I am curious how others measure the H.I. of the total station.
Do you eyeball the reference mark on the standard against a tape?
Do you do the same as above and calculate the correction for the hypotenuse you just measured? Or do you estimate the correction? (someone once told me they were taught to subtract 0.02', regardless of the tripod height)
Do you measure to the tripod platform and add the height to the axis?
Or do you do something completely different.
This question is directly in reference to some very short distance settlement monitoring measurements to permanent targets from a tripod. The project is urgent and short lived so a post mount is not in the cards.
Thanks!
My opinion is that it is irrelevant HOW you measure it as long as you measure it the same way ALL the time. All those calculations introduce another source for blunders.
The Trimble S6 has a lower mark on the instrument that is easy to measure the slope HI to, and then the DC corrects it to a vertical measurement to the telescope axis. Very little error, I believe it is possible to consistently measure to 0.001 m (1 mm, 0.003') accuracy.
Calculate and tabulate a range of HI's and their corresponding corrections to slope HI's. You'll see its non-linear and that the variance in correction gets smaller for the taller HI's.
One thing I do sometimes to reduce introduced error is that the control point I set the instrument up over is simply a TBM of no matter. That way the measurement to true control and the other various measurements are all measured at the rod height, the HI does not enter the equation.
> I am curious how others measure the H.I. of the total station.
> Do you eyeball the reference mark on the standard against a tape?
> Do you do the same as above and calculate the correction for the hypotenuse you just measured? Or do you estimate the correction? (someone once told me they were taught to subtract 0.02', regardless of the tripod height)
> Do you measure to the tripod platform and add the height to the axis?
> Or do you do something completely different.
> This question is directly in reference to some very short distance settlement monitoring measurements to permanent targets from a tripod. The project is urgent and short lived so a post mount is not in the cards.
> Thanks!
check with your instrument's available accessories. if there is a height hook or similar device, that would be useful, considering the nature of your project (monitoring).
as an alternative, buck in a level on the edm's hi, and take a d.i.e. on a different mark. takes a little time to get used to bucking in, but could help. mi dos centavos
We measure to the reference mark on the standard with a steel tape measure.
I measured from the center of the instrument to the side of the instrument and found it to be about 0.35' (if memory serves correctly). At 4.5' (which is kind of low) that would translate to a difference of 0.014'. At 5.0' (which is about right for me) it's a difference of 0.012'. At 5.5' it's still about 0.011'. Generally I just round down on the splits for my purposes.
> I am curious how others measure the H.I. of the total station.
For one person operation, a slant height with an ordinary 8 ft. tape (graduated in decimal feet) from top of ground mark to mark on side of total station reduced by 0.005 ft. to correct for eccentricity does the trick.
For a refined answer, set a point some convenient distance from the instrument and measure height difference from control point to auxilliary point. Occupy control point, measure HI by above method and check elevation of auxiliary point to see whether HI needs (small) correction before continuing.
What Shawn said.
If the HI is critical and/or needs to be precise you should NEVER (IMHO) directly measure the HI.
Set a bench close to each instrument station (but far away enough to allow you to focus (+/- 10 ft.)). Then set vertical to 90 degrees (or 0 degrees) and read the rod on the bench.
Anything else is SWAG.
My $0.00
Yes; what Kent said.
Steve - I like that answer.
I am looking at an intense short term monitor and I will probably not have the same person on it every time. That made me start thinking about the possible discrepancies in technique resulting in false results. I have issued to all staff a little cheat sheet on the corrections measured directly on our standard instruments and the range is 0.020 at 3 feet, not that absurd in Colorado mountains, to 0.008' at 8 feet.
Your solution is easily repeatable by multiple parties. Thanks!
With a tape or a 6' folding rule to the axis point.
I never ever thought I was doing any order of precise leveling with my total station.
Paul in PA
I have some instruments that are easy to measure, some no so easy, but if the HI is important (sometimes it really isn't) then I will measure it with a foot tape and write it in a book, then with a meter tape and enter it into the DC, if the two are in sync then I'm good to go.
The S6, and the R10 have special measure points, I've checked them and they work well.
I seldom require the precision this method would yield, but I like it quite a bit. I'll be tucking that one in my bits of useful knowledge. Thanks.
> Set a bench close to each instrument station (but far away enough to allow you to focus (+/- 10 ft.)). Then set vertical to 90 degrees (or 0 degrees) and read the rod on the bench.
To correct for the fact that different level rods may be used by different survey parties over the course of the monitoring, I think I'd add add actually measuring the level rod with a tape to verify that the rod foot is a true zero on the level rod scale. A reflectorless shot direct to the benchmark would probably be operationally easier in many cases to check work out the HI.
Good suggestion. I can control that the same instrument is used but had not yet considered the use of different rods. Direct measurement is possibly the best way.
As with most monitor projects the hardest thing to control is the client expectation!
I'm loving all these suggestions, and can see where many would work well if you're measuring to the top of a concrete curb or something else hard, but in the woods, I can swipe the grass/dirt next to a station with my foot and be .05 down from where I was, in a heartbeat. Not sure I see the need for more precision than that.
But if there's a hub or a pin or a pipe, does one measure to the top of it, or to the dirt immediately adjacent to it?
If settlement is all your are monitoring, leave the Total Station in the box and use a good automatic level or even better a digital level. Use the same level and the same rod every time to take error there out of the equation. If you are also monitoring horizontal displacement, mount 4 or more prisms on something very stable with good geometry and do a 3-D resection each time you do the survey. If you have to take the prisms off your control, use the same prism in the same orentation on each control point each time you do the survey. By doing a resection, you take the centering error of your instrument out of the equation. Again, use the same instrument every time.
I don't, it's all relative
Someday when I enter the 21st Century this may be needed. As for now, that's other people's problem.