# Correction of Directional Effects in VEGETATION NDVI Time-Series
## 4 Results  
The primary purpose of this work is to describe the methodology developed to remove directional effects from the long-term VEGETATION NDVI time-series rather than present a validation assessment of the BRDF-adjusted NDVI product as this can be found in the Copernicus Global Land Service Quality Assessment report [39] ([[https://land.copernicus.eu/global/sites/cgls.vito.be/files/products/CCLOPS1_QAR_NDVI1](https://land.copernicus.eu/global/sites/cgls.vito.be/files/products/CCLOPS1_QAR_NDVI1) km-V3_I1.10.pdf]([https://land.copernicus.eu/global/sites/cgls.vito.be/files/products/CCLOPS1_QAR_NDVI1](https://land.copernicus.eu/global/sites/cgls.vito.be/files/products/CCLOPS1_QAR_NDVI1) km-V3_I1.10.pdf), accessed on 15 March 2021). This section describes the results of the evaluation of the performance of the algorithm implementation. The methodology described in Section 3 was applied to a selection of land surface reflectances tiles (\(10^{\circ}\times 10^{\circ}\)) acquired with the VEGETATION series of sensors. A subdataset from the Copernicus NDVI 1 km V2 products (MVC NDVI), was prepared for the same study regions to assess the impact of the BRDF adjustment on the NDVI time series. A notable example of directional effects on MVC NDVI time-series is a site located near Bukedea (latitude = 1.25\({}^{\circ}\), longitude = 34.08\({}^{\circ}\)), Uganda. Data for a pixel near the location above is extracted from the MVC NDVI product and the 2019 NDVI profile evolution along with its long-term average (computed from 1998 to 2020) are shown on the top panel of Figure 3. Despite the viewing angle constraints implemented in the synthesis of MVC NDVI product, directional effects on the 2019 NDVI time-series can be identified as saw-like patterns. These latter can seriously hamper the interpretation of observed NDVI temporal changes and derived information. For example, the computation of NDVI anomalies (comparison of current NDVI values to long-term statistics via a z-score) identifies whether vegetation growth conditions are ordinary. In this respect, the bottom panel of Figure 3 reveals the NDVI anomalies for a pixel located near Bukedea where bands denoting favorable (light green), very favorable (dark green), unfavorable (light red) and very unfavorable (dark red) conditions are also displayed. From the bottom panel of Figure 3, it can be seen that the temporal evolution of NDVI anomaly follows abrupt changes with timescales shorter than the characteristic vegetation activity timescales. A specific example is the NDVI anomaly fluctuations between dekad 12 and 19. Within this short period which lasts about two months, the evolution of the NDVI anomaly shows a significant variability. The erratic switching of NDVI anomaly values, as seen in bottom panel of Figure 3, is related to the saw-like patterns in the NDVI time-series acquired with a changing viewing and illumination conditions. Thus, the Bukedea site illustrates the occurrence of directional effects in NDVI time-series (see Figure 3) and allows us to verify whether our methodology can successfully minimize directional effects (short-term variability introduced by changing viewing and illumination conditions) in NDVI time-series. The NDVI profile and anomaly temporal evolution for the Bukedea pixel is shown in the top and bottom panels of Figure 4, respectively. At first glance, the saw-like pattern is not noticeable anymore in the BRDF-adjusted NDVI profile evolution ( top panel of Figure 4). The uncertainties provided in the BRDF-adjusted NDVI dataset allow us to identify less reliable normalized NDVI values. The later can occur during periods of persistent clouds or in the presence of outliers associated with misclassified clouds. The uncertainties on the NDVI are used as a weight in the computation of the long-term NDVI average, allowing down-weighting of the impact of less reliable NDVI values. The uncertainty associated with the current NDVI values is further propagated through the anomaly computations.