Record battlefields with photogrammetry-Memory of war to the next generation-August 15, 17:28
The Ogasawara Islands are known around the world for their clear blue waters, where a wide variety of creatures live.
Although it is registered as a World Natural Heritage Site, in fact, there are "battlefields" that convey the memory of the war, such as the ships of the former Japanese army and Zero Fighters, left behind the beautiful nature.
However, many of them have deteriorated severely due to the wind and rain for many years and are about to decay.
This time, NHK's underwater reporting team has begun an initiative to preserve these battlefields for future generations using a method called photogrammetry, which creates 3D CG models based on the footage.
* The 3D model that can actually be manipulated is in the middle of the article.
(Video center cameraman Shinya Yokoyama)
There is a sunken ship at a depth of 40 meters...
The first place we headed for was a world of pure blue water in the ocean about 1km offshore from Anijima.
When I dived to a depth of nearly 40 meters with an underwater camera, a huge ship that had sunk to the bottom of the sea appeared.
The hull was severely damaged, and bombs and other items were left inside the ship.
It is believed to be a civilian ship requisitioned by the former Japanese army to transport goods, but its name has not been specified.
It is a ship that is tentatively called "Fukachin" because it is sunk in a deep place.
↓ It's a video
Battlefields found 75 years after the war
Next, we headed to the mountains of Chichijima.
It takes more than an hour to go on a road without a road.
Because it is located deep in the mountains, I arrived at a cave whose existence was not known until 2020, 75 years after the war.
When you enter the cave, there is a passage that is more than 20 meters long, and there are seven brick stoves in the back.
It seems to have been a kitchen used by the former Japanese army, and the bottles and cooking utensils were left as they were at the time.
↓ It's a video
Many battlefields remain in Ogasawara, where fierce battles took place
Ogasawara was heavily attacked by the American army during the Pacific War, and there are many battlefields that convey the memory of the war.
According to the Ministry of Defense's National Institute for Defense Studies, Chichijima played an important role for the former Japanese military as a relay point for transportation to the front line such as Saipan, so it is believed that it was targeted by the US military.
It is said that nearly 100 ships that were requisitioned from the private sector to carry goods are sunk in the surrounding waters.
Air raids and naval gunfire also took place, killing at least 4,400 people around Chichijima.
3D model of the battlefield with photogrammetry
In order to leave these battlefields of Ogasawara to the next generation, we worked to create a 3D CG model using the latest technology called "photogrammetry".
↓ It's a video
"Photogrammetry" is a method of creating a detailed 3D model by cutting out images taken from various angles with a high-definition camera and stitching them together.
It is attracting attention in the field of underwater archeology.
This time, we used a 6K camera to record the remains of the battle on land and sea in Ogasawara, and created a 3D model.
The 3D model faithfully reproduces the entire battlefield down to the smallest details, and you can freely manipulate the location, angle, and size you want to see.
↓ It's a video
I want to leave the battlefield before it rots away
What inspired me to create a 3D model was something I heard from a local diver.
Mr. Nobutoshi Kasai, who has been introducing tourists to the battlefields of Ogasawara for nearly 40 years and conveying the history of the war, says, "I want to at least clarify the name of the ship for the sake of those who died in this sea." told us.
A full-scale survey has not been conducted on the civilian ships that were requisitioned to sink in the Ogasawara sea, and little is known about the details, such as the name of the sunken ship.
Under these circumstances, Mr. Kasai formed an investigation team with his friends three years ago in 2019, dived into the sea of Ogasawara, took pictures of the battlefield, measured dimensions, and independently investigated.
Mr. Kasai has a growing sense of crisis over the fact that many of the battlefields have deteriorated severely and are on the verge of decaying.
Mr. Nobutoshi Kasai, who has investigated the battlefields of the Ogasawara Islands,
"If things continue like this, they will continue to crumble and become piles of iron. I think this is the last time we will be able to leave evidence of what kind of ship there was. When I think about the people who lost their lives on this ship, I think it should be like this forever."
The intensity of battle that can be read from photogrammetry
What can be learned from the 3D model created by photogrammetry?
We asked Tatsushi Saito of the National Institute for Defense Studies of the Ministry of Defense to analyze a 3D model of a kitchen that was discovered for the first time 75 years after the war (2020).
Mr. Saito analyzed the 3D model at various sizes and angles, and pointed out that the thickness of the cave walls and the structure of the curved passages could tell the intensity of the battle on Chichijima.
Mr. Tatsushi Saito, Military History Research Center, National Institute for Defense Studies : "First of all, this is the
entrance
. You can tell by looking at the thickness of the concrete here, but even if you get hit by a direct hit from a naval gunfire, this place can withstand.
(Because it is bent), when a bomb explodes nearby, the blast wave will not enter.With so many things in the cave, it is quite dangerous to go outside. You can easily imagine what happened."
Photogrammetry is effective for recording and analyzing battlefields at sea
Some experts point out that 'photogrammetry' is a more effective way to record and analyze the remains of battles sunk into the sea.
This is a team led by Professor Hironobu Suga of Kyushu University, who is researching the remains of battles on the seafloor.
It usually takes about 10 minutes to stay underwater at a depth of 40 meters, and it takes a huge amount of time and money to take detailed records. Because it can be recorded in time.
We asked Professor Suga's research team to analyze the 3D model of the "Shenzhen".
The research team focused on the status of the ship's cargo.
When I magnified and looked at the pointed out place, I found that a large amount of supplies such as bombs and building materials were left behind.
Mr. Shin Yoshizaki, Research Team:
“There are quite a lot of them.
In addition, looking at materials such as battle records of the former Japanese army, the name of the ship that sank at this place was written without unloading the cargo.
The ship's name is the Osaka merchant ship "Shima Maru", which was requisitioned by the Japanese army and where two of her crew were killed.
Based on his own research, Mr. Kasai also thought that the deep sink might be the Shima Maru, but an expert's analysis further increased that possibility.
The research team believes that if more detailed materials are found in the future, it will be possible to identify the name of the ship by matching it with the 3D model.
Professor Hironobu Suga, Kyushu University
“We have to uncover various facts from these battlefields. I think that's the thing."
“A tombstone and a mark for bereaved families”
I was able to talk to the bereaved family who lost their grandfather on a civilian ship that sank around the same time as the Shima Maru.
Toshinobu Kakizawa's grandfather, who lives in Kyoto, was the captain of the Enju Maru, which was drafted into service by the former Japanese army.
Since he died at sea, there are no remains of him, and the only belongings he has are a photograph and a medal given to him after his death.
When Mr. Kakizawa looked at the 3D model of "Shenzhen", he said that photogrammetry would be meaningful for many bereaved families.
Toshinobu Kakizawa, who lost his grandfather on a requisitioned ship:
“There are probably people around this ship who died just like my grandfather.
In that case, at least (if you have a 3D model), I think you'll be able to recognize again how he died here.It's not a grave marker, but it's a mark.
Feel the reality of the fact that there was a war in Japan
This time, we dived 22 times and photographed the battlefields.
He also recorded the remains of the battle on the island.
I was surprised at the fact that the battlefields, which still tell us that there were victims of war, are now decaying without being investigated in detail.
By preserving the battlefields with photogrammetry, I would like you to feel and think about the fact that there was actually a war in Japan in the past.
Imaging Center Cameraman
Shinya Yokoyama
Joined in 2006
After working at Oita and Okinawa stations, he is currently a member of the diving team, covering the sea and coral reef ecosystems of the disaster area.