You know the ones I’m talking about. I’m walking to work today and come upon this bright yellow tripod, mounted on top of which is some sort of reflector. No electronics that I can see, just a little reflecting disc. I hesitate to get too close to it for fear that somebody down the street is shooting laser beams at it or something.
Then, two or three blocks further I come across a man peering into a little monocular device thingy mounted on the same sort of bright yellow tripod. I can only assume he is examining the previously mentioned reflector, but why? What is so interestng about that little reflector that he needs to peer at it through a scope three blocks away?
What kind of information can he gather with this setup? The distance between the two tripods? You think it’d be easier and cheaper to use one of those wheel-on-a-stick distance markers and just walk the three blocks. I guess that, with the help of Pythagoras, the surveyor can tell how much higher the refector his than he, but why is this information valuable or desirable?
Let’s say that Joe Surveyor measures, calculates & concludes that 12[sup]th[/sup] street is 1000 feet long between K and M Streets, and that M Street is 3 feet higher than K Street. That comes out to a .17 degree slope. Why is this information useful? Do they just record it and compare it to last year’s measurements? What happens if, ten years ago, M Street was only 2 feet 10 inches higher than K Street- do they draw up plans to repave three city blocks to keep everything properly aligned?
Someone else will answer this better than I can but what they are looking at (through their transit) is the markers they have set up. What they are measuring is angles. (Although the modern, high-tech transits have range-finding as well.)
They are gathering the basic data to do a little trigonometry to accurately locate the point they are surveying. They use a known distance (before range-finding they used chains of known length). They measure the azimuth and elevation, do a little math and – PRESTO! – they know where they are relative to the measured point.
Others who really now can fill in the details or amend this explanation as needed.
I think one of things they measure is the relative heights of the two pieces of land that the tripod and reflector rest on (but that ties in with the angles suggested by pluto).
On a side note I wanted to mention that here on campus the surveying students always measure the same little field I walk past on my way home. Semester after semester, there are new generations of students out there surveying that field. I got to thinking that it has got to be one of the best surveyed places on earth. The probably know every nook and cranny of that bad boy.
Whatever you call it, the guy peering through the thing is sighting on the reflector to measure the precise angle. Yes, the difference in heights is very important. I leave it to the civil engineers in the crowd to explain how important, and how the data is maintained and used.
Woah! They’re looking for meteorites, acid rain and checking to make sure no volcano is going to spring up at the corner of 12[sup]th[/sup] & K? Way to go surveyors! :rolleyes:
Just as I suspected. Probably on my tax dollars, too.
I have always had an interest in this type of instrument and just purchased one on ebay but I am having problems with the seller (but that is another thread).
The original instrumen was called a theodolite. Then, if it could revolve the full circle around the horizontal axis they called it a transit. Then they used the word transit for all or them whether they did revolve all the way or not.
You need surveys so you can plan roads, define property limits etc.
potentially, you could be seeing a surveyor use any of several instruments but your observation of a reflector thingy three blocks away tells us that it was a theodolite with an EDM (electronic distance measuring device). These are highly accurate tools and in this case was probably being used to locate a property line for building purposes or locate utilities.
Elevations are determined with levels which also sit on tripods. There’ll probably be someone nearby holding a level rod (stick with numbers).
Transits and theodolites do the same jobs. Transits have an external verneer for reading angles and some have a compass built in. theodolites are used almost exclusively now. They tend to be more accurate, have an internal verneer and usually an optical plummet or plumb bob for setting up directly over the point on the ground.
As for why you’d need to know the subtle elevation changes while drawing a map, you need them in order to determine the exact horizontal distances you’re interested in. If you’re on a hill and you measure out 300 feet with a tape, and you draw that segment on the map as 300 feet horizontally, you’re wrong.
If you sent two teams of surveyors ignoring elevations from point A to point B through different routes, the maps they come up with wouldn’t have point B in the same spot.