Even though the available satellite data were corrected for the effects of aerosols, thick clouds veiling a pixel for 8 days make it impossible to correct the surface reflectance of that pixel. To remove data affected by thick clouds, pixels whose band 3 reflectance was more than 10% were removed as abnormal data.
The MODIS data used here had already been resampled systematically to a uniform spatial resolution (500 m). However, in raw data before geometric conversion, the spatial resolution of a pixel depends on the sensor zenith angle. The larger the sensor zenith angle of a pixel, the lower the ground resolution of that pixel. For this reason, a VI with a broad sensor zenith angle includes a strong neighbor effect. It is essential to consider this effect of sensor zenith angle on the detection of phenological stage. We used the threshold of the sensor zenith angle to remove pixels with lower resolution, which give misleading results in time-series analysis. Because the grid resolution of the digital land-use map is about 1 km, we defined the threshold of the sensor zenith angle as 32.25° (ca. 750 m). Pixels with a sensor zenith angle of more than 32.25° were removed.