Cross-sensor compatibility of spectral vegetation indices (VIs) between Suomi National Polar-orbiting Partnership Visible Infrared Imaging Radiometer Suite (VIIRS) and Aqua Moderate Resolution Imaging Spectroradiometer (MODIS) was investigated using their near-coincident observation pairs obtained along overlapped orbital tracks across the globe for the year 2015. The “top-of-atmosphere (TOA)” and “top-of-canopy (TOC)” normalized difference vegetation indices (NDVIs), TOC-enhanced vegetation index (EVI), and TOC two-band EVI (EVI2) were investigated. For all four VIs, VIIRS and MODIS VIs were subject to systematic differences in which VIIRS VIs were higher than their MODIS counterparts. The overall systematic differences and uncertainties (measured as mean differences and root mean square differences, respectively) were small (0.010 to 0.020 VI units and 0.015 to 0.022 VI units, respectively). TOA NDVI cross-sensor differences were neither seasonally nor view zenith angle dependent, whereas TOC NDVI cross-sensor differences slightly varied seasonally, but were not view zenith angle dependent. TOC EVI and TOC EVI2 cross-sensor differences were view zenith angle dependent, where systematic differences increased with increasing view zenith angle and, for large view zenith angles, they were higher during the summer seasons. These results support the normalization of view zenith angles as a required step to extend the MODIS VI record with VIIRS data.
We developed a unique methodology that spectrally translates the enhanced vegetation index (EVI) across sensors for data continuity based on vegetation isoline equations and derived a moderate resolution imaging spectroradiometer (MODIS)-compatible EVI for the visible/infrared imager/radiometer suite (VIIRS) sensor. The derived equation had four coefficients that were a function of soil, canopy, and atmosphere, e.g., soil line slope, leaf area index (LAI), and aerosol optical thickness (AOT). The PROSAIL canopy reflectance and 6S atmospheric models were employed to numerically characterize the MODIS-compatible VIIRS EVI. MODIS-compatible VIIRS EVI values only differed from those of MODIS EVI by, at most, 0.002 EVI units, whereas VIIRS and MODIS EVI values differed by 0.018 EVI units. The derived coefficients were sensitive mainly to LAI and AOT for the full- and a partial-covered canopy, respectively. The MODIS-compatible EVI resulted in a reasonable level of accuracy when the coefficients were fixed at values found via optimization for model-simulated and actual sensor data (83 and 41% reduction in the root mean square error, respectively), demonstrating the potential practical utility of the derived equation. The developed methodology can be used to obtain a spectrally compatible EVI for any pair of sensors in the data continuity context.
Area-averaged vegetation index (VI) depends on spatial resolution and the computational approach used to calculate the VI from the data. Certain data treatments can introduce scaling effects and a systematic bias into datasets gathered from different sensors. This study investigated the mechanisms underlying the scaling effects of a two-band spectral VI defined in terms of the ratio of two linear sums of the red and near-infrared reflectances (a general form of the two-band VI). The general form of the VI model was linearly transformed to yield a common functional VI form that elucidated the nature of the monotonic behavior. An analytic investigation was conducted in which a two-band linear mixture model was assumed. The trends (increasing or decreasing) in the area-averaged VIs could be explained in terms of a single scalar index, ην, which may be expressed in terms of the spectra of the vegetation and nonvegetation endmembers as well as the coefficients unique to each VI. The maximum error bounds on the scaling effects were derived as a function of the endmember spectra and the choice of VI. The validity of the expressions was explored by conducting a set of numerical experiments that focused on the monotonic behavior and trends in several VIs.
Moderate-resolution imaging spectroradiometer (MODIS)-derived vegetation indices (VIs)-the Normalized Difference Vegetation Index (NDVI), the Enhanced Vegetation Index (EVI), and the Land Surface Water Index (LSWI)-are evaluated in terms of their sensitivity to the seasonal greenness pattern and moisture regime of the tropical forests of Hawaii. The annual mean NDVI and EVI signals most rapidly saturated as total annual rainfall increased to a mesic condition, but LSWI was responsive to a much wetter environment. Ecoregional analyses of biweekly VI time series revealed that all three VIs followed the typical pattern of summer-low and winter-high rainfall for dry forests and shrublands. However, NDVI and EVI did not show any significant seasonality of wet forests while LSWI represented a summer-high and winter-low greenness pattern. The three VIs did not respond to the Leaf Area Index (LAI) very well as LAI reached 4, but they sensitively responded to the fraction of photosynthetically active radiation (fPAR) and leaf moisture content. Especially, LSWI responded most sensitively to fPAR and leaf water content in the wet environment, where fPAR and leaf water content were >0.6 and >40%. Greenness seasonality was more strongly represented by LSWI than by NDVI and EVI for all ecoregions considered in the study. In short, it is believed that LSWI is more appropriate for a canopy phenology study than the other two VIs in wet forests of the tropical environment.
Vegetation indices (VIs) are widely used in long-term measurement studies of vegetation changes, including seasonal vegetation activity and interannual vegetation-climate interactions. There is much interest in developing cross-sensor/multi-mission vegetation products that can be extended to future sensors while maintaining continuity with present and past sensors. In this study we investigated multi-sensor spectral bandpass dependencies of the enhanced vegetation index (EVI), a 2-band EVI (EVI2), and the normalized difference vegetation index (NDVI) using spectrally convolved Earth Observing-1 (EO-1) Hyperion satellite images acquired over a range of vegetation conditions. Two types of analysis were carried out, including (1) empirical relationships among sensor reflectances and VIs and (2) decomposition of bandpass contributions to observed cross-sensor VI differences. VI differences were a function of cross-sensor bandpass disparities and the integrative manner in which bandpass differences in red, near-infrared (NIR), and blue reflectances combined to influence a VI. Disparities in blue bandpasses were the primary cause of EVI differences between the Moderate Resolution Imaging Spectroradiometer (MODIS) and other course resolution sensors, including the upcoming Visible Infrared Imager / Radiometer Suite (VIIRS). The highest compatibility was between VIIRS and MODIS EVI2 while AVHRR NDVI and EVI2 were the least compatible to MODIS.
Current earth observing satellite sensors have different temporal, spectral and spatial characteristics that present
problems in the establishment of long term, time series data records. Vegetation indices (VI's) are commonly used in
deriving long term measures of vegetation biophysical properties, which have been shown useful in interannual climate
studies and phenology studies. While significant improvements have been made with new sensors, and algorithms, and
processing methods, backward compatibility of VI's is desired so that the long term record can extend back and utilize
the AVHRR record to 1981. Conversely, any reprocessing of the AVHRR record should consider steps to allow forward
compatibility with newer sensors and products. In this study we evaluated the use of sensor-specific enhanced vegetation
index (EVI) and normalized difference vegetation index (NDVI) data sets, using a time sequence of Hyperion images
over Tapajos National Forest in Brazil over the 2001 and 2002 dry seasons. We computed NDVI, EVI, and a 2-band
version of EVI (EVI2) for different sensor systems (AVHRR, MODIS, VIIRS, SPOT-VGT, and SeaWiFS) and
evaluated their differences and continuity in the characterization of tropical forest phenology. We also analyzed the
influence of different atmosphere correction scenarios to assess noise in the phenology signal. Our analyses show that
EVI2 maintains the desirable properties of increased sensitivity in high biomass forests across all sensor systems
evaluated in this study. We further conclude that EVI2 can be extended to the AVHRR time series record and
compliment that current NDVI time series record.
Long term data records require the effective integration of new sensor technologies and improved algorithms to better characterize global and climate change impacts on ecosystems, while preserving the fundamental attributes of the existing data record. In this study, we investigated key determinants in the spectral translation and extension of MODIS Vegetation Index products across current sensor systems and to the NPOESS (VIIRS) era. We used simulated sensor-specific data sets derived from hyperspectral data using field spectroroadiometers and Hyperion sensors to investigate inter-sensor translation and continuity issues of the normalized difference vegetation index (NDVI) and enhanced vegetation index (EVI). We also investigated the use of data fusion of satellite VI time series with in-situ flux tower time series measurements of photosynthesis, and the use of data fusion with tower-based continuous measures of broadband/hemispherical VI's as possible reference data sets for the inter-calibration of satellite VI time series from different sensor systems. Preliminary comparisons are presented with actual satellite VI measurements from SPOT-VEGETATION, Terra- and Aqua-MODIS, and AVHRR sensors. We found that with a consistent atmosphere correction scheme and a generalized compositing procedure, translation of multi-sensor datasets can be achieved with certain limitations.
Cross calibration of data products across generations of satellite
program is indispensable to facilitate continuous data products by
satellite observations. To investigate long term environmental change
through vegetation monitoring, it is required to use data from
different platforms, e.g., normalized difference vegetation index (NDVI) from
NOAA-AVHRR series with the ones from TERRA- and AQUA-MODIS. In this
context, cross calibration of spectral vegetation index (VI) is an
important factor which determine the accuracy of such changes. In our
previous work, we introduced a way of deriving analytical relationships
between two vegetation indices based on an equation of vegetation
isoline. The functional form of the relationships was found to be a
ratio of polynomials. On the other hand, most of the studies that
investigate relationships of NDVI products between two sensors simply
assumed first- or second-order polynomial to describe the relationships
of the two data products. In this paper, we discuss the relevancy of
using higher-order polynomials by relating those coefficients
implicitly to biophysical parameters, atmospheric properties, and soil
optical properties. The order of polynomials sufficient to approximate
the relationships is clarified from both analytical and numerical point of view by conducting numerical experiments in addition to analytical derivations.
Cross calibration of data products among various satellite sensors has been paid much attention due to ever increasing needs of continuous environmental monitoring by past, current and future sensors. In this context, the studies on the relationships among spectral vegetation indices (VI) of different sensors will put more value on the existing long-term data records, e.g., by the NOAA-AVHRR series of sensors and LANDSAT-MSS, TM, and ETM+ sensors. This study will shade the light on the relationships among vegetation indices in the framework of VI cross calibrations over various optical sensors, including inter-VI relationships of different index formulations. The derivations of equations which describe VI relationships will be introduced based on the recently reported 'vegetation isoline' equations which relate two reflectances sampled at different wavelength regions. A general form of VI relationships will be introduced and then applied to specific cases of two VIs by assuming differences in spectral band-passes and differences in VI formulations to clarify influences of those differences on the inter-VI relationships. The derived expressions imply the necessity of cross calibration activities over various land cover types for the purpose of VI cross-calibrations, which may require major efforts involving both simulation and field activities.
There is increasing interest among the user communities for using satellite data products from multiple sensors for improved environmental monitoring. Spectral vegetation indices (VIs) are one of the more important products in observing spatial and temporal variations of vegetation biophysical properties and photosynthetic activities as well as in biogeochemical cycle modeling. To accomplish this goal, VIs from multiple sensors need to be normalized for differences in sensor characteristics and algorithms. In this study, we evaluated several empirical strategies in cross-calibrating VIs from different sensors for the spectral band pass filter differences. A satellite-borne hyperspectral image was obtained with the Earth Observing-1 (EO-1) Hyperion sensor over a tropical forest-savanna transitional area in South America. The image was first spectrally convolved to simulate Advanced Very High Resolution Radiometer (AVHRR), Moderate Resolution Imaging Spectroradiometer (MODIS), and Landsat-7 Enhanced Thematic Mapper Plus (ETM+) band passes and corrected for atmospheric effects. Data were then extracted from six land cover types with a wide range of biogeophysical conditions and used to empirically derive cross-calibration (translation) equations for the Normalized Difference Vegetation Index (NDVI). The empirical strategies examined included: cross-calibration at the VI level using the NDVI as a predictor variable, cross-calibration at the reflectance level using the reflectance as a predictor variable, and cross-calibration at the reflectance level using the NDVI as a predictor variable. We also examined a two-steps approach in which the cross-calibrations were performed first at the reflectance level and then at the NDVI level. Overall, all of the cross-calibration methods performed well, resulting in root mean square errors less than .05 NDVI units. In nearly all the cases, however, the translations resulted in large residual bias errors with their values reaching .16 units for dark, little or non-vegetated land targets. Depending on cross-calibration methods used, both the magnitudes and directions of bias errors varied significantly. Although the NDVI-based cross-calibration of the NDVI produced the best results with small RMSE values (< .01 unit), there still existed small bias errors. These results indicate that data continuity studies require a theoretical basis in developing a mechanistic understanding of discontinuity and that cross-calibration results need to be evaluated from a real application point of view in order to assess the impact of persistent bias errors and to establish acceptable difference, or error levels in multi-sensor data sets.
Vegetation indices (VI's) are important tools in the seasonal and inter-annual monitoring of the Earth's vegetation. In this study, the vegetation index products from the Moderate Resolution Imaging Spectroradiometer (MODIS) are evaluated over a preliminary set of validation test sites, including a cerrado and rainforest site in Brazil, and two grass/ shrub sites in Arizona and New Mexico, U.S.A. Ground and airborne validation experiments were conducted to assess the performance of the normalized difference vegetation index (NDVI) and enhanced vegetation index (EVI) for vegetation monitoring. Calibrated spectroradiometers were flown for top-of-canopy reflectance retrievals. Vegetation sampling provided the data needed for a biophysical validation of the VI's. Both single-day and 16-day composited MODIS data were processed and corrected for atmosphere at 500m and 1 km resolutions. The MODIS data compared quite well with the validation data with most of the uncertainty associated with the compositing process. Results show the MODIS VI products to offer enhanced sensitivity for land use discrimination and monitoring at both regional and global scales. The EVI was fairly well resistant to residual cloud and aerosol contamination and had a good range of sensitivity over the high biomass, forested areas.
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