More about reflector prisms

@bill93 Great stuff, Bill. I started with the appendix, and I wonder if two corrections are needed near its end. In the next-to-last sentence, should "diameter" be "radius?" In the last sentence, should 52.7356?§ be 54.7356?§?

Posted by: @bill-c

@bill93 Great stuff, Bill. I started with the appendix, and I wonder if two corrections are needed near its end. In the next-to-last sentence, should "diameter" be "radius?" In the last sentence, should 52.7356?§ be 54.7356?§?

Of course.?ÿ Wonder what else I mess up on.

?ÿ

There's a new version of my PDF at the Dropbox link, having a few minor typos corrected, tweaks to the graph labels, and a different comment.

In going through the Leica paper linked at SURVEYEQUIPMENT.COM I notice that Figure 17 shows a wide range for angle of incidence. I tried a mini prism and can't get anywhere near that range before losing the signal. I should have tried a big one, too, and maybe will. I thought I understood a theoretical limit that was much less than that.

What is the maximum angle from straight-on that YOU can get an EDM measurement?

@bill93 I tested my 62mm Seco 6400-00 nodal prism with my Trimble S6 a while back. I found that I could turn the prism until the prism housing blocked the path to the center of the prism before the instrument failed to return a distance. Even at max deflection horizontal, vertical or diagonal, I saw less than a few thousandths of error.

@stephen-ward

Stephen the correction for a OLD STYLE AGA PRISM that had a minus 30mm constant (not really constant

but close) was around -0.2 mm if you had the prism NOT pointing at the inst. by 10 to 15 DEGREES.

There was no pointing device on the prism's. You would site down the side and many test were done showing

that you could site less than 5 deg. (about half that was the average of many people). The big difference 

was in the vertical (non tilting prism). Here is what Dr. Ragnar Scholdstrom said about this:

"Correctly mounted and aligned the old type AGA reflector with 30mm constant and the new type tiltable 0-constant 

reflector will not introduce any significant error for normal geodetic and land survey applications. A much greater source 

error is the accuracy of the vertical angle at steep slopes. For example, a 10 " error in a vertical angle of 30 degrees at 300 feet

will cause an error which is three times larger than the common untilted reflector".

Remember that Bill is trying to learn something that is over 50 years old and by his own omission

does not understand the math.

For modern day tilting reflectors(corner cube) you CAN POINT WITH EASE both horizontal and vertical and not worry

about anything.

Also back in the 70's almost all EDMI were +/- 5mm or more so 0.2mm at 15 deg. in Horizontal or vertical made no big deal.

JOHN NOLTON

@john-nolton He mentioned losing the signal ie. "not being able to get a measurement" if his prism was significantly off angle.  I was trying to communicate that with that prism I can turn it off angle until the prism housing blocks the sight line to the center of the prism before losing the ability to take a measurement. Typical prisms result in significant error if off angle with a robot or an instrument man who hasn't been taught to sight the center of the rod.  My point was that with the nodal prism I saw very little error in H or V even when extremely off angle and sighting robotically.

I read your tome and cites.?ÿ Is my takeaway correct; a moderately misaligned cheap prism or 360?ø prism could result in distance errors of up to 5mm??ÿ That's less than 0.02'; not a concern when land surveying but worth accounting for if doing precise monitoring of dams, etc.?ÿ If a tilting prism is hand/eye aligned to the instrument I doubt it's off by more than 15?ø.?ÿ Concerning robotic 360?ø prisms we're at the mercy of the manufacturer's specifications, which I assume are ballpark accurate.

?ÿAccurate angle pointings are of greater concern.?ÿ Non-robotic pointing at a prism target using an eyeball theodolite has not been a problem for me because I've only used Target Frame prisms.?ÿ When the image of the prism apex (inherently?ÿbad target concerning phase) is sighted instead H&V angle accuracy could degrade because of misalignment, but I don't have a handle on how important that is.?ÿ Seems like bad practice when as you pointed out the prism trunnion axis & baseplate are plainly visible for angle measurements.

In my 20 year dance with prisms the?ÿ nasty problem has been the wrong offset set into the gun or somehow a mix of prisms is used.?ÿ Everything's fine till you get to the office and closure is bad but *not that bad* unlike a blunder of tens of feet which triggers another field session.?ÿ Best is to take all newly purchased?ÿ circular prisms to the baseline and determine the offset yourself; some of the cheap Chinese peanut prisms are poorly labelled.?ÿ They're fine for construction work once validated, but for me Target Frame prisms are the way to go when boundary surveying.

Of course GNSS made terrestrial observations obsolete when lines are over a thousand feet, they're sigma 2 good to 0.1'.?ÿ But the local vertical, there's the rub, mostly below 0.2' but not even close to terrestrial observations with theodolite/total station/level techniques. Important concerning drainage.?ÿ But I digress.

@stephen-ward

Exactly Stephen. You should not see any difference even when the nodal point prism is off by 15 degree's and you have 

done the experiment to prove it. Good work.

By the way what is the angle and EDM  specification on the Trimble S6?

Thanks in advance,

JOHN NOLTON

Posted by: @john-nolton

Stephen the correction for a?ÿOLD STYLE AGA PRISM that had a?ÿminus 30mm constant (not really constant

but close) was around?ÿ-0.2 mm if you had the prism?ÿNOT pointing at the inst. by 10 to 15 DEGREES.

That's about what my plot shows, only a little larger because I probably used a little larger prism.

Posted by: @john-nolton

Remember that Bill is trying to learn something that is over 50 years old

... for which I haven't seen this kind of detail written up.?ÿ The Kivioja paper looks at sighting the line intersection in the prism, and I wanted to look at sighting a target.

Posted by: @john-nolton

?ÿby his own omission does not understand the math.

My omission admission was that I hadn't plowed through the whole Kivioja paper.?ÿ I understood as far as I went, and then got involved with the target sighting case.

My problem with the maximum angle of incidence was that I was ignoring coated prisms and the possibility of higher index of refraction.

The PDF at the above link has been revised for that and some additional sections.

May be some more nuggets in here?

Posted by: @lukenz

May be some more nuggets in here?

Isn't that one of the links in my original post?

@bill93

Yes, sorry for the double up. Had read this post when it went up but not followed all links and the with lockdown here had been going through all old FIG conference papers for professional development and found the paper and thought of the post.