Activity Feed › Discussion Forums › Strictly Surveying › Mini prism school of thought (Help)
Just an idea. Set up in a small area, where you can use an autolevel, and a box tape. Attach prism to floor. With a tripod over it. Take your time. And, do the best you can with it all. I suspect you will find “your own constants” for that prism. I’m not being funny. We did this long ago, to determine prism constants. It was eye opening. One prism was some 0.018′ different than the rest.
It is not as accurate as a lab test, (box tape and all)
But, it does let you monkey with pointing the inst, and it lets YOU do your own testing.
Since its your licence, this will improve you. Jmo (just my opinion) take with a bit of salt.
N
- Posted by: @nate-the-surveyor
Just an idea. Set up in a small area, where you can use an autolevel, and a box tape. Attach prism to floor. With a tripod over it.
It is not as accurate as a lab test, (box tape and all)
Set 3 points in a straight line. Put the TS and prism with well-calibrated tribrachs on tripods over two points at a time, and adjust the tripods to so the line of sight is horizontal. Set the EDM to 0 offset. Measure AB, AC, and BC.
Since AB+BC=AC in truth, and you have included the prism constant twice, the prism constant can be calculated as
MeasuredAB + MeasuredBC – MeasuredAC = contant
(AB+const) +(BC+const) – (AC+const) = const
. - Posted by: @bill93
The more I study this document, the more confused I get. Has anyone else tried to figure it out?
It seems Figure 1 assumes the theodolite is sighting the image of the prism apex (intersection of lines), seen through A2. Sighting a target frame around the prism, plumbed over the monument, would simplify the situation, allow accurate sights at greater distances, and give less vertical error. When we get to tilting the prism (which I haven’t yet) it would be MUCH simpler.
In Figure 1 and the equations on (original, not PDF) page 6, with sighting to the image, he gets the slant distance to point S. Then his horizontal offset calculation using the angle of incidence (i) and that hypotenuse may be right. But neither S nor S2 is a physical point that anyone will measure to for height above the ground point. The recorded height is probably to O or Pn, and that leads to a LOT of vertical error.
. - Posted by: @bill93
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.
I have to withdraw that statement as it is not universal. I have been looking at some mini prism pictures (both Leica and Chinese) and notice that some of them have target marks that are NOT aligned with the plumbing point.
For these, you may indeed be better off sighting the image of the prism vertex, the intersection of lines in the reflection. Or not?
With all the variations, you may just have to do some testing on what you have.
.
Log in to reply.