I can figure a string line, but if you are using a 500 foot string line and you are even off by 1/2, that will end up being in a completely different part of town or on someone’s property by the time you are done. Do they just guesstimate and then when they see they are coming off they make curves or adjustments in the road? I know most roads arn’t straight, but the process still fascinates me.
In: 90
Surveying. Seriously, that’s it. It’s just basic geometry. Surveying has been around for thousands of years.
Surveying. Surveyors use special equipment, such as lasers and GPS, to mark out land and make sure the builders are building in the right place.
Long before they build the road a team of surveyors will ensure the path marked out is straight, or is according to the designs. The builders aren’t guessing or making adjustments as they build, they’re following the work of the surveying team.
They use a tool called a theodolite, which is used in land surveying and allows you to measure both horizontal and vertical angels. The Theodolite has a telescope on it that pivots in 3 dimensions, and allows you to measure in very precise amounts the angels being marked. You can also determine elevation with this tool.
When you setup the Theodolite, it comes with a plumb that allows you to level it, then Someone else carries a sighting point (a big stick with a target on it), and the person using the theodolite directs them were to mark off the location on the ground.
Or….today they use GPS.
Surveying tools. Take a spyglass and then put two targets as far as you can see with it and have them stand at a certain distance. Then from that one point you are standing take the angle of the other target from your perspective. Then walk to the other side of the road and take the measurement again.
You now have 2 triangles with which you can get all the dimensions and you can make square with cross distance to ensure you have it straight.
Although nowadays you have a dude with high precision gps receiver who places markers. Follow the markers with sufficient tolerance, check periodically and you will stay straight.
There GPS based marking tools are with in a millimeters. I have had a pleasure of working on a site where the positions of steel structures pillars were given us with this. There were studs on the concrete foundation. They were more accurate than our tape measures.
I see you’ve been told about surveying… As an aside, ideally you don’t build roads straight, at least not for very long. They used to but it’s now considered poor design because without something to keep the driver engaged with their task, people tend to fall asleep or lose focus. This tends to become a real problem at the point where the road finally does do something different (intersections, corners, stopped vehicles, animal on the road etc) because people aren’t paying attention anymore
Since this ELI5, I’ll do it this way….put a telescope on a tripod, put it directly over a nail on the straight line, level it with a bubble, point the telescope directly at another point in the line, now flip the telescope over(it’s on a bar directly through its middle). You can now put another nail in the ground on that line. Move your telescope to that nail, rinse and repeat.
Have you ever watched the movie Cars?
So this is about surveying a straight border not a straight road, but it gets at your question. The tldr is that long straight lines probably aren’t, at least if they were surveyed pre-GPS.
https://bigthink.com/strange-maps/colorado-is-not-a-rectangle/
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