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Prism constant question  

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James Johnston
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Leica manufactures a mini prism with 0mm constant and a large prism with 0mm constant. Crew used large prism on some of its layout shots but kept the mini prism selection in the TPS configuration. Could this have introduced a systematic error in the measurements as the ATR system was configured for a mini prism but shot a large one. I know they are both 0mm constant, so should be okay, but I am not 100% sure. Access to site is not possible.

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Survey Sue
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FNG
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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

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Dave Ingram
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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.

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party chef
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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.

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Jimmy Cleveland
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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

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Bill93
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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.

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