I have a project to do next week that requires getting the height of threee exhaust stacks above finished floor. Usually not a problem if I had access to the stacks, in this case I don't. My plan was to setup the TS (old TS, reflectorless shot is not an option)over a nail tie in another nail, turm my horizontal angle to center of stack and zenith to the top of each stack. Set up over my other nail BS to previous setup and repeat procedure. Any other ideas?
Cy
Sounds like it should work if the geometry is reasonable.
You won't sight the center of the stack, I hope, since that is probably a wide target. I'd sight both edges of the top and average the angles. Do it with lighting conditions as balanced on the sides of the stack as possible.
Doing every measurement Direct and Reverse will be important unless your instrument calibration is perfect in every respect.
If you turn to the center of stack from both perspectives, the horizontal distances derived by triangulation will be longer than the distance to the edge of stack, resulting in a false elevation.
You need to find a distinguishable point on the lip at the top edge of the stack for your pointings.
I have always determined the height difference between by two setup points, and computed the height of structure independently from each setup to get the warm and fuzzies.
Sounds like a good excuse to finally demo a new TS.
What Greg said.
I would only add that if you need to know the center of the stack or the width of the stack-read angles to both edges during each setup and with those you can calulate the center and the width.
How about lean and out of round?
I'm inclined to agree with Mike.
Reflectorless is wonderful and way more useful than you might think.
But don't you only need the horizontal distance to the stack? Then take a zenith angle in both faces to the top. Do that from more than one setup to be sure you get close to the same answer (assuming the top is level).
> But don't you only need the horizontal distance to the stack?
That's why I am turning angles from both points that have a known distance between them. As long as I know one ditance and two angles I can solve for the other sides. As for what to sight on I drove out to the job site today and noticed that all three stacks have a narrow metal attachment at the top. Would love a new TS with reflectorless but the majority of our work is done with via GPS.
Why is the height needed? permitting? new fabrication?
I've not been told. The building is at the airport so i assume it has to do with FAA regs. However it is a paint building and I've been told the EPA requires the stacks to be a certain height. What I don't get is this building and the stacks have been there for four years and this hasn't been taken care of.
Can't you rent a reflectorless T.S?
Why spend the extra money when you have the tools to do the job? I'll admit once at the job site it would be easier and quicker to have a reflectorless but I would lose that time trying to find a local company that rents one out or waiting on one to ship from somewhere else. I was really more interested in knowing if my procedure would work. Thanks everyone for all your suggestions!
Cy
Than if cheap is what you want try an abney level, as little as $4 on Ebay.
Do what you are planning from three different stations if possible. With three, you will get results that check each other.
really no need to rent anything.
FWIW,
Before undertaking the work, I would find out the requirement. The EPA might want to know the height above ground of the top of the stack while the FAA is probably more interested in the height of the tallest structure on the stack (e.g. lightning rod). The EPA would be interested in what happens to the materials exhausted from the stack while the FAA is interested in whether the stack would obstruct the airspace.
The FAA document related to airport obstructions is:linked here. I believe the FAA would want the height above ground and also its NAVD 88 height.
You might want to check the NGS site to see whether an obstruction chart survey has already taken place at the airport and the value already determined.
HTH,
DMM
Thanks for the link. We know the elevation of the finished floor (so we just need a height from F.F. to top of stack), this is a site we have been on for several years with multiple buildings and add-ons. The engineer sending me out there wanted me to line up with eack stack shoot the distance to the building, measure zenith, and use the plans to figure the distance from edge of building to stack. I'm not comfortable with that knowing that things aren't always built as planned.
I'll second that.
A while back I was asked to get some bolt hole positions in a factory site and they were unreachable.
My Nikon has some nice software whereby I can sight to the same point from different locations and it works out ENZ for that point.
I had 3 sets legs setup with targets and read from one to other 2.
I then read 3 different sets to bolts (one from each station) and let the software do the rest.
I am not saying the results were that accurate, but the Nikon gave me ENZ diffs of about 1-1.5mm which was more than good enough for the job. I'm not sure you would get that result with reflectorless,maybe could??
I believe the accuracy lies in forced centreing and picking an easily recogniseable, unambiguous postion to read. Face 1 and 2 even with moderrn gear also.