Nodal Prisms

A few years ago, I replaced all my 0/30 prisms with the Seco -17.5mm nodal point prisms that you pictured. This was for use with my QS-5 robot.

I was finding it difficult sometimes on sideshots with limited accessibility (like a stone bound partly under a stone wall, for example) to accurately aim the prism at the robot, and with a robot, you don't have an instrument man that can tell you if it's pointed correctly.

The nodal point prisms are more forgiving. I like them.

Since Topsurv has problems with using different prism constants for bs/fs, I replaced all my prisms so they have the same constant.

Leica GMP111 (not GMP111-0) is what I use for max accuracy.
This has a -17.5 constant and can be picked up quite cheap 2nd hand.
I sharpen the points using a drill press and angle grinder as well.

For the S6 you can do all of your higher accuracy work with the mini. If you're so far away that it won't track/aim then you're past the limit for accurate work, anyway.

The mini is smaller so should mean less inherent error. Also it weighs less and can take shots in tighter places. Once I started using mini prisms and doing resections (with properly placed/shaped control of course) I was doing tighter work by accident than the competition could do on purpose. 🙂

Thanks G, S, and PB. The mini will definitely be easier to carry, and I can easily switch back and forth between laying out footings with the 360 prism and changing to the nodal mini for bolt patterns.

Dave

Jim,

I've seen prisms like that one, on eBay for dirt cheap prices from China. But I have to wonder about quality. Leica crows about its "higher than anyone else" manufacturing standards.

Here's what I wonder. If a prism doesn't say "Nodal", then is it? I mean, if it's 25mm in diameter and -17.5mm offset, is it automatically "Nodal", no matter who manufactured it?

Dave

Dave,
I survey for a concrete construction company as well. I have been using a trimble rts 600 for about 2 years now. I use a 360 prism almost always. With a marker I marked arrows on top to show where each prism center is. If I am doing anchor bolts I try to point the prism directly to the robot (that's just my logic though...). I am most concerned with ha and va collimation errors and performing the autolock collimation test.

Stupid question.... but have you ever tried rotating your rod about 30 degrees at a time and monitoring the distance and angle? I was getting a difference of about 3/16" and thought it was my rod...I checked my rod using the "true plumb" and then checked it by taking readings in DR and sighting the rod through the scope. The rod was good....lead me to believe I had to perform the collimation test.

I have yet to do one, but intend to do one on Friday (site is not working).

I am a bit confused by the purpose of a nodal point if the instrument has the autolock feature. I have read various conversation on this forum and am still foggy on the topic. (This is actually my first post on the forum).

I still keep my Nikon total station with mini prism in the truck...I have a hard time setting control points with the robot. I think for what its worth, I just grab my old layout partner, a carpenter on site, and set the point with the mini prism.

> I've seen prisms like that one, on eBay for dirt cheap prices from China. But I have to wonder about quality. Leica crows about its "higher than anyone else" manufacturing standards.

Those pictured are Leica prisms, the 3000 series. I don't know if they're of the same measurement quality as the 5000 series, but the plastic construction suggests to me that they're not. I would expect even lower quality from the Chinese knockoffs.

The only reason I posted that animated GIF is because I was at the Leica site and noticed that going between the images of the two prisms -- each with a different offset -- left everything but the glass and its immediate mount stayed in the same place. So I combined them into a single GIF and posted it for fun.

I use the Leica GMP101 for almost everything. I have a GRZ4, but it's way too heavy to haul around all day. Most of my lock loss is due to obstruction passing rather than the gun losing me, so I'm much happier with the smaller, lighter GMP101. I've found the ATR pointing accuracy to it to be excellent, and when I really want to bear down I use it with a mini "pole" with a height of about 0.4'.

Also, I have viewed the thread "Trimble s6 pointing accuracy". The thread doesn't really tackle the question of whether or not a nodal point prism is necessary for robotic layout. Going back to the thread I have to agree with broken lath.....I have reproducible results, with what I consider high precision with my passive 360 prism.

Not to go off subject, but i do sometimes have issues with elevation, some setups my elevations work out perfect...but sometimes they are off by as much as 3/4". On buildings with leveling plates I like to go behind the guys after the job to see how they did....I noticed some plates were showing to be off...I then checked with my level and they were right on, I pegged my level to make sure.

Doka,

"...have you ever tried rotating your rod about 30 degrees at a time and monitoring the distance and angle?"

My buddy Carlos did that with an S6 and MT1000, and he was disturbed by the results. Shots varied by .02' to .06' off of design, both horizontal and vertical. His S6 was set up on a hilltop, with a steep down angle. That's why we're trying to figure out how to reduce pointing errors.

Dave

For tight work nodal prisms pay even bigger dividends with automatic target recognition.

>

Sweet gif action thar Jimbo.

There are a lot of misnomers about NODAL prisms, how they work and why different prisms claim to be nodal yet have differing offsets.
I suppose I could go into the history as to why and when NODAL prisms came into our industry, suffice to say I do have a lot of experience with them.

The benifits for a Nodal prisms is simple, the optical and mechanical center of the return wavelength of light coincide with each other, so no matter if it is horizontal or vertical there is no shift between the light entering and exiting the prism which results in a very accurate angle which the prism occupies. IT HAS NOTHING TO DO WITH DISTANCE.

Thus all the common nodal prisms on the market weather they be -40mm, -34mm, -17.5mm, -35mm have aligned the mechanical and optical axis over the pivot point. The offset is derived from the diameter, thickness of the glass, position within the housing, the housing on the axis and the speed of light as it travels through the glass. Once these calculations are computed the offset is the result. If you are using a zero offset glass you are potentially using the worst choice in offsets.

Someone mentioned that some of the 360 degree prisms on the market caused errors when shooting in steep vertical terrain and flat HZ shots when the instrument locks on a prism left/right of the center prism. Remember that most 360 deg. prisms are designed for topo shots, not traversing or precise angle measurements. We have had many people try to use these incorrectly and it continues to be an education issue. Thankfully manufactures are aware of this and many have changed their 360 prisms to give better HZ/VT accuracy, Wait until you all see what is coming from the wonderful folks at TRIMBLE and the MT1000 replacement!! it is awesome!

I love working in our industry, have seen many changes over the years and I am always amazed how the simple things often trip people up. 😉

PSCS,

"There are a lot of misnomers about NODAL prisms, how they work and why different prisms claim to be nodal yet have differing offsets."

I'd really like to know more about Nodal Prisms, and how to be able to pick out the best one. Any helpful links out there?

"Thus all the common nodal prisms on the market whether they be -40mm, -34mm, -17.5mm, -35mm have aligned the mechanical and optical axis over the pivot point."

So unless a manufacturer says a prism is Nodal, it isn't? What quantifies Nodality?

"Wait until you all see what is coming from the wonderful folks at TRIMBLE and the MT1000 replacement!! it is awesome!"

Are you sworn to secrecy, or can you share?

Dave

So here is the nodal test on any prism:

Set up a 0 (zero) offset prism on a rotating tribrach adaptor
Set the gun up around 30' away and put the cross hairs in the center of the prism, rotate the prism 10 to 20 degrees and see how much the center of the prism is offset over the point.

If the prism is a nodal prism the cross hairs will show very little movement from the center of the prism.

As for the MT1000 replacement it is called AT360. It was shown at Dimensions and will be released sometime in December / January.
Imagine a cylinder about 6" long and 2" in diameter made out of the reflective target material you see on the sticky targets. It is surrounded by a very tough glass to protect it. In the center of the cylinder there is a slit that goes around the circumference of the cylinder and has the diodes for tracking and channel, it is powered by the same R8 battery the MT1000 is.
Will have a list price around 2,395.00.
The advantage over the MT1000 will be a true zero for AZ angles over the 360 prisms and I was told that the vertical is much better as well. All in all if it preforms as good as it looks, it will be a great solution. Oh, yeah it is 1/2 the weight of the MT1000 🙂

PSCS,

"If the prism is a nodal prism the cross hairs will show very little movement from the center of the prism."

You mean I have to buy it first before I can test whether it's Nodal or not? And how Nodal does it have to be to qualify as being Nodal?

I'll keep an eye out for the new AT360, but at $2395 it'll be a hard sell.

Dave

I have tried rotating the MT1000 and measuring the deltas.
In passive mode I found up to about 4-5mm, same with the standard trimble 360.
In active mode I found zero. I think this is because it points to the strongest LED brightness.
For this reason you need to me careful that LEDs aren't obscured in this mode if you want accurate results.

I saw a tweeted picture of it from Trimble dimensions, I will be keen to test one out.

The prism constant tells you whether it's nodal or not. I think the paper you referred to originally explains. Some circular prisms have 2 positions so you can do the test above and measure the difference.
The AT360 is a bit different as it presents the same view as it rotates. The S6 will track the diodes anyway on this one. I don't think this one will have a passive mode.
Bit cheaper than the MT1000 anyway. Although the price for the MT1000 has recently dropped here, I imagine this is why.