Measurement of HI with a metric steel tape - ? of Does it Matter?

The stuff we do doesn't require that type of accuracy but I also want to know. It also reminds me of something I started to do recently...we don't always need the HI as I'm either not doing elevation for certain builders or I'm going to hit a benchmark anyway, but I often forget to measure it before leaving the gun and realize I'm on an elevated point.?ÿ

If I'm backsighting an elevated point I change Hi to zero look at my backsight error and then punch that error in as the HI. My first thought was I bet the office won't like this (even though the required accuracy level for vertical is a few centimeters on the construction layout I do). Then I realized I'm essentially removing the taping error as you cannot hold the tape straight up against the mark on the total station (especially on the Leica 1200 - the knob gets in the way - so they included a special tape which someone lost a piece for). I have to subtract like 5mm from what I appears to be get it dead on.?ÿ

Slightly off topic as you're talking GPS but either way the tape gives some error when measuring HI.

If the mm measurement is being scrutinized, buy the HI rod from Trimble. The end of a steel tape purchased at the big box stores is likely to provide 2mm +/- error due to what you mentioned, the thickness, and also they aren't calibrated or certified so there's that.

And, if you're measuring up to a Trimble antenna there is no slant distance if your measurement is to one of the precalculated measure up point ( Middle of the bumper, top of notch, bottom of notch, the bottom of quick release etc.....)

That being said, why are PhDs asking such low level questions, just curious.

Additionally, if you're occupying the NGS point, unless you're making an adjustment to the network you're using, the "here" button adds ambiguity, its for an unknown location to be included in a network. Key in the NGS data for the point, and collect your data.?ÿ

2 mm sounds like more error than I would expect or accept in a tape, but still much less than a GNSS elevation accuracy.

Most box tapes have a sliding hook that attempts to make outside and inside measurements with equal accuracy.

Check your tape by holding it against a ruler, matching cm marks but avoiding the end of the ruler (e.g., 3 cm on tape at 5 cm on ruler). Push and pull the hook and see what the error is each way. Outside of hook when pushed and inside when pulled. If excessive, get a new tape..

HI error measurements are random and in a large group of measurements they tend to cancel out. Whether or not I need elevations for a project I carry elevations on all traverse and SURVEY points. Agreements in elevations are a check on your overall precision.

GPS elevations are less accurate than GPS positions for several reasons. 1/ All GPS measurements are from points above the horizon introducing a mathematical bias in the errors. 2/ GPS measurements are affected by variations in the atmosphere and the atmosphere varies much more through various elevations than in NSEW directions. 3/ There is also N/S elevation bias in the atmosphere due to the earth's rotation but I am unaware that any atmospheric correction algorithm accounts for it.

I am personally satisfied to make all my elevation measurements with a 6' folding engineers rule. Measuring up with a steel tape even on a mildly windy day just does not work. Checking HI before breaking down is also a requirement. Any mention of HI includes HA, antenna height, HV or HS.

"I would hope we all do the best we can on every measurement", but when is the last time you had to use an Invar steel tape?

If you cannot explain to a PhD what you are doing when you are surveying then you should not be surveying. Truth be told, most PhDs do understand small words. Let them know that all your work is a statistical mean of multiple measurements not single events.

Paul in PA

As Bill said most box tapes have a moving metal hook so that when you measure to the outside it pushes back to give the correct length and alternatively when you are measuring inside the hook it pulls out to compensate for the thickness of the hook so if you use either inside or outside the measurement should be the same.

We aren't building clocks???

Measure with a micrometer and build it with bulldozers.

I just now compared 3 box tapes.?ÿ All properly adjusted for inside vs outside measure.?ÿ Two were well within 1 mm versus the ruler when testing the last few cm of tape, but my favorite tape was short 1 mm either way.

Another test you can do with a tape is to loop it and compare it with itself to see how consistent the graduations are at various places.?ÿ Most hardware store tapes show differences of as much as one least division (1/16" or 1 mm) over a span of a meter or yard at some point.?ÿ The variation is probably periodic, and the discrepancy is controlled so it does not accumulate along the overall length of the tape.

My "favorite tape" was the one that I had found to be most self-consistent, an old metric/English Sears Craftsman.

Those little advertising givaway tapes may not have the sliding hook, and are often much less consistent.

I've checked all of this with our measurements, what have you seen?

I see <0.01', when care to measure is taken.?ÿ

My advice is run your own tests.?ÿ

Don't know what type of receiver you are using but take the R-10 as an example. It has a lever to measure to, or you can measure on the head or the bottom of the unit.?ÿ

Put it on a known rod.?ÿ

Use the known measure to the head and enter it into the DC, then the lever.?ÿ

Then look at the adjustments done in the DC and see if the same number is returned after the DC adjusts the measurement.?ÿ

Then do the same process on a tripod.?ÿ

We've gone through all of this endlessly with each unit. There isn't much error in the measure up when it's done correctly.?ÿ

?ÿ

@paul-in-pa

I completely disagree with your last statement. If you can't explain to a phd (or anyone for that matter) any aspect of surveying you shouldn't teach that aspect of surveying to that group. A surveyor lacking well rounded experience and teaching skills shouldn't excerise responsible charge over things they don't know, but saying they shouldn't survey is just inflammatory crap.?ÿ

@jed

My surveyor says that to keep us from actually doing that.

@thebionicman

I assume the original poster is not a surveyor based on his name. I also don't know a single surveyor that works in the field. The 3 at our company are busy in the office checking plans and drafting so we can get the field work done. "Field technicians" do the the field work.

I do construction layout and I'm the least experienced party chief we have, but I can guarantee that some of our legal surveying party chiefs may not know the answer to the question.?ÿ

As everyone knows there are many different types of surveying. We do mainly boundary, topo and construction layout. We're not laying out bridges or skyscrapers. If you were then you should know the answer to this question.

I think that saying that if you can't explain every single thing that you're doing in the field you shouldn't be doing it... might as well send all the crews home and shut down the operation. Paul must be one of those guys that never admits he doesn't know the answer to something.

The tape I use is metric and tenths of foot. I measure and record the metric reading to the millimeter. The metric value is entered into Trimble access and it converts it to feet. I then do a blunder check and verify the foot measurement. Height of instrument and target errors are hard to correct.

This method was drilled into me doing GPS surveys in the late 80's. I know overkill for some but it works for me.

The Trimble software reduces the hypotenuse measurement to vertical, the difference can be 0.01+/-.

Yes I'm trying to build a piano ;-)?ÿ ?ÿ?ÿ

Posted by: @paul-in-pa

HI error measurements are random

While ones ability interpret measurements between marks on the tape may bed random, measurements take by an inaccurate tape are most definitely systematic in nature and a blunder if not corrected for.?ÿ

@350rocketmike

Well said. Blindness to our own faults and shortcomings is a malady common to all humans. I find it particularly acute among surveyors (myself invluded at times).

As a PLS I like to see every site of any appreciable size or complexity. I enjoy passing down tricks and tips, and even learn a thing or two from the youngsters around me. Had I quit the first time I didn't know an answer I wouldn't have lasted the first day. When I stop learning someone better call the coroner.

Posted by: @350rocketmike

I also don't know a single surveyor that works in the field.

Unless that's where they want to be then that's kinda sad, imo.

@350rocketmike

Working in the field with some regularity keeps you sharp and aware of the real side of surveying.?ÿ I'll take a day of that over a day of office work any time.?ÿ I do both, so speak from tons of experience.?ÿ I am also the primary client contact.?ÿ Getting stuck in just one side of the business can limit your future potential if you must go job shopping to feed the family.

BTW, I detest the tediousness and boredom of small construction projects so have avoided them for the past 20 years, with a few exceptions, of course.

@holy-cow

Very true, and I wouldn't want to be stuck in the office. My boss (owner of the company) doesn't have a choice though.?ÿ

Im working 12 hour days lately to keep up and I think he is as well.?ÿ

@350rocketmike

The workload is one of the things I can manage as I am the one who decides which and how many jobs to take on.?ÿ I have turned down more projects this past month than I have taken on.?ÿ In a way, this is a good thing.?ÿ People are learning the true value of what we do.?ÿ I had one of the wealthiest people I know call this week to have me cut off a house and buildings from a much larger tract.?ÿ He told me that he realized it would probably be over two months before I could get to it, but that was fine.?ÿ He would work with the buyer to let him and his family move in whether or not the deal had closed.