The _____ sketch is useful when no camera is available to document a crime scene.

The perspective sketch is particularly effective for documenting a crime scene when a camera is not available. This type of sketch allows the investigator to represent the scene as it would appear to an observer, depicting the spatial relationships between the various elements. This can include the positioning of objects, the layout of the environment, and specific details that are important for understanding the scene's context.

By using a perspective sketch, officers ensure that critical details are captured in a way that reflects their actual appearance, which can be vital for understanding how events occurred. This method combines both artistic representation and analytical approach, giving a clearer picture than a simple flat or top-down view might provide.

In contrast, the other types of sketches listed serve different purposes: top-down sketches typically provide a bird's-eye view, often used for layout purposes; 3D sketches offer dimensional representations but may not be practical for quick documentation; and digital sketches rely on technology that may not always be accessible on-site. The perspective sketch is thus the most suitable option in situations without photographic documentation.