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# Prism constant question

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(@squowse)
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March 23, 2014 11:22 am

> 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.

(@paul-in-pa)
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March 23, 2014 12:43 pm

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

(@james-johnston)
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March 23, 2014 2:21 pm

Prism "Constant" Or "Offset" ?

:good:

James Johnston
Survey Technician
Field & Office

(@bill93)
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March 23, 2014 2:58 pm

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.

(@dave-karoly)
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March 25, 2014 8:04 am

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.

O death, where is thy sting? O grave, where is thy victory? -1 Corinthians 15:55

(@james-fleming)
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March 25, 2014 8:19 am

> Someone posted in the Leica world zero=-34.4mm

Foreigners 😉

“Civilization is not an endless succession of inventions and discoveries, but the task of ensuring that certain things last.”
— Nicolás Gómez Dávila

(@plumb-bill)
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March 25, 2014 9:34 am

I learned this lesson the same way you did, the hard way. This was several years ago, and I believe it was directly responsible for me developing my current "I don't believe you until I do/try/research/learn it for myself" attitude.

I was going on the say-so of someone I thought knew what they were talking about. I have since encountered numerous firms and crews using Leica hardware with a Seco prism glass (not that there's anything wrong with Seco prisms). Roughly 10% (just guessing) knew the exact correct offset to apply, the others had anything from zero (which only creates a 4.4 mm error, but still) some of the others had -30 set!

(@ssnider)
FNG Member
Joined: 6 years ago
Posts: 3
April 7, 2014 4:41 pm

Yes this will effect the measurements. ATR (Automatic Target Recognition) was designed to find and acquire a prism in a passive mode. There is a white page available on the AT401 unit (Leica's industrial Measuring Total Station) that provides more detailed information.

In short, ATR was designed to find specific prisms quickly and adjust accordingly. If you enable ATR and set the prism correctly, powersearch you will notice the crosshairs slightly offset (upper left quadrant). when you press "ALL/MEAS" or "DIST" the cross hairs will center on the prism....NOTE the Angles will not changes because of the ATR applying the correct offsets for the CORRECT Prism selected. If you do not have the correct prism selected, the wrong offsets will be applied.

Go into Leica Geo Office and change the prism to reflect the correct prism shot and recalc the setup.

(@survey-sue)
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December 5, 2018 12:29 pm

Most every prism I’ve ever seen can be changed to have two different constants.  And my guys are like kids in candy stores with new equipment . Even my little peanut can be zero or another offset like 30mm.  Just set up on a few good tight cp’s and see what it gives you. Also it does not come from factory all set for your particular prism and constant.  There are millions to choose from. Most extreme I’ve seen was +50 and -60. Not the same prism.  Two different ones on same day

(@crashbox)
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December 6, 2018 4:07 am

The Leica 0.0mm = -34.4mm thing is frustrating and nonsensical IMO. Ideally, Leica should either state THEIR prism constant equation of 0.0mm = -34.4mm on their glass, or go with the norm.

A couple of years ago we were tying out a number of mon's on I-5 south of Bellingham, and we borrowed a second crew from another office for the evening. The PC had a Topcon gun but Leica glass (we have Leica everything), and I asked him what he had set for a prism constant in his gun. He replied "-30", apparently unaware that Leica glass had a different prism constant. I was able to show in Star*Net that by applying the additional -4.4mm, the results improved quite significantly. He's held -34.4mm ever since as far as I know.

There's just NO need whatsoever for Leica or anyone else, to muddy the waters when precision and accuracy are paramount.

The only superior evidence is that which you have not yet found.

(@jph)
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December 6, 2018 5:33 am

We did the same thing, years ago.  I was a fairly young party chief and believed it when they told me what the offset was.  Funny thing is we laid out more than a few major large buildings and additions using the -30mm os, and the steel all went up and fit together fine.

(@plumb-bill)
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December 6, 2018 8:47 am
Posted by: Jim Frame

> 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:

I'm not seeing the graphic?  How are they not the same if both "0"?

(@plumb-bill)
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December 6, 2018 8:52 am
Posted by: A. Sellman

The Leica 0.0mm = -34.4mm thing is frustrating and nonsensical IMO. Ideally, Leica should either state THEIR prism constant equation of 0.0mm = -34.4mm on their glass, or go with the norm.

A couple of years ago we were tying out a number of mon's on I-5 south of Bellingham, and we borrowed a second crew from another office for the evening. The PC had a Topcon gun but Leica glass (we have Leica everything), and I asked him what he had set for a prism constant in his gun. He replied "-30", apparently unaware that Leica glass had a different prism constant. I was able to show in Star*Net that by applying the additional -4.4mm, the results improved quite significantly. He's held -34.4mm ever since as far as I know.

There's just NO need whatsoever for Leica or anyone else, to muddy the waters when precision and accuracy are paramount.

I agree, but there's a fun little history lesson here - if what I was told is correct.  Story was related to me that everyone "copied Wild's tribrach" design so that things would be interchangeable.  Then later the tilting prism was somewhat generically adopted to match the vertical offset of the old top-mount EDMs.  Since everything tended to gravitate towards interchangeability - Wild declared their prism constant of -34.4mm to be "zero" becuause it directly offset the EDM constant (because of it being mounted and/or machined in such a way that it was a +34.4mm constant).

Other manufacturers, probably rightly so, determined that EDM technology would change - and that using the absolute constant was the preferable route.  Both make sense depending on the viewpoint, but this is why there's always been two versions of offsets for Leica glass.

(@plumb-bill)
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December 6, 2018 9:18 am
Posted by: Plumb Bill
Posted by: Jim Frame

> 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:

I'm not seeing the graphic?  How are they not the same if both "0"?

Funny, nevermind.  I just realized this a pretty old thread!  Probably got lost in an update.

(@jmh4825)
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Posts: 43
December 6, 2018 10:31 am

I'm fairly new to robotics, with that said, is it poor practice to use a circular prism (large or small) while using a robot?  I'm not concerned about distance offsets,  I'm concerned with angular error due to pointing at different prisms.  I have used a circular prism as a BS and a 360 on a FS to turn sets just because I only own one 360 prism.  The BS prism is always pointing directly at the robot.  Will the robot not center in on the prism correctly even if the correct prism is set?

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