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# Prism constant question

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Posts: 632
Member
(@james-johnston)
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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.

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(@john-hamilton)
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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.

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(@jim-frame)
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> 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:

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Member
(@plumb-bill)
Joined: 9 years ago

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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"?

Member
(@plumb-bill)
Joined: 9 years ago

1,000+ posts
Posts: 1429
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.

Posts: 632
Member
(@james-johnston)
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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

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(@squowse)
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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.

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