I dont get that crazy here. Most of my work is large boundary surveying where we do not need topo. On those jobs we just run horizontally. We do topo on state subdivision jobs but only need 5' contours. Trying to split hairs between slant height and HI is just counter productive.
Having said that, occasionally I do take jobs that require precise vertical layout. Then we do get fussy and most of the time remote bench mark and do a tape check.
Check out the two items at the bottom of this page. This is Leica's solution to the problem. I prefer the older bracket that measured true vertical and compensated for the offset, but this is smaller.
Usually, though, if height matters that much I resection. Remote BM still has the same inherent measure-up inaccuracies, just applied to your solved elevation and not the resulting obs. Of course this doesn't matter if you're on a known elevation. With newer robotics that are well-adjusted and well robotically collimated you can do some nice tight vertical work in a hurry.
> The vertical is usually .01 less than the slope with a 5'+/- HI. Deduct .01 form your slope distance and you should be good for most applications.
If you workout the trig on this you will find that the vertical component is always 0.01' less than the slope measure for HIs between 4.0 and 6.0 feet. So simply deduct 0.01' from your slope measure, and you will be as good as the method needs to be.
The real trick is to get that slope measure to +/- 0.02'.
> .. the difference between slant vs vertical height ... is about 3mm.
:good: Which is near enough to 0.01'.
Why, oh why can't the instrument makers make the laser plummet into a distance meter? I'll even put a reflective tape on the setup point.
Or how about pointing a Leica Disto into an optical plummet?
Or duct taping a Disto to the side of the TS pointing down?
Was it here or the old site that Scott Z showed a Wild pole calibrated for the slope distance to a total station axis point. It came with a table for temperature corrections to the 0.01 mm, I think.
I have been using Leica's height tapes for years now. Thew work great but being Leica are a little spendy.
In my professional opinion, I feel that obtaining an accurate HI or HR is worthy even if the possible error is negligible. I prefer to minimize the systematic errors I can to allow for a large portion of my error budget to be used by those that I can not control.
John
> In my professional opinion, I feel that obtaining an accurate HI or HR is worthy even if the possible error is negligible. I prefer to minimize the systematic errors I can to allow for a large portion of my error budget to be used by those that I can not control.
Took the letters right off my keyboard. I try not to let it slow down the project (within reason), but my thinking has always been "why do the work twice?". More often than not I wind up needing elevations on the project later, anyway.
Also, if you use TBC and network adjust - having the height/elevation components helps the combined factor.
How does the Leica tape work? It's hard to tell from their photo.
The plastic claw/hook piece snaps around a trunnion on the tribrach. It then has a sort of slide-in "buckle" that holds the hook end of tape. It's nice because it is pretty wind-proof and allows for modest tension.
The tape also has a probe that folds out and this is what you use on the point to obtain a precise measurement.
Do you have to then measure from the tape bracket up to the scope or is the tape calibrated for that already?
If it works as good as the tapes they had on the older GPS setup it sounds like it would be very handy.
It's calibrated to take into account the instrument axis difference, and the slope adjustment.
It's a piece of cake to use.
:good:
For Leicas, anyway.
Thanks for the info guys.
One last question. Can you get the TPS tape in feet? (all our GPS tapes are metric)
> :good:
>
> For Leicas, anyway.
And Trimble. Only problem is getting the clip onto the tribrach. My tribrachs are all Leica or Topcon. Topcon tribrach needs some electrical tape to increase the diameter of the thumbscrew cover.
Is the offset to center of axes the same? I didn't think they are.
> Is the offset to center of axes the same? I didn't think they are.
Yep Trimble S6 is 196mm above the tribrach plate, same as Leica.
Hey hey, I just checked and you're right. I never realized that. Thanks.
I don't think so. Millimeters is more useable than hundredths anyway, though. On most data collectors these days you just punch in 1.565m and it converts for you.
Thanks Adam.
I didn't know that the DC's would convert the measurement.
Even if I had to convert to feet it still seems like a much more precise method than the whole "hold the sloped tape while trying to guesstimate the measurement to the center of axis...and then subtract a 0.01" all while the thing is blowing in the wind.
My current method always seems like more of an estimation than an actual measurement.
You may have noticed that slight wobble in the Earth's axis?
That's because of that time I inaccurately measured the instrument's HI.