Prism constant question

Don't know the answer to your question, but the simple thing to do is take them outside and measure the same distance with both prisms.

Zero equals zero regardless of what other attributes or names are attached to it.

To reassure yourself you could check the gsi, or, as suggested already take a breath of fresh air and a couple of shots.

James,

It has been about 7 years since I have used Leica equipment, so I am going off of memory.

I seem to remember that Leica 0mm offset settings in the software actually have a (-34mm offset) physically. I would think that if you used a Leica mini prism with a zero offset setting, and then used a larger Leica prism with the same setting, you SHOULD be okay.

I second what Dave said. I would take the instrument outside, and set two points a known distance apart that you can measure with a tape. Shoot the distance with the mini prism, and then shoot the same distance with the larger prism. Compare the results. They should be the same. If they are not, you should have the constant needed to adjust the shots.

As I mentioned, it has been a long time since I have used Leica gear, so please double check the information I have stated regarding the Leica prism offsets.

I used the same method above when checking my prisms before starting a very large construction project. We used zero offsets on that entire project.

Good luck, and please let us know how things go.

Jimmy

Since Leica decided to hide the prism constant in the instrument settings, thus making true 34 (-34.5?) appear to be zero, I wouldn't make any assumptions about the small prism. If they have a selection for it, it would be most likely they have hidden a different number for it.

As suggested, you need to test it. Set up two points and measure them with a trusted configuration, and then try all the other combinations of setting and prism.

Thank you folks - will test Monday.

I think the distance component of the measurements will be okay, both prism having the same 0mm constant. My thoughts are more about the ATR technology. Does the automated system react differently in the angle observed based on the selection of a particular prism? The system uses an offset calculation to center of prism.

You do not need a known distance to determine the prism offset (actually combined instrument and prism, but typically called the prism offset, as the instrument should already have the instrument constant applied).

Set three co-linear points (A, B, and C), line does not need to be long. Place B about halfway. Set the instrument for a zero offset. Shoot AB, BC, and AC. Then

offset=AC-(AB+BC)

If you have more than three points on the line, you can solve by least squares and get a more accurate determination.

> I know they are both 0mm constant, so should be okay, but I am not 100% sure.

They're not the same. I put together this graphic to help me manage my prisms:

Thank you for the picture, very handy.

In this particular case, the mini prism used was a GMP111-0.

Leica GMP111-0 (0mm)
Leica GMP111 (+17.5mm)

Both prisms are part of the same kit which contain 4 extensions of 0.3m; for five fixed total HI possibility: 0.1m 0.4m 0.7m 1.0m 1.3m

There are 2 Leica mini prisms that look very similar/identical but have different prism constants.
1) GMP111-0 (current model) has a Leica 0.0 constant (so -34 on other instruments)

2) GMP111 (older model) has a Leica +17.5 constant (so -17 on other instruments)

I suspect the mini-prism setting on your instrument may match the second type. You will need to get into the prism settings on it to check this.
To complicate matters further (although the idea is to simplify I'm sure) - on newer Leica instruments like the TS06 you will find the "absolute prism constant" specified as well. This is the same as other manufacturers use. It can't be set independently, it just changes with the "prism constant".

ah - cross-posted I see you've got the GMP111-0 so you need to check that the prism settings in the instrument aren't for the older GMP111. I have been caught out exactly this way with using "mini-prism" setting on the imstrument but with a newer GMP111-0 with zero constant that I was not aware of. Caught it on the first backsight luckily, and used "leica round prism" setting for it after that. That was on a TCR805 I think so may be updated on newer models.

> > I know they are both 0mm constant, so should be okay, but I am not 100% sure.
>
> They're not the same. I put together this graphic to help me manage my prisms:
>
>

used another company's GMP101 the other day and found prism constant errors of about 2mm.
we were using it on 10no. spigots mounted on the walls of a tunnel. When I free-stationed from a different position, the prism positions that were being observed from the opposite side to the original observation gave errors 3-4mm. (basically the same test as described above). I was using a constant of -17 on a trimble instrument. (17.5-34.4).
Swapped back to using leica circular prisms and no problems - tight freestations from all positions.

As the GMP101 is meant to be the most accurate mini prism in the range I was surprised and wonder what I was doing wrong.

> I suspect the mini-prism setting on your instrument may match the second type.

We already checked that on TPS1200 and confirmed in LGO raw data. The correct constant was used. My unresolved question was more in regards of the ATR and how different sizes prisms affect its automation.

Unlike the Leica round prism the GMP111-0 isn't a nodal point prism so you would get small pointing errors if the miniprism wasn't perfectly facing the instrument. At steep vertical angles there will be a (tiny) level error with prisms that aren't nodal even if they are pointed correctly.

I don't see that it could affect the ATR centring.

> In this particular case, the mini prism used was a GMP111-0.

I wasn't aware of that model. Good to know.

I never did care for Leica's decision to hide the true offset from the user. It's been a source of confusion for anyone who steps outside of Leica's walled garden.

I was thinking along the line of the ATR using the radius of the prism in its calculation to adjust to true center. If the instrument is configured for a large prism / large radius and a small one is used, any effects on the computation?

Anyway, mm stuff which will not create a problem on this here job, just good to know for future reference.

> You do not need a known distance to determine the prism offset (actually combined instrument and prism, but typically called the prism offset, as the instrument should already have the instrument constant applied).
>
> Set three co-linear points (A, B, and C), line does not need to be long. Place B about halfway. Set the instrument for a zero offset. Shoot AB, BC, and AC. Then
>
> offset=AC-(AB+BC)
>
> If you have more than three points on the line, you can solve by least squares and get a more accurate determination.

I think the point with this test is that the instrument is set in 2 different places, once inbetween the two points A and C (ie point B) and once outside of the points (point D?).
Points B and D don't need to be marked but should be close to a line running through A and B.

Having re-read your post I am sure we are describing the same thing.

Prism "Constant" Or "Offset" ?

As I recall the prism constant has to do with the speed of light within the prism. Leica "glass?" is different.

Prism offset has to do with the measured prism return distance versus the prism/rod physical center.

Paul in PA

Prism "Constant" Or "Offset" ?

Primer on Prism Constants

:good:

Prism "Constant" Or "Offset" ?

Leica glass material is not necessarily much different. It's their method of labeling that is different.

The speed of light comes into play because any glass is slower than vacuum or air. The travel time from the front face of a typical prism to its rear tip and back to the face is about the same as for the light to travel 30 or 34 mm in air. So to get a 0 offset prism assembly they mount it with the rear tip 30 or 34 mm in front of the mounting axis. Then the effective reflection point is over the pivot.

But that means when you swivel the prism so it doesn't point directly at the instrument you get errors. To alleviate that, the alternative is to mount the glass with the rear tip on the pivot axis and correct for the ~30 mm travel time in the instrument.

Leica does the same thing but relabels it as 0 mm relative to their standard instead of the absolute offset.

We wrote the actual, true offset (Prism Constant) on our Leica Prisms.

We wrote -34.4mm on our Leica circular prisms, what Leica calls zero.

We wrote -11.3mm on our Leica 360 which has a Leica label marked +23.1mm (go figure).

Then we created targets in the Trimble Access software which uses the actual prism constant (that is just nuts, I mean why on Earth wouldn't you make 0=-34.4mm???)

Some sarcasm herein, folks.

I had my first experience with Leica at my previous job about 12 years ago. We went out and ran a simple 4 sided traverse and it didn't close worth spit. I thought, this Leica gear is JUNK!!! The previous generation had purchased a Leica TCA1102+ total station but no Lecia prisms because they had ancient AGA zero offset prisms(only Surveyors would purchase a Maserati but go to the junkyard and get wheels off of a wrecked Ford Pinto to save money). They assumed and the Leica dealer (the gawl derned dealer) confirmed they could just use zero offset in the Leica instrument which entered a 12 hundredths error into every distance measurement. I posted about it on RPLS, why in the heck is this simple traverse not closing? Someone posted in the Leica world zero=-34.4mm and here's how you test it (the simple A-B-C test above). Well my reaction was what the swearword? After that the results recalculated were much better, to say the least.