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Optical/laser plumb with plexiglass

Hi guysThanks to those of you who helped me out with a similar question I asked a while back.When plumbing up through penetrations using laser/optical, do you guys fix the plexiglass on the floor above permanently or do you draw round it or something once positioned correctly in order to replace it in the same position when sighting down on it later on for plumbing up to the next floor?Also does the plexiglass technique work when setting out on floor above before slab has been poured.I have done this a long time ago for a couple of floors. And as I remember we just had 4 penetrations and used the plexiglass glass points as resection control.Thanks guys.

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  • Hi James, NO target is permanent, until you are sure it is not moving anymore. As your building grows, the bottom floors will load and the upper will lean and sway. The method you are using without any external off set scale face targets to check your internal targets. Now you must check your lower targets for movement. Marking where the plumb center is on any upper floors then removing it , then placing it back does not mean it has not move. You must re- target a permanent lower target ,that you know has not move to replace , that target again, then move up to the next floor or targets. So what is the distance  range of your laser and the Accuracy Factor, you must stay in that, to set up what floors have a permanent target to extend upward, and these still have to be checked, because your building will move. External Face Targets offset scale checks would help you if they are possible. If your laser has any error the farther you use it ,the more you will be out so Turn it 90, then 180  to make sure it lines up.  Corrections must be made, because your building is going to move. Next how far up are you going. how wide is your building Foot print. I will also give you a link to a Civil Engineering site on Control of Verticality  this  has a lot of good info. Every Building is different,so every one of them is a leaning Curve.

     I have never used resections off of Targets, , can you explain?

     I hope this Helps.



    James all this google book is here ,just scroll up then back down and you will see the whole book, the pages you need.

    • Land Surveyor
      Thanks for your advise guys much appreciated.

      Also thanks Billy for you're info & Links.
      I realise with movement in the concrete that plumbing control & grids will have to be reestablished from the ground up when internal works such as windows are to be set out.

      For the concrete plan of the building I would obviously plumb down to the lower floor to check before establishing the target on floor above.

      My thinking was to do a free station setup resectecting off of the 3 or more plumb targets once setup on he the deck prepour/postpour. Or maybe just setting up over one plumb target and backsighting another? I should be able to see one fixed target on neighbouring building all the way up so can check bearing & coordinates.

      For prepour penetrations Setout, with the tolerance not being as high and to save time, I was going to plumb up and bang nails through the ply deck before plumbing cutouts made for postpour plumbing. Hope this makes sense lol
      • Hi James ,you sound like you have everything in control, I did not mention the nails ' I knew it was covered in both of the links i gave you. if there is no other way, to see your Target  controls, a resection will have to do, but i suggest  doing it manual and solve these triangles, yourself,   Resections programs use best fit, they will give you a closure, but they also places these coordinates as best fit. if every thing is very close , than that is fine. to compare all your calculations, SAS or resection 2 or3 pt's, do a distance ,distance intersection from all your controls, to compare, there is no program adjustment in it  the same with SAS.. hope this helps.  I  prefer everything at Right Angles, If you can offset your controls to do this.

         also thank you, for your thanks,  and you are a very good Surveyor.

  • Land Surveyor
    I used a similar method many years ago. The plexiglass had a cross scored into the surface from edge to edge so that reference marks can be made on the floor so that it can be repositioned later in the same place.

    As for plumbing up, just remember that the decking can be prone to movement and may need to be checked as it is being completed...

    Good luck
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