Using 3D photogrammetry models to identify wrecks

Tyler recently “re-identified” the Corsair that we dove last October. While we cannot be 100% sure, we are pretty sure based on accident reports that the original team mis-identified it (newer accident reports were released in 2012).

In the search for more clues, Tyler started reviewing the photos I had taken.

The “version” of the Corsair is different between the two accident reports and so is the type of propeller. The original report had the type of aircraft as a F4U-4 and we believe that the right identification involves a FG-1A.

The F4U-4 (“Dash 4”) was the first Corsair to utilize a four bladed prop which would put the blades at 90 degree angles. The FG-1A had a three bladed prop which would indicate a 120 degree angle between the blades.

Photographic Evidence

Tyler and I started pouring through photos that I had taken of the prop to see if we could positively identify whether it was a four prop or three prop plane. Since only one prop remains, it is “tricky” to judge the angles between the mount points when looking at a two-dimensional photograph. Below are several photographs of that area (as you can see, one of them has been color corrected in post processing):

Based on the photos, it is really hard to definitively say whether it is 90 degrees or 120 degrees between the props.

The one important aspect to note is that you can see in a couple of the photographs that the prop is cracked. That makes it hard to tell what the “natural” angle would be. It looks to be pointing slightly downwards which would imply an angle greater than 90 degrees to the mount point on the top.

3D Photogrammetry model

This was an exploratory dive (we had no idea what we were going to find). Therefore, my plan was to photograph whatever we found for further analysis later. The point behind that is that I was not shooting specifically to build a 3D model. While processing photos, I noticed that I might have enough to build a “decent” model of the prop:

The cool thing about a photogrammetry model is that it is 3 dimensional so you can really get a better sense of angles and space.

Here are some screenshots of the prop section of the model:

Based on the pictures and the model, it looks to me like the hub for the prop that is on top is angled slightly to the left (counterclockwise looking towards the engine) and the prop that is in the sand is angled slightly down (clockwise).

If the angles were truly 90 degrees between the blades and the hub up on top is pointing slightly left, then the prop that is attached would be out in the water and not angled down into the sand.

Our conclusion is that this is a three-bladed prop and not four. I don’t think either Tyler or I would be as confident with only two dimension photos.

Video Clip

In addition, I sometimes will put a GoPro on top of my camera housing to document the scene in case my camera malfunctions or I just want to go back and review something. Luckily, I had the GoPro rolling during part of the dive and captured the prop area as well. Below is a 79 second clip of the engine and prop:

From the video, it also looks as if the angle is greater than 90 degrees.

Follow-up and Conclusion

If and when we visit the site again, I will take photos to build a better photogrammetry model and we will find a way to measure the angle between the propeller blades and mount points to be 100% sure.

For now, we believe it is a three bladed prop and supports the evidence that this wreck is is the one piloted by S.E. Brodie (FG-1 #13701).