Found the Reiganjima Water Level Observatory!

I was interested in the reference point for the elevation of Japan, so I went to see the Reiganjima Water Level Observatory. I was also able to stop by the ruins of Ino Tadataka’s residence, so it was a fun walk for map lovers.

Ino Tadataka Residence Site

The other day, after taking a walk along the Nihombashi River from Monzennakacho Station, I was looking at a map and discovered that the site of Tadataka Ino’s residence was near Monzennakacho Station. I thought I should have stopped by.

This time, when I looked up nearby stations to visit the Reiganjima Water Level Observatory, Tsukishima Station, Monzennakacho Station, and Tokyo Station came up as candidates, so I chose the route from Monzennakacho Station to Tokyo Station without hesitation.

Stone monument of the former residence of Ino Tadataka

First, I exit Monzennakacho Station. I took a different exit from the other day. Whenever I exit a subway station, it usually takes me a while to figure out where I am going. This time, too, it was hard to figure out. But when I saw the highway, I knew it was to the north. I walked along a relatively wide street and found a stone monument in front of an eight-story building that indicated the site of Ino Tadataka’s residence.

Next to the building is Menya Endo. It must be a popular restaurant, because people were lined up in a row, and I felt some stares as I took pictures.

Tokyo University of Marine Science and Technology

第一観測台（東京商船大学旧天体観測所）

I returned to Monzennakacho Station and headed southwest again. Crossing the Kurofune Bridge, I reached Etchujima. Shortly after, I saw the first observatory (the former astronomical observatory of the Tokyo University of Mercantile Marine) on my left.

The building was built in 1903, and there was an explanation that it had the best astronomical telescope in the world at that time. According to the website of the Tokyo University of Marine Science and Technology, the hemispherical dome on the roof was manually rotated 360 degrees. Since it is written in the past tense, I guess it doesn’t work anymore.

A hemispherical dome like this one is a common feature of astronomical observatories. Before I entered elementary school, I had been taken to the Goto Planetarium in Shibuya, and I thought that there was a planetarium inside this hemispherical dome, and that it could also show movies. However, when the NAOJ Mitaka Campus opened its doors to the public, I found out that there was only a telescope inside the dome, and I finally had an accurate knowledge.

Nowadays, the area around the first observatory is so thickly covered with trees that even with a telescope, you cannot see the sky.

The walls of the plaster section are somewhat unnaturally dirty. I wonder if this was to keep it inconspicuous and prevent it from being targeted by airplanes during the war.

Meiji Maru, a national important cultural property

About 100 meters from the first observation platform, I saw the Meiji Maru. This was the first time I learned about the name and existence of the Meiji Maru.

It is said that the Meiji Maru was the ship that the Emperor of Japan boarded when he went on a pilgrimage from Aomori to Yokohama via Hakodate, and that July 20, the day he arrived in Yokohama, became Marine Day. So it was a ship that created history and a holiday.

The First Observation Deck and the Meiji Maru are located on the grounds of the Tokyo University of Marine Science and Technology. There are many other interesting things to see, and I’m sure they’ll let us visit them, so I’ll come back again when the time comes.

Tsukishima

Aioi Bridge

After passing the Tokyo University of Marine Science and Technology, the Aioi Bridge appears. The truss bridge itself is not unusual. But it is unusual for the entrance and exit to be at an angle to the road. It looks like a parallelogram in the aerial photo.

Crossing the Aioi Bridge, I arrived at Tsukishima. It is a place where monja is delicious. But before the stores appear, turn right and cross the Chuo Ohashi Bridge. I crossed the Chuo Ohashi Bridge on the right (east) side of the sidewalk. But if you want to see the Reiganjima Water Level Observatory, I recommend crossing on the left (west) side.

After crossing the Central Bridge, I finally reached Reiganjima. There is a path that goes under the bridge, and I was able to get to the water level observatory side immediately.

Reiganjima Water Level Observatory

Reiganjima Water Level Observatory

I found the Reiganjima water level observatory! Here, they observe the water level, so I think it’s basic to take pictures with the water surface. However, since the observatory is located near the corner of the land and there is a railing in the foreground, it is difficult to get a good angle.

The triangular frame is an image of an inverted triangle indicating height, with the bottom vertex pointing to A.P. 0m (Arakawa Peil 0m = reference height of Arakawa River = sea floor). Based on observations from the 6th year of Meiji to the 12th year of Meiji, the Tokyo Bay Mean Sea Level (T.P.0m) was determined to be 1.1344m as the height from A.P.0m. That’s about the same depth as a swimming pool. It is surprisingly shallow.

Huh? Why is it based on the Arakawa River? This is the Sumida River, right?
The answer is that this used to be the mainstream of the Arakawa River.

Huh? If you look at the map, you’ll see that the Reigishijima Water Level Observatory is still in the middle of the Sumida River, right? Can we say that it is the average water level of Tokyo Bay?

The Sumida River flows between Reiganjima and Tsukishima. Tsukishima is an artificial island that was created by land reclamation since 1892. So it must have looked like the ocean until around 1879, when the water level was observed.

Huh? The sea level is always wavy, so how can it be determined to sub-millimeters?

That’s because they observed it many times, every day for 6 years and 3 months. Even if the accuracy of one reading is 10 mm, if it is measured 2,280 times, the accuracy will increase to the reciprocal of the square root of the reading, so it can be read with an accuracy of 10/47.8 = 0.21 [mm].

By the way, I also found the following written on a signboard nearby.

In May 1891, the height of the origin of the Japanese level in Nagata-cho, Chiyoda-ku, was set at 24.5000m, based on T.P. 0m. However, due to the effects of the Great Kanto Earthquake, the height was revised to 24.4140m in 1928, and it is still there today.”
But the current Order for Enforcement of the Survey Act, Article 2, Paragraph 2, Item 2 says, “Origin value: 24.390 meters above the mean sea level of Tokyo Bay. That’s right. The value has been changed since October 21, 2011 due to the effects of the Tohoku Earthquake on March 11, 2011.

There are only 224 days between the earthquake and the change in value. Even if it is observed every day, the accuracy can only be improved to about 1/15. The fact that the number of significant figures seems to be one digit less than when it was revised in 1928 may be due to the fact that there are not enough observation days.

By the way, the elevation of the origin of the Japanese level is getting lower every time there is an earthquake. If this speed continues, the area around the Diet Building will become an ocean in 26,607 years. Komatsu Sakyo’s “Japan Sinks” does not seem to be a joke, although the time span is completely different.

First order bench mark Koumugou

First order bench mark Koumugou

You are tired of all the quibbling. While searching around, I found the first order bench mark “Koumugou”. The description says that the elevation of the Japanese level origin was determined by surveying from Koumugou. However, it doesn’t say how to determine the elevation from the mean sea level (0m above sea level) of Tokyo Bay to Koumugou. I wish it had clearly stated whether the elevation was determined directly from the water level mark at the water level observatory or from the geographic pole level marker stone (the former point of Reiganjima) near the water level mark.

The elevation written on this sign is about 3cm higher than the one you can check on the GSI’s reference point results browsing service.

The topography is constantly changing, and surveying is a never-ending job.

Chuo Ohashi Bridge

When I crossed the Chuo Ohashi Bridge, there were many people on the bridge and buildings in the background, so I couldn’t take a good picture, so I took another one.

Tokyo Station

Kameshima Bridge

I crossed the Kameshima Bridge and left Reiganjima.

On the way to Tokyo Station, I looked for the remains of Utagawa Hiroshige’s residence, as it seemed to be there. But I couldn’t find it this time.

Tokyo Station Yaesu Exit

I arrived at Tokyo Station. It looks like they’ve been under construction for a while now. I wonder if they were putting up these eaves.

The grid lines of the eaves and the edge of the transparent wall that says “JR Tokyo Station” are not parallel, so it looks distorted. Am I the only one who thinks that the design is not very Japanese?

Walking data

Course: Tokyo Metro Tozai Line Monzen-Nakacho Station -> Reiganjima Water Level Observatory -> JR Yamanote Line Tokyo Station
Distance: 5.4km
Time: 1h7m