Caveats in calculating crop specific pixel purity for agricultural monitoring using MODIS time series

Monitoring agriculture at regional to global scales with remote sensing requires the use of sensors that can provide information over large geographic extends with a high revisit frequency. Current sensors satisfying these criteria have, at best, a spatial resolution of the same order of magnitude as the field sizes in most agricultural landscapes. Research has demonstrated that crop specific monitoring is possible with such coarse spatial resolution instruments (such as with MODIS, 250 m at nadir) if a selection purer time series is isolated. To do so, a mask of the target crop is necessary at fine spatial resolution in order to calculate the crop specific pixel purity at the coarser scale. Pixel purity represents the relative contribution of the surface of interest to the signal detected by the remote sensing instrument. A straightforward way to compute pixel purity is to calculate the area of the target crop that falls in the coarse spatial resolution grid. However, the observation footprint is generally much larger than the squared projection of the pixel. Furthermore, the relative contribution within this footprint is not homogeneous and depends on the spatial response of the sensor. This study analyses the consequences of calculating crop specific pixel purity in the straightforward way with respect to using a model of the MODIS spatial response to do so. The effect of whether the MODIS sensor is on-board of the Terra (descending) or Aqua (ascending) platform, and the repercussion on the calculation is also explored. The further deviation that can occur due to the high view zenith angle that can be reached by MODIS is quantified with respect to the spatial response at nadir. Finally, the consequences of underestimating the spatial response when calculating pixel purity is illustrated by analysing the effect on the quality of daily MODIS time series.JRC.H.4-Monitoring Agricultural Resource