They’re surveying monuments. Used for horizontal and vertical location. USGS has a database that would contain this monument’s lat/lon SPC etc.

You’ll see them for other buildings, utilities, etc. What Toronto said.

Is there a reason these are branded with Metro specifically?

Illuminati clearly look at that

I don’t think it’s referring to “Metro” like “the Metro”, I think it’s referring to “Metro” like “Metropolitan Police Department” does. It’s referring to the city and their land survey group, not the public transportation.

I don't know, but I found this article on old manhole covers in DC. Some are from the 1800s! Pretty cool.https://ggwash.org/view/62399/theres-history-to-behold-on-some-of-dcs-old-manhole-covers

Literally has information upon it to help you determine what it is

I think they just might be "the Metro" survey pins. It seems the placing agency labels their pins (e.g., USGS) which helps identify the owner.

Metro obviously has critical infrastructure around and in between stations.

It is referring to WMATA Metro.

My guess is because they’re for doing monitoring on/around wmata property. Probably used for some combination for new construction and or vibration/settlement monitoring.

They are indeed survey markers… explore this map, it’s pretty interesting.

https://geodesy.noaa.gov/NGSDataExplorer/

It is indeed referring to "Metro".

So what do they go back and re-measure where they are at some interval?

Yes, the exact location of the marker was recorded when it was put in, the X, Y, and Z (elevation) coordinates. They use it as a control to measure from. You might see things like this, which are temporary measuring targets, periodically. They're really important for having a baseline to go from to figure out if things have shifted/moved over time.

Usually these are used to show changing elevation over the years

Much of the metro’s miles of tunnels were constructed using cut & cover, digging down from the surface, pouring concrete and laying steel, then covering it back up. (Not the deep stations along the redline, those were bored into bedrock). Disrupt that much soil and there will be compaction that occurs years later, and that can damage infrastructure built above. So metro monitors surface elevations around tunnels for that problem.

Survey markers.

Even when very far away from Metro entrances on the surface you are still standing above or near a station, tunnel, or other Metro structure. Metro stations are massive.

The survey marker serves as a verified control point. When on site, it’s the one point you know it’s location to within acceptable tolerances. Everything else on site — everything — is measured trigonometrically relative to that point.

(These are not used anymore because we have GPS now. It provides a virtual survey marker almost anywhere, any time. We are living in the future.)

Correct

A survey datum

You'd have to expand on that. I would say the main use of a survey marker is for a surveyor/contractor/engineer to have as a control point. As mentioned by others it gives you an x,y,z based off of a known datum that you can use as a reference point.

The tree is 100' west of this survey marker and 10' higher. We know that the survey marker is at 10000,10000,100 then the tree would be at 9900,10000,110.

That's an easy example and what I would say is their most common use.

When you reference changing elevation that sounds like a high water mark, which is not what is shown in OPs picture.

https://en.wikipedia.org/wiki/High_water_mark

https://en.wikipedia.org/wiki/Survey_marker

Can confirm, have seen some of the sewer manholes stamped with 1880-90 dates around the Navy Yard area.

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Looks like survey level points.

There's registery that marks the elevation point if these so when someone does a survey they start at a marker and measures tge difference in elevation surrounding areas are compared to it

The A06 on it indicates that this picture was taken near Van Ness.

Can the OP confirm?

We are the control group and the survey is on us 😂

It was right in front of the Zoo entrance on Connecticut Avenue

Does Metro maintain these discs alone, or do they keep their measures in tangent with USGS?

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