Peanut prisms - not as accurate as Traverse prisms?

How steady can you hold a peanut prism on a point? I think this is where some inaccuracies would occur. Also, because error increases with distance, an out-of-adjustment traverse prism would introduce more error than an out-of-adjustment peanut prism. Finally, you could consider the effect of the microclimate; the peanut prism height is only 100 mm. I don't completely trust Topcon traverse prisms because of how they're constructed. I don't know how they're manufactured nowadays, but they used to have foam rubber inside to fill the space between the housing and the prism. In south Florida we would experience very brief rain showers that would cause us to disassemble the housing, remove the foam rubber and prism, and let everything dry out. I never completely trusted the accuracy of such prisms after that. How do I know the centerline of the prism is aligned with the housing and therefore the "candy cane?" Just a pet peeve of mine.

Some ideas:

Have you checked the bubble adjustment?

Is it consistently long, or short, or is it random?

If consistently one direction are you using the correct prism offset constant for that model?

Do you point the prism horizontally and vertically at the gun within several degrees?

The offset is either different from your larger prism or else made the same by setting it back, where it will be more sensitive to pointing.

What is the accuracy spec that is provided by the manufacturer for the peanut prism? It should have a +/- mm that may explain your error.

Also, you can only really check your traverse with the equipment you used to run it (if said equipment is properly calibrated)

It would be like using a steel tape for distances less than 100 feet from a traverse point on level ground for doing layout and then wondering why you're not hitting your distances when using an edm to check your work.

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I've found them to be more accurate.

It's our go to for tight building layouts.

Posted by: @bill93

I re-checked my study paper on prisms, and think that it would have to be seriously mis-pointed to cause that much error.

@steven-metelsky

I completely disagree regarding use of the same equipment. That is true for deformation monitoring where the goal is to screw it up the same way every time. For actual surveying you want your chwcks to be as independent as possible.?ÿ

The GMP111, as pictured has a centering error of 2 mm per Leica.?ÿ The GMP101 mini prism has a centering accuracy of 1 mm.?ÿ Unless you are going to drop $2K on the GPH1P at 0.3 mm, 1 mm is the best you are going to get.

For my part I use the GMP101 for most of my control work.?ÿ I do have a set of GPR1/GPH1 prisms but rarely use them any more.

Don't forget that in addition to whatever standard errors are present in the peanut prism, you have your instrument height, centering, levelling, horizontal angle, zenith angle, and EDM standard errors to factor in as well. The peanut prism helps reduce centering errors at the observation point, but will do nothing for the other error sources.

Not to mention the error inherent in your occupation and backsight points. How good is your control network to begin with? What are the standard errors for those points, and were they based on direct ties between the points you are checking?

Add up all those error sources, and 4-5mm x 7mm for a single F1 observation is (usually) well within expectations.

If you are using a Leica prism with non-Leica equipment you might like to review the prism offset you are using. It's not going to be what's printed on the prism. Subtract 34.4mm from what is printed. So for the GMP111 you should be using -16.9mm (for non-Leica instruments)?ÿ?ÿ

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thanks guys

The offset I have in is fine. I use Leica big round prisms for traversing. I bought the peanut prism with the same offset to save switching and to cut down on errors.

I reckon the x and y might be caused by a lack of verticality and an unsteady hand but I can't explain the consistently different elevation measurements I get. I might lose a mm or 2 as the point of the mini pole sits in the centre hole of the nail. I use this Leica tape measure to get an accurate target height for traverse prisms so I'm confident I've got accurate station elevations.

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100mmtarget height at 100m? Grazing rays.

Both the prisms I use have Leica offsets of 0 which in the rest of the world is -34.4mm

Those particular style mini prism holders have differing offsets. I know since I have two that are not the same. They are not handy right now but first read the prism holder to confirm that it is the same as your normal traverse prisms. Also find out all you can about each particular prism constant which varies based on content of the prism glass?

Vertical height error should be verified with a rule because the middle of the prism is clearly visible.

?ÿI have taken shots up to 1000' with a mini prism. Then again I have driven back 80 miles to pick up my mini prism that I left a few hundred feet away on a curb.

I occasionally set up a tripod over a point, check all my tribrach mounted prisms, hang a plumb line to check those prisms and then check all my poles. If they do not agree within 0.005' I do something about it.

Paul in PA

Sub 100th difference in peanut prisms vs a prism on a tripod.

After verifying the bubble is good, if you're getting greater than that, work on technique.

Posted by: @john-putnam

For my part I use the GMP101 for most of my control work.?ÿ

Same here.?ÿ Every few years I'll want to take a half-mile shot, so I keep a GPR1 in the truck, but the GMP101 is my go-to for everything else.

@martin_au

This is what I'd go with too. Was doing a house monitoring survey so was trying to use the GMP111 with only the point under it but had tracking problems with ATR, stuck a section off pole under it and problems went away.

@fobos8

What instrument are you using to shoot said mini prism?

@jim-frame

Same approach here. It was some of your other posts that helped me realised that for most applications the GMP101 is better to use than the heavier and more expensive 360 prism which only comes out now for the odd open topo or rough but not GNSS rough setout.

@fobos8

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I am guessing that the zero offset mini is the source of your problems. It is not the accuracies of the prism that are fouling?ÿ you up but the pointing, especially if you are using ATR/Robotic aiming.?ÿ

Set up the gun in tracking and run a trial where you point the mini directly at the scope and then rotate it a little side to side and up down, you will see the departures. You can do this with your eyeball also.

Would have been better to go with the 17.5...(Or full robot with a mini 360)

It might be because the points are at ground level and what you are getting is a ground refraction, which would explain why the height changes. Any shot which passes within 1 ft. of ground level is potentially at risk, especially if the ground is a hard surface. You don't need the sun to be shining to get the refractive problem - just a significant difference between ground and air temperature.