blipsman: They are measuring the land (the actual dimension of plot, etc.) and the topography, or changes in elevation. They need to do this so they know exactly what work needs to be done, make sure they comply with set-back and other zoning considerations, know how much dirt needs to be cleared to level the site, and so on.
new-man2: I was a surveyor for a time.
If you are talking about a road or construction, they are marking off exactly where everything is supposed to go. Both location and elevation. The place where a road is supposed to go is surveyed before any plans are made. This lets them know what space is available, etc to place the road. Then a plan is made that says, “the road should go right here”. The surveyors come back and put in stakes and other markers that let people know, “The center of the road should be here at this height” and other things about where construction should occur. As they go, new surveying is done to make sure everything is where it is supposed to be.
Also, when someone buys a piece of land, they also mark off where the piece of land is that the person is considering buying.
The way all of this is done is by comparing to things that you know where they are. The little tripod thing- it shoots out a little laser light that reflects off a mirror on a stick held by a second person. They find location by determining the angle that the laser light was shot and the time it takes for the light to come back.
The laser shooting thing is called a theodolite. Here is a pic
The mirror thing is called a traverse prism. It is usually on top of a stick held by a person. Here is a pic
If you have any specific questions, please do ask. I’m pretty sure I can answer them. Be happy to tell you about any of the specifics of pretty much anything.
sik-sik-siks: Roads and pipes and things need to be built at certain slopes to allow drainage to happen according to plan. So all the water that builds up on a roadway during a rainfall can roll off the road and into the storm drains where it can be properly carried away. If the road is sloped wrong the water may pool up and cause a serious hazard to traffic. When the road is being made, layers of sand and gravel are laid down and compacted. Surveyors measure the height of the land and the slope and tell the equipment operators where to shave a little off here or add a little there, until the numbers are all correct.
There are markers all over the place, laid down in the past by surveyors for the government, that have known heights above sea level. These are used as a constant reference point to gauge the height of the measuring stick. There is a telescopic device on a tripod that the surveyor looks through and the assistant holds a very tall ruler and stands at the spot where they want to know the height of the ground. The surveyor looks through the telescope and reads the numbers on the ruler and through some simple math using the number obtained from the known marked point, the exact height of that particular location is now known.
Imagine you want to build a curb, sewer, bridge or even just lay down some new pavement. You want to design them accurately so they’re easy to build and match into the existing conditions perfectly. Remember not everything you build starts on a clean plot of land, likely there are lots of man made objects you need to avoid or tie into.
So to get your existing conditions, you need survey. Accuracy of topographic mapping (flying a plane and taking a picture) is accurate depending on the height of the plane, but say +-0.5 feet. That’s usually only precise enough for large scale grading, not bridges, pavement, utility work. So you’ll need higher accuracy. That means boots on the ground and survey usually by GPS. So you’ll have a surveyor with a big rod, the bottom end is a point, the middle has a small computer, and the top has a super accurate GPS. This instrument gives an accuracy of +-0.005 feet in all directions. The surveyor will walk the entire site and mark all objects and a grid for anything in between. The surveyor will see an object, determine what the object is (say a manhole lid) then describe the object by inputting a code into the computer, then physically put the pointy end on the object to measure it’s exact coordinates. Once you get all the coordinates and codes, you send it back to the office to process and produce a map of the site.
As you can imagine this is a laborious process for big sites, but to make an engineer’s work easier and more accurate a good survey is where it all starts.
gloggs: I’m not a surveyor, but when installing large equipment that needs to be precise we do surveys. For us it’s to find out if the equipment is completely level, not just square to the floor. We have also surveyed to check the squareness of a wall, again on a very large scale (tens of metres). Both instances are for when an extremely accurate measurement is required for an extremely large distance. I’d just like to reiterate that I am not a surveyor, I’m just trying to give you an idea bc the thread was empty.