Cosmicdave wrote regarding using fiducials to measure: If you don’t know how far the module is how can you take accurate measurements, or did they take a tape measure?

The region was mapped out beforehand, often from Lunar Orbiter photos. The stations were preplanned. Thus, one generally can determine how far the camera was from a given feature.

The Apollo 15 Preliminary Science Report shows some of the measurements the fiducial marks were used for. Some captions for photos of the far wall and floor of Hadley Rille:

"Large layered block is 30 m long..."

"Outcrop near center with a dark layer at top is 18 m long..."

"Width of area is approximately 150 m..."

"The largest block is 15 m across..."

The mission video shows where the Scott or Irwin generally were when taking photos. The maps indicate how far the camera was from the rille wall. The focal length of the camera is known. Camera rotation can be estimated. The fiducials help accurately measure the sizes of the objects on the film. From all this one can calculate the sizes of objects on the rille wall.

(If one knows the size of the object, such as the LM, one could calculate the camera's distance from the LM.)