Comparison of the Continuity of Vegetation Indices Derived from Landsat 8 OLI and Landsat 7 ETM+ Data among Different Vegetation Types
Landsat 8, the most recently launched satellite of the series, promises to maintain the continuity of Landsat 7. However, in addition to subtle differences in sensor characteristics and vegetation index (VI) generation algorithms, VIs respond differently to the seasonality of the various types of vegetation cover. The purpose of this study was to elucidate the effects of these variations on VIs between Operational Land Imager (OLI) and Enhanced Thematic Mapper Plus (ETM+). Ground spectral data for vegetation were used to simulate the Landsat at-senor broadband reflectance, with consideration of sensor band-pass differences. Three band-geometric VIs (Normalized Difference Vegetation Index (NDVI), Soil-Adjusted Vegetation Index (SAVI), Enhanced Vegetation Index (EVI)) and two band-transformation VIs (Vegetation Index based on the Universal Pattern Decomposition method (VIUPD), Tasseled Cap Transformation Greenness (TCG)) were tested to evaluate the performance of various VI generation algorithms in relation to multi-sensor continuity. Six vegetation types were included to evaluate the continuity in different vegetation types. Four pairs of data during four seasons were selected to evaluate continuity with respect to seasonal variation. The simulated data showed that OLI largely inherits the band-pass characteristics of ETM+. Overall, the continuity of band-transformation derived VIs was higher than band-geometry derived VIs. VI continuity was higher in the three forest types and the shrubs in the relatively rapid growth periods of summer and autumn, but lower for the other two non-forest types (grassland and crops) during the same periods.
Keywords: hyperspectral; vegetation index; continuity; Landsat 7 ETM+; Landsat 8 OLI; Time series
In this study, three band-geometric VIs (NDVI, SAVI, and EVI) and two band-transformation VIs (VIUPD, and TCG) were used to evaluate the continuity of VIs derived from Landsat-8 OLI and Landsat 7 ETM+ data in six different vegetation classes and during four seasons. The results and conclusions of this study could be a salutary reference to studies using VIs derived from both Landsat-8 OLI and Landsat-7 ETM+.
The consistency between the on-orbit ETM+ and OLI, as represented by the VIs (NDVI, SAVI, EVI, VIUPD, and TCG) derived from simulation data, was high (R2 maximum value = 0.9997), which suggests that differences in the sensor characteristics of OLI and ETM+ have little impact on Landsat data continuity. Therefore, OLI largely achieves its Landsat data continuity mission.
The band-transformation-derived VIs VIUPD and TCG, which comprehensively utilize all spectral bands, produced higher R2 values for the relation between OLI and ETM+ compared with the band-geometric-computation-derived VIs, as assessed using various vegetation cover classes. Thus, VIUPD and TCG can be used as prior VIs in time series studies that make use of data from different sensors.
There were different correlational relationships between OLI and ETM+ for the different vegetation cover types in different seasons. VI continuity was higher in the three forest types and the shrubs during the relatively rapid growth periods, i.e., summer and autumn (average R2 was 0.947 for three forest types in summer and autumn, while the average R2 was 0.931 for shrubs in summer and autumn). In contrast, two non-forest types, grassland and crops, had less continuity during the rapid growth periods, when growth was highly variable.