A SEVIRI active fire pixel is confirmed, and no omission or commission error is considered to occur, if a MODIS active fire pixel is found either (i) within a SEVIRI active fire pixel; or (ii) within a 3 \(\times\) 3 window surrounding a SEVIRI active fire pixel. Such a scenario is labelled as (A) in Figure 5. A SEVIRI omission error is considered to occur if no SEVIRI active fire pixel is found at the centre SEVIRI image coordinate containing the MODIS active fire pixel or within the 3 \(\times\) 3 surrounding window. If the centre and the eight surrounding SEVIRI quality flags are all classified as \"non-potential fire pixels\", then the MODIS active fire pixel is considered to be unconditionally missed by SEVIRI. This scenario is identified as (B) in Figure 5. If, however, SEVIRI fails to detect a MODIS active fire pixel (_i.e._, there are no SEVIRI active fire pixels within the 3 \(\times\) 3 surrounding window), and the processing status of any SEVIRI pixel within the 3 \(\times\) 3 window is anything other than a \"non-potential fire pixel\", then the SEVIRI omission error is categorized according to the artefact that may have prevented the detection. For example, SEVIRI omission errors can be attributed to sunlight, vagaries in the background characterization, or clouds, as depicted as (C) in Figure 5. Seasonal SEVIRI omission errors in each ROI are calculated as the ratio of the number of MODIS active fire pixels not adjacent to a SEVIRI active fire pixel _versus_ the total number of MODIS active fire pixels detected in the ROI. For each concurrent observation, a SEVIRI active fire pixel is considered to be a commission error if no MODIS active fire pixels are found within the SEVIRI active fire pixel or within the 3 \(\times\) 3 surrounding window, as illustrated as (D) in Figure 5. Seasonal SEVIRI commission errors are calculated in each ROI as the ratio of the number of SEVIRI active fire pixels detected in isolation (_i.e._, not adjacent to a MODIS active fire pixel) _versus_ the total number of SEVIRI active fire pixels detected in the ROI. ROI. Thus we confirm that the ROIs span a strong geographical gradient of observed fire activity, and we assume that fire behaviour and observation conditions are relatively uniform within each ROI. ### SEVIRI Errors of Omission and Commission  
Seasonal SEVIRI omission rates relative to MODIS increased from a minimum of 25% in ROI 1 to a maximum of 74% in ROI 4 (Table 2). By contrast, SEVIRI commission rates relative to MODIS were less variable, remaining within \(\pm\)8% of an overall commission rate of 24% calculated across all ROIs (Table 2). In the remainder of the results, we examine the design features of the SEVIRI and MODIS active fire detection algorithms--as well as the observation conditions--that affect sensor-to-sensor comparisons and thus the evaluation of the SEVIRI FTA detection algorithm relative to MODIS. \begin{table}
\begin{tabular}{l c c c c c c c} \hline  & \multicolumn{2}{c}{**ROI 1**} & \multicolumn{2}{c}{**ROI 2**} & \multicolumn{2}{c}{**ROI 3**} & \multicolumn{2}{c}{**ROI 4**} \\ \hline  & \multicolumn{2}{c}{\(\phi_{FRP}=0.75\)} & \multicolumn{2}{c}{\(\phi_{FRP}=0.62\)} & \multicolumn{2}{c}{\(\phi_{FRP}=0.50\)} & \multicolumn{2}{c}{\(\phi_{FRP}=0.37\)} \\ \hline  & **SEVIRI** & **MODIS** & **SEVIRI** & **MODIS** & **SEVIRI** & **MODIS** & **SEVIRI** & **MODIS** \\ \hline Sum of FRP (GW) & 256.8 & 341.4 & 164.9 & 267.4 & 72.4 & 145.6 & 34.0 & 93.1 \\ Min. FRP (MW) & 9.1 & 5.1 & 9.3 & 6.0 & 14.8 & 5.0 & 15.2 & 4.7 \\ Median FRP (MW) & 51.4 & 41.8 & 51.1 & 35.4 & 43.1 & 22.2 & 35.6 & 21.3 \\ Mean FRP (MW) & 73.0 & 73.7 & 68.7 & 61.7 & 52.4 & 35.3 & 42.7 & 32.4 \\ Max. FRP (MW) & 467.5 & 1609.3 & 429.1 & 1432.8 & 397.7 & 626.2 & 239.8 & 647.2 \\ \hline \end{tabular}
\end{table} 
Table 1: Summary of concurrent SEVIRI and MODIS active fire pixels detected in each region of interest (ROI) shown in Figure 1a between 1 October 2008 and 30 April 2009 using, respectively the SEVIRI FTA algorithm [34] and the MODIS active fire detection algorithm [38].