total station prism

For accurate angles the instrument needs to be pointed to the center of the prism.?ÿ If you are using a auto-locking or robotic total station the your pointing needs may vary.?ÿ For my Leica robotic total stations, if the instrument can see the prism then it will move itself to the center of the prism.?ÿ In some cases what the instrument may not appear to be locked on the center of the prism but it is.

To get an accurate angle by manually sighting, you need to point at the pivot, not the center of the glass.?ÿ Targets are designed to be over the pivot and that is centered on your monument.

Basically, if the instrument is pointed so it can get a reading on the prism (and not some other reflection) the distance (not angle) will be pretty close.?ÿ Pointing to somewhere other than the center of, or slightly off the glass should not make a significant difference.?ÿ Depending on how the prism is mounted, its pointing at or off the instrument can make a measurable difference.?ÿ

A typical ~3 inch prism will give you a distance reading to an effective reflection point behind the rear apex of the glass because light moves slower in the glass than in air.?ÿ The EDM gives you a larger reading than the distance to the pivot (lined up with the rear point of the glass) which is what?ÿ you center over the monument.?ÿ Thus you get a -30 mm offset to apply to the basic reading.?ÿ With this mounting, the prism pointing is non-critical, and that is why it is used.?ÿ

If the prism is mounted so the effective reflection point is at the pivot when aimed at the instrument, you get zero offset.?ÿ The rear point of the glass is not over your monument, so aiming the prism away from the instrument may still get you a reading but it won't be as accurate.?ÿ The glass has moved further away from the instrument.

If you use Leica, they have a slightly different offset, and a different way of describing and accounting for it, but that's just a number game.?ÿ Don't mix Leica and non-Leica if you don't fully understand the difference.

So am I understanding you correctly that if I am using one with a 30mm offset than It will still be accurate, and if it anything other than that I will be getting some, albeit small, error? Because my reflector is a 0 mm offset.?ÿ

I like to see at least 50% of the glass and the center. It's best to clear the line of sight completely for control points.

Why not run a test?

Set up your instrument and reflector about 200 feet apart. Point the reflector directly at the instrument?ÿand take a measurement.

Then turn the reflector 10?ø at a time to the right and/or left and see what the differences are.

I suggest you might want a helper for this unless you like to walk a lot.

A tilting prism that can be pointed directly at the instrument is best.

I've seen results from people attempting to run 3D traverse with a fixed prism and the elevations always came out sloppy.

When I am using a prism on a tribrach, I use a 0.3ft stub pole that is just right to sight on for line.

My typical setup

Your -30mm prism should be good for traverse work. As far as if the gun can lock no matter what's in-between, I have a hard time with that! I've done multiple Traverse's with the -30mm and what I've seen is horizontally is more forgiving with minimal branch obstruction, where vertically your rolling the dice. I have found hundredths of error in a station with as little as one small branch in my line of site.?ÿ

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I recently found that Seco makes a prism assembly that looks very similar to the Nikon above (Seco 6400-10).?ÿ It can have an offset of 0mm, -30mm, or by removing a spacer on the rear of the prism it can be -40mm.?ÿ The -40mm offset makes it nodal.?ÿ

I did an experiment where I set the instrument and a traverse kit with the Seco 6400-10 installed on tripods approximately 100 ft. apart. I then measured the remote point by turning several rounds (S6 using autolock).?ÿ Once I'd established it's true position, I set the instrument to stake the remote point and began rotating and tilting the prism.?ÿ I found that even if I rotated horizontally and tilted vertically to the point where any additional movement would cause loss of lock, the errors were sub .01 ft in H and V.?ÿ

I now own several and won't use anything else for traversing.

This is interesting because I am finding something similar. Do you have underlying science behind why that would be, or is it just based on experience?

@moosesamuel?ÿ Someone here has posted a diagram before showing the science.?ÿ The gist is that with many common prism setups the center of the pole is not the same as the optical center of the prism (it can be in front or behind the center of the pole).?ÿ Rotating a prism like this will result in positions that are different than if you shot is straight on.?ÿ A nodal prism is on the other hand is set so that the center of the pole and the optical center of the prism are the same point.?ÿ This results in a prism that is nearly immune to errors from having the prism not pointed directly at the instrument.

I'm sure that there must be other considerations that come into play or we'd all be using nodal prisms for all our work.

Link to Illustration of Nodal prism

I use CST 0 os prisms and have experimented turning it so that it's not pointing at the instrument.?ÿ Both the target zero and the EDM distances vary by minuscule amounts.

I also check my prisms, tribrachs, and OP's pretty often, so most of my traverse error is human error.

Sorry what I meant to say was @Trimbleman "This is interesting because I am finding something similar. Do you have underlying science behind why that would be, or is it just based on experience?"