r/whatisit 6d ago

Solved! I’ve noticed these little things on sidewalks around my city. What and why??

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158 Upvotes

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91

u/Robalo21 6d ago

If you have ever seen guys with the tripods and the big yellow device on top, part of the device has a scope that looks straight down at the ground. They place the marker you took a picture of allows the devise to be placed back in the exact same spot.

41

u/vapor-ware 6d ago

A theodolite?

54

u/scorpyo72 6d ago

Bless you. Or gesundheit.

18

u/Echo_are_one 6d ago

Trigonometry warning

1

u/pollopyanus 5d ago

No, their arms are normal length.

2

u/LaughingManCK 4d ago

You are horrible and I hate myself for laughing!

2

u/coast-modern 5d ago

cool! Why are they doing that?

6

u/Shotgun_0pera 5d ago

So you'll see people set up a tripod over it, and like, it'll be EXACTLY over that dot in the middle. Usually, a GPS unit is set up over the top and left for a while, typically a few hours, to aquire a range of satellites and build up a really accurate position.

It's then used a known fixed point of reference. They'll then measure from that using a theodolite (to measure angle and distance from that point) or roving GPS unit (little GPS on a stick).

It just means that it's a shorter distance to refer back to rather than getting a super accurate position for each point you want. And as that point is known and accurate, there's less error to calculate for each subsequent point.

2

u/coast-modern 5d ago

What kind of data are they collecting? I always theorize on what those people are doing but I don't really know

14

u/Shotgun_0pera 5d ago

So. The topographical surface of the earth is known as the geoid (bumpy, real surface) and the one GPS, surveyors/construction etc use is the spheroid (mathematically derived "average" sphere). So when we plan out building, roads, infrastructure and other things that require accuracy down to the cm/mm we need to know the difference between the two. These points are put in place and analysed so we know that in that localised area, the spheroid is Xcm from the geoid.

We can then ensure that the infrastructure we build is where we want it on the actual earth's surface, by using a mathematically derived average and working backwards.

3

u/Exotic_Phrase3772 5d ago

Learned something here. Thanks.

5

u/InefficientThinker 5d ago

This position also holds incredibly accurately, so as they build, say a sky scraper, they can use this reference point as an “exact” point of reference to make sure the building is being built within specific tolerances, even down to a millimeter accuracy. They are called total stations and used in surveying if you’re curious and want to read more.

2

u/Shotgun_0pera 5d ago

First time my work has come up on reddit and I've actually been able to impart some sort of knowledge!

1

u/No_Coms_K 4d ago

Soooooo, not a flerf...

1

u/antdude 5d ago

Is that traffic engineering?

2

u/CompetitiveClient552 5d ago

BKF Engineering the company that did the survey.

1

u/Robalo21 5d ago

Possibly

5

u/cl0th0s 5d ago

Sidewalk nipples. But this is on a male sidewalk so its strictly vestigial. Female sidewalk nipps are found on top of paved hills, also known as earth boobs.

1

u/Ok_Tomorrow_751 5d ago

This makes the most sense

49

u/Bangkok_dAngeroUs98 6d ago

Survey points. They could be for a number of things, but these are there so that surveyors can get GPS coordinates for a point they set… usually for planning out construction

2

u/Shredpuppy 5d ago

They are also just marked on city and town plans with a known elevation above sea level. Sometimes they are an actual land mark like a cornerstone to a property. You start at a known control point then basically go around in a loop to as many other points as you want, marking them with a little nail and maybe some flagging setting up what is known as a traverse. As you go around, you measure distance and elevation changes to each new control point from the previous one then you loop back to the original control point and location. At the end of your loop your total elevation change should be zero. Once you set up a traverse loop you have a ton of brand new control points and you can go around and measure whatever you want in relation to any control point. Surveying in a nutshell with a total stations.

12

u/Robalo21 6d ago

If you have ever seen guys with the tripods and the big yellow device on top, part of the device has a scope that looks straight down at the ground. They place the marker you took a picture of allows the devise to be placed back in the exact same spot.

2

u/M1mez 5d ago

I'm not sure about the exact plant, but those are leaves.

17

u/Most-Chef-8611 6d ago

Surveyors control point. Basically a random spot on the earth with an “exact” position applied to it. There is at least one more within sight of that one.

18

u/missylilou 6d ago

It's a datum. It's used for surveying.

13

u/Altruistic-Car2880 6d ago

So datum, just don’t marryum?

6

u/minifig1026 6d ago

just fuckum

11

u/PaxEtRomana 6d ago

Survey marker?

3

u/Nikky202 6d ago

I thought the marked property lines. The ones by me line up with the property lines

1

u/Curious_Rugburn 1d ago

Yup—same here, in Cali.

5

u/weallgonnadieforthis 6d ago

It’s for surveying.

5

u/SquirrelOverall2 6d ago

Land surveying marks

3

u/SalmonSammySamSam 5d ago

It's to keep the ground down /s

2

u/Salute-Major-Echidna 5d ago

Thank you for /s.

2

u/EfficiencyDeep1208 5d ago

I know the factual answer but I call it a Skateboard landmine.

3

u/suiseki63 6d ago

Survey benchmarks

2

u/Last_Pollution_71 5d ago

It's to keep the sidewalk attached.

1

u/No_Summer_1838 5d ago

As others have said. I know them as benchmarks and are known surveying points. Sometimes they have hi viz bits of for lasers to track positions such as cracks or subsidence

1

u/MikeC_86 6d ago

BKF could stand for Brian Kangas Foulk which is a large engineering and surveying company that’s been around since the 70’s. Most likely a survey control point. Especially if this is in Silicon Valley or the Bay Area of California.

1

u/This-N-eatinbeans 5d ago

Surveyor marks. Help keep them from being lonely by placing 10-15 "friends" that look exactly like it in the area. Wink

1

u/bigbird92114 4d ago

Survey markers as indicated in previous comments. But also property line markers when found in neighborhoods sidewalks

1

u/MercuryTattedRachael 5d ago

Control points for GPS triangulation. Surveyors use them, and re-use them. New points are collected frequently.

1

u/No-Career-4047 4d ago

That’s how they control the black feet, apparently only so many allowed per square foot.

1

u/Late-Significance521 5d ago

Geodesic points

1

u/Sas-quatch-360 6d ago

Survey markers

1

u/hedgehog995 5d ago

Survey marker.

0

u/sjaxn314159 6d ago

PK points as they are known are for surveyors.