this post was submitted on 16 May 2025
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No Stupid Questions

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Today a friend of mine made me realize that roundabouts have "perfect" road markings around them and i started wandering how a machine would go about making a circle without a compass-like system. So i tried google but every result led to either a janky home made compass or some sort of hand-drawing technique. I assume it should be possible to draw a circle knowing the radius in an analog system without a compass but i can't figure out how. Plus i don't know how the physical structure of a roundabout is made to be round and that is even weirder to think about.

TLDR: How are roundabouts and round road markings made?

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[–] [email protected] 6 points 1 day ago* (last edited 6 hours ago) (1 children)

I used to be a surveyor! The tripods have different tools you can put on top.

A ‘level’ is used to look at rod (some distance away) with measurement gradations on it, like a ruler, to add or subtract height from its current position to determine elevation. You can transfer measurements long distances by leap-frogging positions of the level and the rod. If you start or end at a known USGS monument, you can tie into historicity known elevations. This is how elevations were mapped before GPS (but the survey markers are still used today). They have some really fancy auto-levels that read a qr-style barcode and can measure down to very precise heights.

A ‘thedolite’ is a robot-style machine that uses triangulation to determine elevation, distance, and angle. You benchmark it in place so it knows its location, then uses a rod with a prism it can follow. It calculates degrees it turns horizontally and pivots vertically to calculate where you are with the prism. It will automatically guide you to pre-programmed points to lay out very precise locations. Or you can use it to capture really precise points that are in the field. I haven’t been a surveyor in 20years, but they could easily layout points to millimeter accuracy when I was in the game.

We also used lidar scanners to capture ‘as-built’ maps or calculate volumes of material. A lidar scanner shoots out a laser a few 100,000 times a second as the scanner turns on the tripod. When it bounces off an object, it returns a x,y,z coordinate and a color of the object it bounces off of. When you get a few million returned, you get a point cloud that represents the physical area you are mapping. When you move the scanner and repeat the process, you can map out large areas to a pretty high degree of accuracy. We once used a scan of a statue that was built in the 1800s to compare against the weight of construction materials so we could calculate the crane load expected to move it. Pretty cool stuff!

[–] [email protected] 1 points 11 hours ago