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Would someone please check my math?

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(@dave-karoly)
Posts: 12001
 

"idiotic"

Knew a guy who worked as a handyman in a college town.

He called the Professors who employed him "educated idiots."

 
Posted : 13/11/2014 5:56 pm
 BigE
(@bige)
Posts: 2694
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"idiotic"

> If arrogant condescension comingled with haughty derision and myopic self-importance is affection, I think I'll chose tepid indifference.

That was an incredible piece of master word-smithing Shawn.
Well done!!! Mighty dam funny!!

 
Posted : 13/11/2014 6:15 pm
(@a-harris)
Posts: 8761
 

I like to have a stub pole under the prism assembly when using a tribrach.

When the sites are very close together I will also hang a plumb bob as another option.

There are human errors and instrument errors included in the optical plumb, sighting the foresight and backsight plus systematic errors in play on every setup.

Repetitive angles with flop between sets bring these into the equation and will usually average any errors out or bring large ones to light of day.

😉

 
Posted : 13/11/2014 9:39 pm
(@kris-morgan)
Posts: 3876
 

> One thing that doubling the angle using both faces does for you is to average out the effects if the horizontal axis (vertical motion) is out of adjustment and not perpendicular to the vertical axis (horizontal motion). This is most important when shooting BS and FS targets which are at different elevation angles.
>
> Another benefit, of course, is averaging out some of the error in the graduated circle.
>
> It does not have anything to do with leveling. If you take an instrument in good adjustment and throw it seriously out of level, the Face1 and Face2 measurements will still agree even though the angles have significant error.

So Bill, by the bold statement (emphasis mine), you noted above, would not a misleveled instrument mirror EXACTLY what you noted? I mean, if it's misleveled, then it can't be perpendicular, can it?

Seriously, I'm absolutely right about this. If you disagree, maybe I used the wrong term, but the pragmatic application is spot on. Go try it. But have a control group. Double the angles, by flipping the barrel, with it level, and then take it a quarter bubble out, and see if it's the same. I know the answer, but you should check it out.

There are other reasons also, like the legs are in a bind or something else, but if we can't double the angle in two tries, we break set up, and then re-do it, and it always works. Spongy ground will do it every time.

 
Posted : 14/11/2014 5:03 am
(@mneuder)
Posts: 79
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Just a quick tip for anyone planning on doing this. If you take your mechanical pencil and trace the tribrach on the plate, you can make sure you are getting a 120 degree turn by just rotating and making sure it lines up.

 
Posted : 14/11/2014 6:00 am
 rfc
(@rfc)
Posts: 1901
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I assume you guys are talking about "out of level" beyond compensation, or without compensators at all, correct?
Don't compensators work continuously?

 
Posted : 14/11/2014 6:13 am
(@shawn-billings)
Posts: 2689
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why not just unlock the gun from the tribrach and turn the gun?

 
Posted : 14/11/2014 7:03 am
(@dan-dunn)
Posts: 366
Customer
 

Do you wear eyeglasses?

Could this be a parallax problem?

 
Posted : 14/11/2014 8:14 am
(@bill93)
Posts: 9834
 

Data supporting my statement

I tried it with my old 6" instrument, which has no horizontal compensation.

Level:
60-46-07 direct
60-46-15 reverse

Tilted so tribrach bubble all the way to edge along direction midway between the two targets.
60-44-10 direct
60-44-16 reverse

The direct and reverse measurements agree well in each case, considering the minimal care I used and accuracy of instrument.

Averaging direct and reverse would do nothing to correct the 2 minute error in the angle that is due to being grossly out of level. The principle applies also to being just a little out of level.
----------------
When I talk about correcting for not being perpendicular, I mean the two rotational axes of the instrument not being perfectly perpendicular. That's not depending on orientation or leveling in the world.

 
Posted : 14/11/2014 11:36 am
 rfc
(@rfc)
Posts: 1901
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Topic starter
 

> Do you wear eyeglasses?
>
> Could this be a parallax problem?

No, and I don't think so. Still working on it. More measurements tomorrow.

 
Posted : 14/11/2014 1:22 pm
(@summerprophet)
Posts: 453
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"idiotic"

If the instrument doesn't have a physical plate that releases for doubling the angle, there is no need for doubling the angle. Rather, observe on face left and right, as you have already mentioned.

For very high accuracy, multiple sets on face left and face right (BS, FS1, FS2, BS, FS1, FS2.....)

After doing this a LOT, you will realize that your initial observations are usually outliers. For deformation monitoring surveys, I would record 5 sets, and always ended up averaging the last 3.

If I were you, I would invest in an HP or other easily programmable calculator. It would take about 90 seconds to create a program to average these angles for you, rather than repeating the same 4 steps over and over (with the possibility of fatfingering something with each step)

 
Posted : 14/11/2014 4:32 pm
(@rj-schneider)
Posts: 2784
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That sounds right. Sitting here trying to remember which instrument it was that had the notch cut out of the tribrach for the nine-pin serial connection on the instrument.
It seems i remember it being an older model Topcon.

 
Posted : 15/11/2014 8:58 am
 rfc
(@rfc)
Posts: 1901
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Topic starter
 

> The advantage to this method is you can do the math in your head on the spot and know if your angle is acceptable before breaking down your setup and moving ahead.
>
> If the angles aren't closing satisfactorily, check your gun isn't out of level and repeat as necessary.
>
> Carry on good man.

Williwaw:

I tried this today and IT WORKS GREAT!
Easy to pick up mistakes on the fly. Takes half the time compared to my previous method. It's my new standard. Thanks:-)

 
Posted : 15/11/2014 1:12 pm
(@shawn-billings)
Posts: 2689
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Ah that's exactly right. In fact I'm pretty sure what I suggested will not work. Thanks for the catch.

 
Posted : 15/11/2014 2:18 pm
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