Precision Levelling

[dave-lindell]

I have a set of Invar rods if you want to come to Pasadena, California to get them.

[brendan8762]

I’ll be right there!

(It’s only 2000 miles, one way.)

[john-hamilton]

I still run a lot of precise levels, but we use a digital level and invar rods.

I should note that the micrometer has to be for the same graduations as the invar rod (i.e. 0.02 feet graduations, or 1 cm graduations)

I do have an optical level (tilting, no micrometer) that I bought to be able to use when high vibrations are present, for example when crossing large bridges. But I do not have a micrometer on it nor a conventional invar rod.

I did a 130 mile second order leveling project Pittsburgh, PA to Fairmont, WV along the Monongahela River) that was bluebooked in the late 80’s using an optical level and micrometer with a dual scale invar rod before digital levels came around. I wrote a program for the HP41 to collect and check the data. For each sight you had to read the high scale with the micrometer, then the low scale with the micrometer, then the high and low stadia hairs, and you had to check it all before it was accepted. Using the HP41 made it easy to check, and I stored the data in a binary format on the HP41, and then downloaded it to a PC each day to process and format it for submittal. By doing the checks there were no blunders present in the data.

I have done a bunch of leveling bluebook projects since them but using a digital level. Much easier and quicker.

• This reply was modified 1 month ago by  john-hamilton.
• This reply was modified 1 month ago by  john-hamilton.

[brendan8762]

That sounds like a fascinating project. The man who taught me most of what I know used to work for the provincial government here doing control work and he used to regale me with stories of running levels all summer.

On a much smaller scale I wrote a three-wire levelling program for my HP50 that I use when I run (construction) control levels, it returns the three-wire average and compares it to the booked centre crosshair reading and triggers a buzzer if the delta is greater than 0.5mm.

That’s a good point that you need to ensure your micrometer and rods are in the same units, something I do check for as I see used instruments pop up.

A titling/split bubble level is also on my wish list. Perhaps and old style Wild N2 or a Sokkia PL1. I was thinking today that the last time I used one was circa 2005.

[john-hamilton]

For those who don’t know, a tilting level does not have a compensator. Just before reading each rod you look through an auxiliary telescope at a split level bubble, and make sure that the line of sight is level by turning a screw which tilts the instrument in the line of sight and aligning the two ends of the bubble which are viewed side by side with a mirror. A compensator level is adversely affected by vibration, as on a bridge or around large machines, whereas the tilting level is not.

Here are two pics of the N2 that I purchased.

• This reply was modified 1 month ago by  john-hamilton.

[james-vianna]

<div>Here is a old post from 10 years ago when I sold my Kern NK3 parallel plate micrometer. The link doesn’t seem to work but maybe someone can redirect it. Mine had an inverted image and needed a inverted metric rod to use. The idea of the parallel plate micrometer was really innovative. </div>http://beerleg.com/index.php?mode=thread&id=57039#p57260

[brendan8762]

Thank you for the quick tutorial.

I sometimes forget that there are people that might not be familiar with these instruments, or the nomenclature. I haven’t used one myself in almost twenty years.

[Wendell]

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[pfirmst]

Still handy for windy conditions and vibrations where digitals don’t work, don’t need to charge the batteries either, this is a second order Pentax, with Sokkia micrometer. Digital levels are faster.

+1 More Photos

[pfirmst]

First order Sokkia tilting level.

[brendan8762]

Beautiful.

[chris-mills]

Optical levels have one great advantage over digital ones – they have no time limit on reading. Even if there is a great deal of interruption to the sightlne (waving foliage, heavy traffic – vehicles or mammals) given patience you can always get the reading in the end. In many situations you might just move a digital level to a different set-up, but when carrying out precise monitoring operations consistency in location is always important (keeping back and fore sights equal, any residual ground heat refraction consistent) and in these situations the optical wins out.

In some engineering monitoring applications there might only be a small (couple of centimetres) gap through which to observe; again the digital will give up. The N3 and NA2 might only go out occasionally but they pay for themselves whenever they do.

[party-chef]

Thanks for sharing about the tilting levels, I knew there had to be a better way for bridges.

[chris-mills]

On the subject of tilting levels and vibrating bridges …

A similar situation arises when you need to measure things on an unstable platform. Several years ago I was nvolved in a problem with a ship loading cable on a tidal berth. The deck carousel was having problems and the ship was stuck, anchored to the shore by the cable and going up and down with the tide. In order to safely wind the rest of the cable on, the carousel needed to be monitored whilst turning slowly. Modern instruments with auto-collimation couldn’t cope. I ended up using an old style T2 with manual levelling. Instrument set exactly parallel to the deck in both planes at the top of tide (when ship was most stable). Locked off the bubble adjustments with temporary covers. Then all readings taken were directly related to the deck, regardless of any slope, roll, etc. Instrument recovered its original cost just on that one job!

[Bruce Small]

I read an interesting analysis decades ago on the thousandth digits and the brain’s bias. With enough shots the thousandths should appear with equal frequency, but they don’t. We all have our own bias, so .001 and .009 may appear twice as often as .002 and .008, etc.

[john-hamilton]

I agree about the unconscious bias. I saw it in the 80’s when we were running a lot of three wire levels for photo control for a 100 mile proposed highway. At the time we were not using GPS for elevations, so we ran 100’s of miles of levels. I could see that some numbers occurred more often than would be expected if there was no bias.

I had one level party that would only check the digits in the thousandths place, and not check the hundredths or tenths between the three wires. Once I figured out what they were doing, they had to rerun a lot of lines, because the i-man misread these other digits occasionally and it was not caught. That was not using a micrometer.

I believe that the use of a micrometer mostly removes the bias, since you are no longer estimating as much. You are only estimating between two graduations (i.e. one or the other), not interpolating to a tenth of a graduation.

[brendan8762]

That is interesting. I hadn’t considered that before. I always tended to think that my estimation of millimetres, on a metric E rod (in non-precision applications), was unbiased and therefore and random errors would cancel each out over a run.