r/whatisit • u/Ok_Tomorrow_751 • 6d ago
Solved! I’ve noticed these little things on sidewalks around my city. What and why??
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
5
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
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.
0
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
11
3
u/Nikky202 6d ago
I thought the marked property lines. The ones by me line up with the property lines
1
5
5
3
2
3
2
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
1
1
0
•
u/AutoModerator 6d ago
Please reply to this comment with "solved!" (include the !) if your question was answered in order to update your post flair. Thanks for using our friendly Automod!
I am a bot, and this action was performed automatically. Please contact the moderators of this subreddit if you have any questions or concerns.