free monthly images generated from the maximum values of each signal recorded during the period (assumed to reflect cloud-free conditions), called maximum value composites.[18] These are of much greater use in studying dynamical epidemiological processes.

Data from each satellite channel may be used directly to describe epidemiological events, or may be processed to produce indices related to ground-based variables such as soil surface temperatures. Commonly used products include the middle infrared band, derived from AVHRR channel 3, and land surface temperature (LST), derived from AVHRR channels 4 and 5, both of which are related to the Earth's surface temperature; the normalized difference vegetation index, derived from AVHRR channels 1 and 2 and related to plant photosynthetic activity; near-surface air temperature, derived from LST and vegetation index measurements; and cold cloud duration from Meteosat, which is correlated with rainfall in convective precipitation systems (all reviewed in refs 18, 45).

Data Processing and Application

Monthly composited imagery often shows strong serial correlations, and therefore data redundancy, which may be overcome in two ways. The data may be subjected either to principal components analysis (PCA) and the resultant significant principal components used in further analyses, or to temporal Fourier analysis that describes natural cycles in terms of annual, bi-annual, tri-annual, and so on, components with longer or shorter periods. Temporal Fourier processing removes data redundancy and produces a set of orthogonal (uncorrelated) outputs while retaining a description of seasonality (which is lost in PCA); this is of vital interest in epidemiology.[26][46][47] Windowed Fourier analysis overcomes the problem of serial changes in the mean and variance of the data. Trends are first removed by taking the difference of the time series from a moving average spanning a number of annual cycles, and this de-trended time series is then Fourier-analysed. To deal with changing variances, a fixed aperture window is moved progressively over the de-trended time series, and only the data within the window are analysed. By comparing such analyses across the entire time series, the periodicities contributing most to changes in the overall variance can be identified (Fig. 4 on page 3).

Composited, multi-temporal and Fourier-processed satellite sensor data may be used to describe epidemiological data statistically using linear and logistic regression techniques[48][49] or various discriminant analysis and maximum likelihood approaches.[26][50] Output maps record the similarity of each pixel to the satellite-determined environmental characteristics in a sample set of sites where the epidemiological situation is well documented (the training set). Such maps can record the predicted suitability of each pixel for the presence of a vector or disease (Fig. 2 on page 2), or quantitative data related to the burden of disease (Fig. 3 on page 2).

Predictive accuracy can be assessed using a contingency table that compares the training set data and the suitability category to which the pixels were assigned. From this is calculated the overall percentage of correct predictions, the percentage of false positives and false negatives (that is, false predictions of presence and absence, respectively), and the sensitivity and specificity (proportion of positives or negatives, respectively, correctly identified). The kappa index of agreement, k, measures predictive accuracy compared with a null model (that is, one with no predictive skill); values vary between 0 (fit no better than random) and 1.0 (perfect fit),[28] with a valuse of more than 0.75 regarded as excellent; confidence intervals can be attached to k values. Once robust and reliable correlations between the satellite and disease data are established, real-time monitoring of environmental conditions by satellites can provide valuable inputs into disease early-warning systems.[16]


Acknowledgements

S.E.R. is currently supported by a NERC Senior Research Fellowship. R.W.S. is supported as a Senior Research Fellow by the Wellcome Trust. S.I.H. is currently supported as an Advanced Training Fellow by the Wellcome Trust. We thank M. Coetzee for supplying geo-referenced observations on the African distribution of the A. gambiae complex and D. Shanks for providing malaria incidence and meteorological data from the Brooke Bond Kericho Tea Estate.

Contact

David J. Rogers
TALA Research Group
Department of Zoology
University of Oxford
South Parks Road
Oxford OX1 3PS, UK
Email: david.rogers@zoo.ox.ac.uk


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