Measurement of HI with a metric steel tape - ? of Does it Matter?
I've been asked this enough times, I need to know, in our line of work, we either do HERE positions on marks or hit NGS priority marks then submitting to OPUS shared (and doing the best we can to accurately record the HI to a point on a GPS receiver.
We setup over marks with Tripods and Fixed Heights for Base, or use a steady mount with a GPS rod (Mark Silver's method is what I call it). We always stress checking the length of all the tools using metal metric tapes.
Trimble makes some heavy duty tapes, of course Home Depot has a bunch.
The measurement error in using the outside of the tape end (like when measuring slant Height or checking a fixed height tripod over a flat surface). Is a "surveyors metal tape "zero end" the outside of the end clip, or the inside. The difference from outside to inside of the metal tape end is a couple mm, and errors are additive.
Is it worth sweating or not?
I have some PhD's that want to know.
Excuse my ignorance on this subject.
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.
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.
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.
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