I??m seeing different views regarding using the vertical tilt action on a mini prisms to make it look directly at the total station (I.e along the slope angle) vs keeping it horizontal at all times and not adjusting the vertical tilt action.
Would someone who knows more about prisms give the correct method for using the kit please. I??m more interested in the level accuracy of when it is tilted down to meet the slope angle , wouldn??t the target height and RL be affected. Thanks
Ideally, you should be sighting at the target surrounding the prism so that pointing the prism isn't an issue for H or V angles.
A cube corner prism has an apparent reflection point that is further back than the physical rear point of the glass due to slower light propagation than in air.
There are different ways to mount the prism in the holder that trade off affecting the apparent distance by its pointing and affecting the apparent visual center,
The prism may be mounted so the physical rear vertex of the glass is over the point being measured and aiming of the prism is non-critical. This requires a correction for distance measurements, typically -17.5 mm for mini prisms, -30mm for bigger ones. Leica measures from a different point than about everyone else.
If the design is precisely adjusted for no dependence on pointing way, it is called a modal mounting.
The mounting may be so that virtual point is over the point being measured and there is 0 offset in the distance measurement (ignoring Leica definition). Then pointing of the prism has a noticeable effect on the visual center and a small effect on the distance measurement.
For a mini prism you need to check how it is mounted in the assembly. It may be set up for a common offset like 0 or 30mm and not a nodal mounting for its smaller size.
I hope I got that all correct.
I like to work in the same settings all the time when using prisms where all of them are -30mm offset and I have many different makes of prisms and targets and all pivot at the axis of the prism except the retro prism assemblies and I only pull them out for shooting to points that are over several thousands of feet apart as they can be put together in arrays to maximize the reflection.
thanks , I have attached an image to better get across what I mean, should the prism be pivoted to align with the slope angle of the total station shot or left un-adjusted?
@picklespieandsauce for short distance ~20m I normally pivot it upwards. for >20m I would normally just leave it as it is.
@picklespieandsauce Yes pivot, that is what it is there for.
I know if you don't sweat the little stuff, the big stuff suffers.?ÿ But aren't we really splitting hairs here??ÿ Using that device in the picture, you get more error from the shaky rod man than you would from a non-tilted prism
I know if you don't sweat the little stuff, the big stuff suffers.?ÿ But aren't we really splitting hairs here??ÿ Using that device in the picture, you get more error from the shaky rod man than you would from a non-tilted prism
Agreed. If you are running a mini prism for topo work on a handheld rod, the error from pointing or not pointing the prism will almost certainly be lost in the centering error for each observation. As some said upthread, we should be sighting the target plate rather than the prism line intersections anyways.
However, if we are talking traversing or backsights, then that's a whole different kettle of fish. Then best practices would dictate using a nodal prism and tilting it appropriately.?ÿ
I??m more interested in the level accuracy of when it is tilted down to meet the slope angle , wouldn??t the target height and RL be affected. Thanks
Not if the prism is a nodal prism. This video offers a good explanation of why they are likely what you are looking for.
With the exception of 360 prisms and a very few of our mini prisms, all of ours are nodal. It makes a big, big difference when running tight traverses in tricky terrain.
I've used this type mini-prisms for years. (I guess decades even...)
that video link posted by Rover83 reminded of a couple things. Thanks.
Leica has an interesting white paper on prisms.?ÿ In it they proport that for control work you should be within 10 degrees to perpendicular to the line of site.?ÿ In the case of the GPR1 your pointing error is ~1mm at 40 degrees.
?ÿThat being said, whenever possible I try to get the prism pointed in the general direction of the instrument.
Excellent read right there, nice to see them quantify the effects of particular title angles.
When I worked in the flatlands, I hardly ever tilted the prism up or down, and it likely never made a significant difference. Nowadays is a different story.
For our crews, we tell them to always tilt the prism toward the instrument, because it's just best practices...and because Murphy's law says if we told them to only do it in extreme terrain, they would forget and that would be the one job where we really needed that accuracy.
@picklespieandsauce that prism (GMP111-0) needs to be tilted to get the true height when using autolock at least. If manually aiming then sight the cross hairs. A nodal prism like the GMP111 is much more forgiving if the prism is not orientated correctly
Why is it that nobody makes a -30 offset nodal prism?
Maybe I am looking at this wrong but it seems to me that if a 62mm prism has a nodal point at -40mm offset and a 25mm mini prism has a nodal point at -17.5mm offset, then a prism that is somewhere about 45mm in diameter should have a nodal point that is -30mm offset.
Am I simplifying this too much?
James
@jaro Makes sense to me, but it would take someone who understands optics to tell us if it is just convention or optical considerations that have resulted in the common sizes of retro prisms that we find available.
Maybe I shoulda gotten a patent on it before I asked the question ?????ÿ
When I was at Purdue in the mid 80's my geodesy professor was Lassi Kivioja. He was big into EDM's and in fact had a Kern ME-3000 Mekometer, which was one of the most accurate EDM's ever produced. In 1981 he prepared a technical report on research he had done on this exact topic.?ÿ
Am I simplifying this too much?
Only a little bit. The offset to make it nodal depends proportionally on the size of the prism and also somewhat on the index of refraction of the glass, which doesn't vary greatly among usual glasses.
@john-hamilton Thanks for sharing that report; it's been a tough one to track down.
?ÿtechnical report
That's not easy reading, but what I've waded through really opened my eyes to several complications. I'd guess that few other than NGS, John, and Kent ever do the corrections.
I didn't find clearly stated a principle I think the analysis rests on: all rays entering the prism at a given angle will have the same total path length after bouncing off multiple walls to exit. Thus we can carry out the analysis most easily by using the ray that strikes the physical apex of the glass.
Agreed?