This paper examines the accuracy of audio-video based localization using multiple cameras and multi-microphones. Covariance mapping theory is used to determine the accuracy of audio and video based localization. Both modalities are compared in terms of their ability to provide accurate location estimates of a moving audio-visual source. Relatively, video is found to be significantly more accurate than audio. The problem of audio-video fusion is also examined. The fusion of audio and video location estimates is applied in the audio domain, the video domain and the positional domain. The accuracy of these three fusion strategies for 3D localization are examined from a theoretical basis. The best localization performance is found when fusion is applied in the positional domain. Fusing audio and video data in the video domain is found to exhibit the worst localization performance. This analysis is confirmed by measuring the accuracy of each fusion strategy in localizing a moving audio-visual source.