Does anyone here use a dual prism pole?

You don't need a special pole. Just two shots with the same prism and two different  rod heights. Here is some  info. 

You can use that method with a single prism pole, but I can see a speed advantage with two prisms, especially since you are doing a lot of construction work. For two or three points it would be no big deal, but with 20 or more the time savings will become noticeable.

Topcon has the same routine for two prisms.

@gary_g I'm not sure using two shots on one prism is going to be accurate enough for laying out steel.  For the hidden point routine to work, you need the two prisms and the point to be on a single vector, and any deviation will be amplified at the point.  The single pole w/ 2 prisms will eliminate, with in reason, the source of error.

That may very well be. I was imaging a bipod holding the pole in what ever vector the first shot would be , then extending for the second shot. Sound like some testing to be done.

Thanks for your advise guys. I can see how you can do this with one pole and different heights, but its absolutely critical that you get repeatability in the slant angle of the pole. Using a bipod on a ladder or up scaffolding wouldn't work, and that's when I'm taking these sort of shots.

I'd need to clamp or brace the pole to a concrete or steel structure to keep the pole still for both shots.

I have used them in the past with good results for some specific tasks.

I have heard that they are popular in Germany for doing manholes and to me that sounds like a great application but I have never done it.

@leegreen Good to see you posting, seems like it has been a while

Just saying, the AP20 makes life much easier in these situations.

This 2 prism idea has been around for a long, long time but has never really caught on. The AP20 solves the same problem and does a lot more. The $10k price should be trivial considering the productivity increase over the lifetime of the unit. 

Posted by: @john-putnam

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Just saying, the AP20 makes life much easier in these situations.

And costs $12,000 and is still just a prism. Depending on accuracy needed, you're better off using Integrated Surveying.

Posted by: @robertusa

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And costs $12,000 and is still just a prism. Depending on accuracy needed, you're better off using Integrated Surveying.

I'm with John on the AP-20. It's not $12k, and it goes on sale from time to time. It is far more than "just a prism".  Automatically updates target heights and provides a target id, so you don't shoot the backsight 50 times before realizing it.  Not having to pay any attention to such things, as well as the plumb bubble, topo point collection goes up dramatically. Since your attention is now focused on just the point coding, that accuracy improves as well. Precision is - maybe - just a little short of a properly adjusted and plumbed pole, but still amazing. I'm quite convinced that whatever you pay for it will be repaid many times over in productivity increases. 

One use is to shoot pipe inverts in manholes and catch basins. The newfound precision of the result cannot be beat.

Posted by: @norman-oklahoma

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Precision is - maybe - just a little short of a properly adjusted and plumbed pole, but still amazing.

According to a white paper on Leica's site, the AP20 is actually more precise than a manually steadied prims pole and slightly less than a prims pole w/ bipod, assuming the pole is in proper adjustment. 

Leica makes a 360 prism (MPR122) that has a 5/8" thread on top that has its own carbide tip or you can attach a 1' point on that would give 2 options without needing two prisms. The CRP poles are even equiped with two level bubbles facing up and down.

Just keep track of your rod height and you are good to go!

I don't think I've seen a dual prism rod before.  What is the point of the bottom prism?  If it's to get under something it seems the rod would still be in the way.

> What is the point of the bottom prism?

From two prisms you can calculate the tilt magnitude and direction of the pole. This means the pole does not have to be plumb

You can make your own with a wrap of reflective tape near your pole tip (accurate, but need to allow for prism constants and pole radius) , or with two traverse prisms and a short pole spacer (easier calculation)

I see.  Sounds like it would be simpler to use some sort of peanut prism.

@cvt Theoretically that would work, but only if you could keep the prisms aligned on exact same vector from the point of interest.  Slightest of change in relationship of the points generates a fair amount of error.  And while it is possible to do so, it is not very feasible, and there's the rub.  Now you could mount a section of range pole to the top of the MPR122 and place a second prims on top.

I wonder why Trimble has not release a similar solution to the Ap20 that wants ts13 with total set up 40k too expensive.

αHidden routine in Trimble access can be used also for stake out?