First off I??m not a surveyor and have no formal training but I use a robot for measurements and layout in the oilfield construction industry.
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Recently started using a Leica ICR-70 and the kit we purchased came with both the gmp-111 and also the gmp-111-0. ?ÿI know that the standard gmp-111 has a prism constant of 17.5mm and the the gmp-111-0 has a prism constant of 0.0mm.
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Are there any advantages to using one over the other? ?ÿWhen using the icon software and I select the standard Leica mini prism it automatically sets the prism constant to 17.5mm so I??m curious as to what advantages there are or situations where the 0.0mm prism constant is beneficial other than if you are switching between different prisms or fixed stations that also have a 0.0mm constant and you want to eliminate user error.
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In testing I??ve found that I??m getting the same data shooting reflectorless or with the gmp-111.
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I'm sure this is a very amateur question but want to learn as much as I can to become more efficient.
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Neil
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When shooting straight on to a prism, the choice doesn't matter so long as you are using the right constant for it.
When the prism isn't pointed directly at the instrument, there are small errors which are reduced by using a prism with a particular "nodal" mounting position. One of those will be more accurate (by a few mm) than the other if you can't maintain pointing. I'm guessing the 17.5 is closer to nodal but don't know for sure. If used with a robot that probably isn't a consideration, but it could be if it meant you had to keep running back to aim your backsight when you moved the setup of the instrument.
Here is a link to a .pdf of various Leica prisms:
It shows some info you may find useful.
The prism constant/prism offset is typically a horizontal distance measured from the plumb line (pole) over the point to the internal position measured at the plane of reflection inside the prism.
It can be: short of the plumb line of the pole, perfectly aligned with the plumb line of the pole, or farther than the plumb line of the pole, depending on the manufacturer, etc.
And like @bill93 says, the sighting errors for non-nodal prisms are higher than the sighting errors for nodal prisms. Nodal prisms stay aligned with the centerline of the plumb line even when they are rotated.
Thanks for the replies everyone, makes more sense now for sure. I’ll do a bit of digging and see which of the 2 prisms is more of a nodal unit and stick with that one since I’m mainly using a single prism along with reflectorless.
This thread prompted me to dust off and (I hope) improve my old writeup on prisms. Let me know if you see something wrong.
Dropbox File (Private or Invalid)
This thread prompted me to dust off and (I hope) improve my old writeup on prisms. Let me know if you see something wrong.
Dropbox File (Private or Invalid)
If you're not busy, I can give you some topics for "write-ups"...
And while reviewing, I found this "note to self" amusing:
DO A LOT OF ALGEBRA.
Nate s guide to prism offsets:
Set up 2 tripods. And tribrachs, 10 to 12 ft apart.
And close to level. Set it all up. Shoot the dist to the prism. Measure the distance with a box tape.
Adjust the offset accordingly.
I've only done this with standard prisms. Not those funky robot 360 kind.
But, I did find that not all prisms have the same offset. If you can key in the offset, make it an average of your prisms. As I recall, the average offset for us was 31.5 mm. (Supposed to be 30mm offset).
Whatever you do, do the Nate check.
It's rough, but it works!
N
The precise way to do it is to set up three tripods and tribrachs A, B, and C carefully in a straight line at the same height. Swap the prism and TS around to measure AB, BC, and AC distances.
Calculate: (AB+constant) + (BC+constant) - (AC+constant) = constant
@bill93 : That gives you the system constant.
True, it does not resolve between the instrument constant and prism constant. It is the number you should enter.
Does anybody pay attention to what their instrument constant is and adjust the number they enter accordingly?
