## 3 Results  
### Illumination Condition and Vegetation Indices  
Mean and standard deviation of IC values for the selected images were calculated for each pixel and transformed into an image (Figures 6 and 7). Mean IC values ranged from 0.47 to 0.95, whereas the standard deviation ranged from 0.027 to 0.208. Values for both images were unevenly distributed across the image as a result of the effect of topography and sun-sensor geometry. Pixels with low mean IC values correspond to frequently shadowed areas across the 39 images (dark pixels in Figure 7a). Due to the sun-sensor geometry, these areas are placed obliquely or even opposite to the sun direction and the radiance received by the sensor is lower or null. Pixels with high mean IC values (bright  
Figure 6: Example of computation of descriptive statistics using the ee.Reducer algorithm across the stack of 39 images (mean in the example) (**a**). Each image has three bands IC, Enhanced Vegetation Index (EVI) and Normalized Difference Vegetation Index (NDVI) (**b**). pixels in Figure 7a) are better placed in the sun direction and the radiance received by the sensor is higher. Using the standard deviation as a measure of the magnitude of change in IC for each pixel, we observed that pixels with high mean IC values experience little variation (dark pixels in Figure 7b), compared to those with low mean IC values which experienced greater variation (bright pixels in Figure 7b). The visual effect looking at the images is that, overall, both mean and standard deviation IC images show opposite patterns, except for an area with low-medium mean IC values and low-medium IC variation (green ellipse in Figure 7). This area would correspond to the terrain where the surface is parallel to sunlight beams (Figure 8), therefore, keeping low mean IC values and low variation. Figure 8 shows how mean IC is related to aspect and slope in the terrain. Aspect shows the highest mean IC values in the interval of 80-160\({}^{\circ}\) with maximum values around 120\({}^{\circ}\). The lowest mean IC values are in the interval 320-360\({}^{\circ}\) and 0-40\({}^{\circ}\), with the minimum around 360\({}^{\circ}\) (Figure 8a). Areas with aspect values around 120\({}^{\circ}\) would correspond to areas well oriented to direct sunlight, whereas those around 360\({}^{\circ}\) would be placed opposite to the sun (shadowed). We hypothesize that the areas described as parallel to sunlight beams might be those around 200-280\({}^{\circ}\), where low mean and standard deviation IC values are found. These areas can be spatially located in Figure 2. The slope shows high mean IC values (~0.8) for flat areas (slope = 0\({}^{\circ}\)), and then increasing slope values show a variety of IC depending on how the surface is oriented to the sun (Figure 8b). This effect can be seen easily between the highest mean IC value (0.95) found at 34\({}^{\circ}\) of slope and 119\({}^{\circ}\) of aspect, whereas the lowest mean IC value (0.47) is found at 28\({}^{\circ}\) of slope and 321\({}^{\circ}\) of aspect, both slope values are close, but they have opposite orientations (202\({}^{\circ}\) of difference). Although variations in the IC have proven to be normal as an effect of seasonality and sun-sensor geometry variability across the year, we would expect to find IC variations with a similar degree of change across the whole image. These results suggest that variations depend on terrain conditions (Figure 8). However, if we consider that terrain, topography and cover are constant, the uneven variation in different IC values must be affected by changes in sun-sensor geometry. To investigate the magnitude of these changes and their influence on VI, we computed the correlations between VI-IC. Figure 7: Spatial distribution of the mean illumination condition (**a**) and its standard deviation in the area (**b**). Each pixel shows the result of the 39 selected images. Red and blue circles show areas with opposite patterns. The green ellipses show areas with low mean IC and variation.