When the computing device determines the comparison value is lower than the overlapping threshold value in step S11, that means the camera may have been shifted due to external forces such as the wind or vibration. The computing device then updates the first detection zone with the second detection zone in step S13, and outputs a notification for the display device to display in step S15. On the contrary, when the comparison value is not lower than the overlapping threshold value, that means the camera may not have been shifted, or the amount of the camera being shifted is within an allowable range, the computing device then continues to compare the first detection zone and the second detection zone in step S09.

It is worth noting that, step S13 illustrated in FIG. 4 is performed before step S15, however, step S13 and S15 can be performed simultaneously, or, step S13 can be performed after step S15. The present disclosure is not limited thereto.

In other words, the computing device updating the first detection zone with the second detection zone in step S13 is using the newly acquired second detection zone as the detection zone. The computing device then outputs the notification in step S15 to notify the monitoring center that there might be some abnormal activities with the camera. Accordingly, when the camera is shifted, the detection zone of the image acquiring scene is not affected even when the monitoring personnel doesn't adjust the camera immediately.