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Elevation Calculations

Discussion in 'Jobsite Coordination' started by Dozerman85, Nov 27, 2016.

  1. Dozerman85

    Dozerman85 Member

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    I've been using site levels and lasers for years now. I'm fairly well versed with them. However, I am currently working on a project that has me stumped.

    I have 4 catch basins with 24" storm drain in a line. I am trying to find the elevation of each catch basin.

    My benchmark is set at 157.91. If I set the benchmark on my laser rod and it's set at 3.25(for example), if my 1st basin shoots a 4.50 on my rod, I calculate the difference of 1.25. I then subtract 1.25 from 157.91 and get the elevation of 156.66 for my 1st basin. My question is, if my 2nd basin shoots a 5.50 on my rod, do I subtract the difference between 4.50 - 5.50(1.0)from my original benchmark, or do I subtract it from my 1st box elevation of 156.66?

    I really hope this makes sense as it's kind of hard to explain.

    Anybody? Thanks a bunch in advance.
     
  2. lumberjack

    lumberjack Senior Member

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    Your bench mark is your bench mark, that's what you add/subtract from, you can have an infinite amount of points on the plane, but they don't effect your bench mark.
     
  3. Dozerman85

    Dozerman85 Member

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    That's exactly what I thought. Since the benchmark never changes, that's what I would always refer to.

    I greatly appreciate your reply!
     
  4. Oxbow

    Oxbow Senior Member

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    Same result, but you might consider calculating your HI (height of instrument) and working off of that figure, rather than keeping track of one rod reading, and then working off of that rod reading.

    In your example:

    BM=157.91, Rod Reading @ bench mark = 3.25, Height of instrument = 157.91 + 3.25, or 161.16. Use this and then subtract the designed height of the basin to figure what the rod reading should be if at grade.

    If your first basin is designed to be 156.66, then subtract that from the HI 161.16-156.66=4.5 rod reading. For the next basin subtract the design height from the HI again.

    As I mentioned, you will get the same result but it should cut down on mistakes made from trying to remember rod readings. If you need to move your laser, then shoot the bench mark again and re-figure the HI.

    This method sometimes seems slower than just comparing the previous rod shot, but it has helped me to save time by not making (as many) mistakes. I also have finally got in the habit of writing all these things down rather than trying to keep them in my head, which also has cut down on mistakes. The old noggin ain't what it used to be!
     
    M Silva likes this.
  5. oceanobob

    oceanobob Senior Member

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    I get white primed 2x2s from the lumber yard. When at the bench, I mark the 2x2 at the laser bracket with a 0, or, the benchmark number. Off I go to the next location and move the laser receiver bracket to the new location and then literally measure. A tape measure with inches in tenths facilitates this.
    ~
    Same as the other way, just less math.
    ~~~~
    ~~~~
    Agree on writing down. 3x5 cards help this task.
     
  6. Dozerman85

    Dozerman85 Member

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    This is a big help, too. I've never done it this way, but it definitely makes sense and I understand it. I'll try it tomorrow.

    I wear shirts with pockets, where I keep a pen and paper. I have to write all of these numbers down or it just becomes too much. Thanks for your help!
     
  7. Dozerman85

    Dozerman85 Member

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    I use a tape measure constantly. It's way too easy to screw up the tenths/inches conversion. Thanks for the response!
     
  8. Hoppsxc140

    Hoppsxc140 Member

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    I've worked hard with my guys on using elevations with a Lenker rod. It cuts down on the mistakes by taking all the math out of it as the guys just have to match the number on the plans with the number on the rod. They do cost a little bit, but if taken care of they last and the time saved pays off.
     
  9. CM1995

    CM1995 Super Moderator

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    We do the same when setting storm structures, pipe and grade elevations. I stress to my guys to always reference the bench mark, not the last structure set in order to cut down on mistakes.

    For example the day before we set two manholes and ran 100' of pipe. At the start of the day set up the laser, reference the bench mark and check the last structure set. If there is a discrepancy re-check your rod on the BM and find out what's wrong. It helps to cut down on errors.

    I use this an orange field book like this - http://www.engineersupply.com/elan-...2j19JZRKWPpw4o911fkZ1r6b4MljmPTGw3hoCoXfw_wcB

    Each day of pipe laying is recorded in the book with date and job. All the calculations for the day are written down in the book.

    I like the Lenker Rod, going to check into that. Looks like a simple way to reduce math errors and speed production.
     
  10. Shimmy1

    Shimmy1 Senior Member

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    I agree with checking the benchmark against last structure at the beginning of the day, but if you're dealing with structures such as manholes, once it's set it's essentially a benchmark, unlike floating grade such as a building pad or excavation. Of course, this all depends on how competent your guys are with math. :)
     
  11. bobatack

    bobatack Member

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    Lazer head with slope capability helps to keep calculations to a minimum
     
  12. CM1995

    CM1995 Super Moderator

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    Agreed. If your last structure was out a little sometimes you can adjust the next one depending on the flow line to get back to grade. Don't ask me how I know this...;)
     
  13. Shimmy1

    Shimmy1 Senior Member

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    That's funny you mention that........I was going to ask you about "oops" situations, because they happen to the best of us. I've did it both ways, put just a tad less slope for sometimes a few runs to get back to zero, or just leave it alone if it's not going to matter anyway.
     
  14. CM1995

    CM1995 Super Moderator

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    It happens to the best of us. When it does happen and it will, if the water still runs downhill that's all that really matters.;)
     
    Shimmy1 likes this.
  15. Bls repair

    Bls repair Senior Member

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    When using a laser or instrument remember to have to checked. I have had quite a few that were off,had to redo pipe line or redo slab on grade.