1. A surveillance system comprising: a video capturing unit capable of capturing video of an area, the video comprises a prior image and a current image; a detecting unit configured for subdividing each of the prior and current images into a plurality of subdivisions and calculating a contrast value for each subdivision to produce a contrast value matrix associated with each of the prior and current images; a comparing unit configured for comparing each element of the contrast value matrix associated with the current image with a corresponding element of the contrast value matrix associated with the prior image and determining a compared value matrix associated with the current image; a calculating unit configured for weighting the compared value matrix using a preset weight value matrix to determine a location of a moving object in the current image; and a controlling unit configured for adjusting the aim of the video capturing unit to keep the moving object included in a subsequent image of the video based upon the determination of the calculating unit; wherein the detecting unit counts the number of pixels whose values are greater than a predetermined value for each subdivision, and calculates the ratio of the total number of pixels to the counted number for each subdivision, the calculated ratios are the contrast values of the subdivisions.