New Methods for Invert Measurement

Updated: September 8, 2020

New Methods for Invert Measurement

The Old Method

Invert data can generally be collected on structures through direct vertical measurements such as using a box tape or measuring rod, but these methods do not generally work on recessed or hard to reach structures such as pipes, drop inlets, catch basins and many other drainage structures. These methods lead to estimations, inaccurate data, and often multiple site visits to confirm measurements.

A New Method

Recently a new method has been developed that enables one to collect redundant invert data at any angle by using a lightweight calibrated measurement rod. The vert rod is a four-sided calibrated measurement rod with each side of the rod calibrated to common recessed angles. When the rod is aligned to the angle of a particular side, the vertical distance is achieved rather than the slope distance.

New Methods for Invert Measurement

Introducing the 15ft Vert Rod by Accurate Inverts

The 15 ft Vert rod by Accurate Inverts is calibrated to the cosine of each sides corresponding angle, ranging from 0-30o .  The rod also includes a digital angle finder which allows field crews to use any angle and quickly achieve a redundancy of measurement.

15 ft Vert rod by Accurate Inverts

Redundancy can be achieved by measuring multiple angles or by using the 0 side to measure the slope distance and the angle. By noting the slope distance on a random angle, the vertical distance can be calculated later as a check. If one aligned the angle finder on the 0 side to 30o, read a slope distance of 11.55 and multiplied by the Cosine of 30o the vertical measurement is 10 which is exactly the same measurement you would achieve with the calibrated 30o side of the rod. Each side is calibrated this way, but the 0 side also allows one to measure custom angles. This new methodology allows field crews to quickly and accurately collect invert measurements without the need to second guess measurements also allowing redundant office checks to verify accuracy.

vertical distance is obtained directly using the calibrated 30 degree side of the Vert Rod
Figure 1: the vertical distance is obtained directly using the calibrated 30 degree side of the Vert Rod.
The slope distance is obtained using the non-calibrated 0 side at an angle of 30 degrees
Figure 2: The slope distance is obtained using the non-calibrated 0 side at an angle of 30 degrees. The slope distance multiplied by the cosine of the angle (30o) gives you the corrected vertical distance of 3.93 (see figure 1.)

More Information

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Originally published on September 8, 2020

About the Author

Wendell T. Harness

I’ve been building online properties since the late 1980’s and transitioned to web design in 1999. I formed Harness Media in 2005 to help businesses grow through online marketing. I also talk to cats in a silly voice.

19 thoughts on “New Methods for Invert Measurement”

  1. Looks very good.
    I’ve used a cheaper method for years. I had a series of “probes made up – flat bars of length 0.5, 1, 1.5 metres with a standard 5/8 survey thread stud welded onto the end, facing upwards. Screw on however many target extension rods you need (mine are 1 metre), lower vertically down manhole, stick end of probe into pipe and note length of rods. Much easier to keep clean than an expensive stave and it can be as long as you have extension rods.

    • @guy-townes we have one but its a pain to attach and maneuver into structures and sometimes its unable to reach recessed pipes. it also seems to act like a sewer soup ladel that I’d rather not touch. The vert rod also allows you to quickly get multiple redundant measurements on a structure where you can only get one with the pipe mic.

  2. I carry a Pipe-Mic in my truck and use it when I have a lot of manholes to dip, but assembling, using, disassembling and disinfecting it is too much trouble for just one or two manholes.  For the onesie-twosie projects I use a fiberglass rod, and if I can’t get a vertical measurement I use a carpenter’s angle finder similar to the one shown below to get the angle.  I record the rod measurement and the angle in a field book and reduce it later in the office.  It’s not super-accurate, but good enough for most sewer and drain situations.

    • @jim-frame
      I have been doing that for at least the last 25 years.  Sin of the angle times slope = vert. down.  Protractor fits in my pouch, a simple solution.

    • @jim-frame the cool part about the vert rod is that each side is already calibrated, put the rod at 30 and the measurement you read is the cosine of the angle times the slope, no calculation or reduction needed, measure at 30 and get an invert of 5′ measure at 20 you’ll get the same 5′. This allows you to quickly get redundant measurements and the digital level allows you to pick a random angle if you want to do your own math.

  3. I sometimes use a digital spirit level and a standard staff, however a short spirit level (600mm) is best so it can lay parallel to the staff and not foul any buttons or joints, also you have to eye down the staff to see that it’s not bent and be aware if the measuring side is touching the invert or about 20mm above it. It helps to be an octopus so as nothing falls down.
    I think the best method if the invert is dry, is the V depth tool by Schneider. It looks expensive but if the Laser could be detached and used for other things that would offset the cost. I have only seen pictures.

    • @mark-mayer
      I recently had a job on 0.9m diameter sewers generally flowing about half full. Wanted to avoid putting a staff down them as be forever cleaning to take in and out of truck so used a Lecia disto with tilt sensor to measure soffit and calved invert from pipe size on asbuilt. Measured from timber stake level across MH rim to soffit of pipe but found it hard to get soffits accurately when shooting at acute angle down 2-3m deep (usually 1-2cm variance). Wasn’t for design work just checking for future pipe clashes so spec was 6cm which I was well within.
      Was great at giving a vertical distance to invert in stormwater manholes just in anything with constant flow I assumed shooting through the liquid would affect the values.  I’m assuming from your comments that you checked it with some other method and worked ok?

      • @lukenz
        I checked mine against a wall about 15m away.  I did the following:
        -held it steady on my desk
        -shot the bottom of the wall
        -shot the top of the wall
        -added those two measurements
        -checked that against the disto shooting from the ceiling to the floor
        -checked that against a tape
        Worked out within +/-1″ between disto to disto (I wasn’t using the disto stand and was just “free handing it” so that’s about what I expected) and about less than a 1/8″ for disto to tape (and I trust the disto more than the $3 Home Depot tape I was using).
        We put notes on our plans that our inverts are measured from the surface and that to obtain highly accurate values they should commission someone w/ confined spaces certifications.  If the tolerances required for inverts are greater than +/-2″, the client can pay accordingly for that.  Heck, looking at old plans guys just used to measure to the sump/benching and called it a day.
        When I’m designing sewers, the first thing I do is set the bottom end at least 6″ higher than what the invert is supposed to be.  If possible, I then put 2″ to 4″ of fall across the structures.  It’s pretty easy to work out everything to the mm in CAD but, well, someone has to build it in the real world.  Might as well design some “schluff” into it so that it works easier in the field and makes the construction easier IMO.
        ie. sewer crew is setting the structure; if they’ve got 1″ or 2″ of play on either side of the structure, they save time installing it and there is less chance of a “ugggh, how am I supposed to sign off on this?!” moment where the client and contractor are pressuring you to accept something so the municipality “goes away.”

  4. Most handheld laser meters today have built-in trigonometric functions where you measure the 2 sides (slope and hor) and it will return the vertical distance.

  5. I sometimes find drain lines dry, but mostly not (and when they are they often  have a tenth or two of muck on the bottom), and sewer lines almost never.  I don’t think a Disto would be much use to me for MH measurements.

    • @jim-frame
      Fair point.  However, many of the people asking for our services do not state the required accuracy/precision/methodology.  We simply note the methodology used and tell them to contract a utility location company for precision measurements.  We do not have that liability insurance/in-house capability so we’re not going to pretend that we provide beyond what we should.  Different strokes.

  6. The accurate invert solution is neat, but would become less reliable if you have to use multiple sections of the rod.  They can flex quite a bit.  The other problem I see is in the throat inlet example getting a reliable pipe size would be pretty hard, and for the engineers I’ve done these for the size is as critical (or more) as the invert elevation.  I would like to see an abstract of size certainty relying on the difference between two measurements using the digital angle measurement device.  Would also need to compensate for 1/2 rod thickness when measuring inside top of pipe.  If that is reliable they would sell easily.
    For standard manholes the pipe mic is hard to beat, but it seems like every site has at least one problem child like the shown example.

    • @plumb-bill the rod is made from aircraft grade aluminum not fiberglass, so even when fully extended it still does not flex any noticeable amount. it also has a slide on the bottom to help visualy confirm pipe sizes.


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