Given the substantial costs of installing and maintaining closed-circuit televisions (CCTV), scholarly attention has increasingly focused on the spatial heterogeneity of their effectiveness. However, spatial variations in CCTV impacts have often been neglected in prior research. To address this gap, this study introduces a refined spatial delineation framework that classifies CCTV-influenced areas into three categories: the inside viewshed, directly visible to cameras through line-of-sight analysis and capable of capturing primary crime scenes; the outside viewshed but within coverage range, where the scene is obstructed but nearby activities may still be recorded; and the area beyond CCTV coverage, outside the expected range of CCTV surveillance. This study utilizes CCTV deployment data from Detroit’s Green Light Project from the first half of 2016, along with police-reported crime incidents, to evaluate CCTV effectiveness. Specifically, each CCTV is assessed twice, treating the inside viewshed and the outside viewshed as separate target areas, to examine its impact on reducing all crime, violent crime, and property crime counts. The delineation of the viewshed was based on WorldView-3 remote sensing imagery. Results reveal significant spatial variation in CCTV’s deterrent effects on crime counts, along with evidence of diffusion from the inside viewshed to surrounding areas over time. These findings challenge the prevailing practice of evaluating only the inside viewshed, which may underestimate or misrepresent overall effectiveness. The proposed framework enhances the accuracy of CCTV assessments and offers a novel methodological lens to understand spatially differentiated surveillance effects.