Leica Mini Prism used as a peanut prism constant?

I'd first do some physical measurements while the prism is in the holder - easiest way would probably be to put it at its lowest possible rod height.

Place the rod tip on the floor with the back point on the prism up against a vertical wall, with the prism facing horizontally and the rod plumb. Mark the rod point on the floor and measure from there over to the wall face. Add that value to the +17.5mm offset and that should be the adjusted offset to use.

Then you could test it out by setting up a TS, and marking a point on a wall (making sure angle of incidence is 90 degrees H/V), then measuring it reflectorless and with the adjusted prism offset. If they are matching within tolerance, you're good to go.

I guess you could do the TS/wall setup to begin with, measure reflectorless and just adjust the constant by trial & error too. But I always like verification if possible.

Leica prism constants always mess with my head.?ÿ Mark a point on a wall, and take a reflectorless shot.?ÿ Then use the prism with whatever constant you think.?ÿ See how they compare, and adjust accordingly.

Hopefully, when you find the right one, it'll make sense with the stamped prism constant numbers

The prism constant is going to remain 17.5 mm.?ÿ This is a physical characteristic of the prism's reflective facets in relation to the path of the light beam both in and out.?ÿ What you are looking for is the offset from the prisms datum surface to the tip of the point on the back of the prism.?ÿ For the GMP-101 that is 20 mm, I'm not sure about the GMP-111 since my diagram does not show that it has a point.?ÿ I think the datum surface is the back of the prism casing at the larger diameter on the 101.?ÿ I'll check when I get back in the office.

Thanks guys for the help!

@john-putnam made a great point about the constant being a physical characteristic of the prism, but more importantly the offset also in my case.

I'll keep digging around to find out this offset.?ÿ I guess it would have to be added to the constant also.?ÿ Clear as mud. ; )

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On the GMP-101 the reference datum is the is the front side of the non-threaded ~1 mm notch that the target fits over and is coincident with the center of the mount's vertical pivot axis.

The prism constant is going to remain 17.5 mm.?ÿ This is a physical characteristic of the prism's reflective facets in relation to the path of the light beam both in and out.?ÿ

Hmm?

The GMP111 has the +17.5mm constant. (orange forward protruding mount)

The GMP111-0 has the 0mm constant. (black rearward set in mount)

They are both the same prisms, but different mounts, the 0mm constant GMP111-0 is set back further than the GMP-111 in the same orange holder.

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Seems the distance forward/ backward determine the total constant?

Looks like I may be back to the initial suggestions.?ÿ I.e. in order the find the constant of a target if nothing stamped on it or unknown.?ÿ Guess I could also just shoot to a known prism in a tribrach, then swap out with unknown and shoot for the difference.

Here is a catalog for some general prism offsets/errors information and when using Leica Prism with Trimble Station page 3-6.

https://bohnenstingl.de/wp-content/uploads/2022/10/Bohnenstingl_Surveying_Catalog_2023_221016.pdf

There is a Mini Prism MP24 that looks similar like on GPM-111 Miniprism at Page 31 but i dont know if it is the same.

Also there are offset spikes to get accurate 5 or 10 mm offset when using mini prism at?ÿ page 34.

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But easy way would be a test measurement like @Rover83 and @JPH said.

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Some other information for leica Prism here:

https://leica-geosystems.com/-/media/files/leicageosystems/products/white-papers/leica_surveying_reflectors-_wp.ashx?la=en-us&hash=955F4596E3E0B8AE9A546BB467354082

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Like @John Putnam said for GMP-101 it is 20 mm but for GPM-111 it doesn??t show in PDF.

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I am getting error in latest Fieldgenius also after creating new user target.

As far as the FieldGenius error I was getting with their demo when trying to create a user target, I figured it out.?ÿ Just a simple settings.xml edit. (ProgramDataMicroSurveyFieldGenius11)

Just FIND the user target made (i.e. test) change 0 to 1:

<Target Name="test" Manufacturer="1">

VOILA!

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@john-putnam?ÿ

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FWIW

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https://geomatics.cc/articles/prism-constants-in-survey-targets-explained-259#Prism_constant_vs_prism_offset

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mostly for the table attached for many prisms.

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carry on.

Iƒ??ve always heard that Leica prisms were odd constant but I never worked with one. Iƒ??m used to the peanuts that are 0 offset at the point on the back of the prism.?ÿ

@on_point

Iƒ??ve always heard that Leica prisms were odd constant but I never worked with one.

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It's not really the constants that are odd, but the fact that the "official Leica" offset numbers are equal to the true offset plus 34.4mm.

So when Leica literature or datasheets tell you that a particular Leica prism is 0mm, that means that if you are using any software other than Leica software, you must enter -34.4mm as the offset. Because that is the actual, true, real offset. But Leica doesn't tell you that.

I worked with Leica for the first 5 years of my career, never got told this little nugget of knowledge, and promptly screwed things up when I switched to a different employer, who mixed and matched gear.

So I wonder after removing the prism (peanut prism) from the GMP111 holder (+17.5mm Leica constant), that the sharp point of the rear of the peanut prism is "meant to be used" for placing into corners maybe??ÿ So maybe the constant as @john-putnam mentions, is ALWAYS +17.5mm when removed from holder or when used with GMP111 model holder (GMP111-0).

The same goes for when using the GMP111-0 0mm??ÿ But when prism peanut REMOVED from holder the prism peanut itself is "constant" +17.5mm Leica constant. (plus 20mm (pivot to rear spike))

I guess what I am getting at is "depending on the swivel holder used" (GMP111 orange or GMP111-0 black), when prism is secured in either holder, determines the constant to be used.... until it is removed it is +17.5mm? (plus 20mm (pivot to rear spike))

Maybe the peanut prism when used by itself is in fact initially for inserting into corners?

When I can, I will be following @rover83 and @jph advice also as if process for trying to determine constant for an unknown prism constant.

I don't have a GMP101, but have the GMP111 (+17.5mm Leica constant).

But it appears the removable prism peanut via large thread is the same used in all 3 models which going to "assume" is +17.5mm Leica offset WHEN REMOVED AND USED BY ITSELF. I.e. corners.

GMP101 (17.5mm Leica constant when used in holder)(20mm offset shown in diagram)

GMP111 (17.5mm Leica constant when used in orange swivel holder)

GMP111-0 (0mm Leica constant when used in black swivel holder)

EDIT: plus 20mm (pivot to rear spike)

@timd1971?ÿ

It isn't complicated.?ÿ Take the specified constant in the?ÿ normal position. That gives measurements to the vertical axis of the plumbing point.?ÿ Measure from the plumbing axis to the end of the projection behind the prism.?ÿ Add algebraically that distance to the usual prism constant and use that value when held against a wall, etc.

Prisms ƒ?› Bohnenstingl GmbH - Special Surveying Equipment

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There are tools designed especially for such tasks.

I usually just use RL or DR as Trimble likes to call it.

If in fact the GMP111-0 uses the same prism, then you would specify the 17.5 mm prism constant and add a 20 mm distance offset to the measurement.?ÿ The 20 mm for the GMP101 is from the from reference plane (the pivot point).

@john-putnam?ÿ

Thank you very much John.

If in fact the GMP111-0 uses the same prism, then you would specify the 17.5 mm prism constant and add a 20 mm distance offset to the measurement.?ÿ The 20 mm for the GMP101 is from the from reference plane (the pivot point).

Problem with that is that the alignment has to perfect (corner-instrument). For these reasons i shoot all of these things reflectorless through an intersection of 2 lines.?ÿ

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