Which type of sketch is described as a three-dimensional representation useful when cameras are unavailable?

The correct choice for this question is the perspective sketch. A perspective sketch is designed to depict a scene in a way that simulates how an observer perceives it in three dimensions. This type of sketch captures the relative scale and depth of objects in relation to one another, making it particularly useful for presenting a clear view of a crime scene when photographic equipment is not accessible.

Perspective sketches offer a visual representation that combines the height, width, and depth of objects, allowing investigators and jurors to understand the spatial relationships within a scene. This is essential in criminal investigations as it aids in conveying the scene's layout and the position of evidence or other relevant features in a realistic manner.

In contrast, other types of sketches like projection and schematic sketches may lack the three-dimensional qualities necessary to convey depth and scale effectively. A detailed sketch, while thorough in describing specific details, may not capture the overall three-dimensional appearance of the scene as effectively as a perspective sketch. Therefore, the perspective sketch serves as the best option for creating an accurate and informative representation of a scene in the absence of photographic evidence.