Back in February, Boundary Lines posted a topic on this concern at [msg]49189[/msg]
I thought the discussion that ensued was good, and included reference to a system developed by Leica for measuring the height of the instrument
See the descriptions on the bottom of this page
Here is a picture of the GHT196 bracket that sits atop the tribrach:
And, here is a picture of the specially designed GHM007 tape measure which is used with the bracket to obtain the HI:
As I understand it, this tape measure is designed to read the true vertical height to the optical center of the instrument, and accounts for A) slant height, and B) height of instrument above tribrach.
Unfortunately, every instrument has a different height to center of telescope and also generally has a different width, so the scaling using on this equipment will only work with the Leica instruments.
Still, I like the concept. I'd like to have similar equipment that matches my instrument (Sokkia SRX3 robot).
I presently measure HI using a Pocket Rod. I usually subtract a hundredth from what I measure as the slant height, but I should better calculate that difference. Maybe subtracting more like 1.5 hundredths is more accurate.
Some contributors on the old post from February indicated that they measure the height using a rod, that apparently wears at the tip. What sort of rod is used?
Are there any other reasonably priced systems out, or a different methodology, that can result in improved HI measurement? Seems with all the technology utilized to measure distance, horizontal and vertical angles, this one measurement seems to be done with tools that characterize the work of 100 years ago.
Its a shame that you cant just plunge the scope on the danged ole thing and shoot a reflectorless HD to the point your set up on.
Of all the possible problems to encounter during a day of surveying, I think I'd count this one as close to the bottom of the list. Besides, in many cases, a direct shot to a BM negates the need for an HI at the instrument.
On a typical survey, I use a 6 foot folding ruler, and subtract 0.01. I find it hard to justify buying another piece of equipment to obtain this measurement.
That said, I have always wished for an illuminated optical plumb which would provide the HI when the light focuses on the set-up point.
Yup, I'd love to do that too!
I know, I'm getting fussy. Here we are in 2011, with instruments that are capable of reading angles to the nearest second, distances to the nearest hundredth, least squares processing engines capable of adjusting our redundant observations so that errors are now sub-hundredth, and yet we continue to make our HI measurements in a way that I would say is plus or minus 2 hundredths. Just doesn't seem to complement the precision we have grown used to with the rest of the system....couldn't there be a better way?
I support the KISS system, gimmicks that some will grow dependent on, not needed.
jud
Maybe - but would it really matter? If I need to measure vertical that accurately I use the tool that was designed for that prurpose, a level.
Big Al makes a good point. You'd think in 2011 we could get something more precise than a pocket tape...but it works.
I also agree with Jim, if vertical requires that degree of precesion, break out the level....of course your measuring height based on a rod than comes in pieces, has a plastic locking feature with a certain amount of play in that locking feature..
I've wondered for a while why manufacturers don't put a reflectorless distance measuring device in the optical (or laser) plummet. Integrating a Leica Disto into the total station somehow...just push a button, and wha-laa...accurate HI.
I use a Trimble S6, which has a notch at the bottom of the instrument. When you tell the DC you measured to the notch, it does the reduction to the center of the scope. I consider this method to be accurate to ±1 mm. I use a folding 2 m rule or a pocket tape graduated in mm.
I should note that I also have a Zeiss S10 and a Wild T2. All three have the same height above the tribrach plate, namely 196 mm. I have some wild targets that have the same height. Also, the prisms I normally use (seco mini), along with the adapter (aka hockey puck), are 0.083 m above the plate. Knowing these values enables me to carry very accurate elevations when using forced centering. Measuring the height of the prism is not nearly as accurate as measuring the height of the S6. So, if needed I can correct the HT measurements in the office to get better results in the adjustment.
Leica has a (different, but) similar device for measuring up on their GPS devices. It seems decidedly un-Leica-like to have a different device for one of their other pieces of hardware.
If you want it precise and accurate with a minimum of fuss, measure the distance between the point and the top of the tripod through the hole in the tripod without the instrument attached. Then, with the tripod head level (as we all always set it up), measure from the top of the tripod to whatever point on the instrument. Add 'em up for your total H.I. The latter measurement is almost always a constant.
I use the GPS height hook for my TPS setups too, you just have to do that PRIOR to setting the instrument in the tribrach.
One thing Leica has done AFAIK is keep the axis height at 196mm above the tribrach for decades going back to at least the 1980's and the T-2 days, if you use ALL Leica hardware the targets are also 196mm, forced centering works slick because once you get rolling, the BS-INST-FS all have the same exact height.
SHG
is it the more sophisticated we become the more we lose our old senses and the need for more gadgets is a reality not to be missed?
Being tall I just knock 2mm off my sloped instrument height and call it done.
For 99.99% of the time (at heights I set up - 1.7m±) this is accurate enough.
If I want ultimate accuracy (whatever that is) it is easy to do a maths calcs or read to a known BM and get the RL from that.
How many of us are sure our prism poles are accurate (pole tips wear) and our instrument is in good vertical adjustment and how much refraction is causing odd things.
Refraction is my main worry on a hot day even over short distances along roads, concrete ect.
I use Leica height meters for my Leica total stations and my Leica GPS/GNSS. Of course one can readily compute for the theoretical HI but I still prefer the measured HI.
If I can remember it correctly, last two to three months ago, Horizon ( http://www.horizon.sg) announced the new way of measuring HI.
Thank you guys!
Regards.
That sounds like a reliable way to improve precision without needing more gadgets and without spending too much additional time.
I checked out that link to Horizon but didn't find what you are referring to.
I guess it is in one of their press releases.
Thanks.
I'm a dinosaur.
Like several others, I use a six foot folding ruler.
About the only precaution I take is to keep a relatively small maximun distance for shots.
SAME AS SICILIAN
.