## 4 Results
### Behavior of the Landfill at Pixel Level in Both Spatial and Time Domains
The overall heat index (I) of the landfill for the study period is obtained as explained above and shown in Figure 10b. Figure 10: (a) Accumulated heat index (AHI) and (b) equivalent I for the entire landfill by end of 2016. By the end of the study period, we observed that 10% of the landfill was experiencing high temperature, mainly in the southwest of the South Quarry, and 25% of the landfill was showing temperatures three times greater than the standard deviation. As shown in Figure 11a, cells/pixels with I = 5 tend to exhibit increasing behavior during the period of study. The pixels in Figure 11b with I = 4 show the same increasing trend at lower levels of standard deviation. As indicated in Figure 11a,b, the heat elevation in this area of the landfill is consistent for many years and is also expanding to neighboring areas. The other I (1-3) indices show different trends, with some pixels showing an increasing trend and others a decreasing trend for the same index. However, all the rising indices reach their peaks beginning in year 2010 and maintain that level until mid-2013. I = 3 in Figure 11c shows the same increasing trend at different levels of standard deviation until 2013, but some pixels continue to increase while others start to decrease. A few I = 2 pixels in Figure 11d continue to rise to nearly two standard deviations and most are above one standard deviation. However, some pixels show decreasing temperatures until 2013 and then start to rise again. Similarly, pixels with the index I = 1 pose no concern as they remain around the mean. We are only concerned with the beginning of the temperature rise at the end of 2013 in Figure 11e. The SSE reported in 2010 can be inferred from the rising temperature indices shown in Figure 11, wherein all pixels/cells reach peak values beginning in 2010, which they retain until the end of 2011, except for I 4 and I 5, which continue to increase rapidly [21; 23]. Following the same reasoning described above, Figure 12a,b shows the thermal status of the landfill between February 2010 and September 2011 (19 months), in which the locations of all pixels that continue to rise in temperature are evident. Figure 12c shows the location and degree of difference over the 19 months. A comparison of Figure 10b, which shows the thermal status of landfill by the end of 2016, with Figure 12b reveals that there is greater heat elevation and that it has continued to spread in the southern part of the landfill. Figure 11: (a,b,c) I (5, 4, 3), respectively, showing an increasing trend; (d) I = 2; (e) I = 1. Figure 13 shows a plot of the X-bar (XBAR) control chart for the AHI matrix (115,306), which contains indices 1-5 for all years of the study (time domain) and for all pixels (spatial domain). The XBAR chart detects violations based on those that exceed \(\pm 3\)\(\sigma\) between the two red lines in Figure 13a,b. We ignore violations below \(-3\)\(\sigma\) as they indicate low temperature, which is not a matter of concern. Figure 13a shows violations in the time domain (observations from January 2000 to December 2016), of which there were many during the period of study. Similarly, Figure 13b shows violations at the pixel level by matching the pixels in Figure 13b with 1 (3-5). The x-axes indicate the pixel locations depicted in Figure 9a.