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# Instrument Constant question

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Posts: 5519
Member
(@bill93)
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Joined: 9 years ago

I don't have wide experience, but I thought that Leica was the only one to hide the constants and pretend they are zero. My old old Topcon needs to be set to -30 for common prisms.

How many prisms did you use in your test, and if they are reconfigurable, do they appear to be set in the nearer or further position? Misinterpreting the prisms could confuse the results.

You might try one of those red reflectors that look like a taillight, that people set beside their driveway to help find it at night. Some brands are good past a hundred yards with my old instrument. They will have a very small prism constant, and give you a way to determine by comparison whether your prisms are set to 0 or -30 position.

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(@dave-karoly)
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The classic test is set up three tripods/tribrachs (A, B & C) in line with A to B being about 50' and B to C about 50' and B on line between A and C.

Put the instrument in tribrach A and prism targets in B and C.

Measure the distance to B and C.

Put the instrument in Tribrach B and prism targets in A and C and measure the distance to A and C.

A-B + B-C should equal A-C. If not then you have your prism constant is Not set correctly.

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(@kent-mcmillan)
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> I'm checking the instrument constant of my ancient TS (AC+BC-AB).
>
> AC is 139.035
> CB is 124.308
> AB is 263.344

> (All three measured 6 x 3(instrument averages)for a total of 18 measurements each.
>
> That = -.0010'
>
> The Instrument Constant set in the instrument is -31.1mm. That's also written on the sticker inside the battery compartment. The Prism constant is currently set to -30mm.
>
> -.0010', which is -.3048 mm certainly seems fine to me, but I think I've read that if you're using Topcon prisms (I am) with a Topcon TS, the prism constant would be set to zero in the instrument.

Why not just examine the prisms to see whether they are -30mm offset prisms or not? It should be marked on the prism assembly somewhere. Otherwise, take a closeup photo of the prism from which it can be identified and post it. 0mm offset prisms have the nodal point of the prism forward of the centerline of the prism assembly mount which a sideview should show well enough.

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(@thebionicman)
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The Standard ABC test is incapable of determining the instrument constant unless you use a correctly identified prism. It is also imperative to use the three tribrachs to get the most benefit. You all but eliminate centering errors...

Posts: 102
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(@rundatline)
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Joined: 7 years ago

That seems too high for an instrument constant. Perhaps someone entered the prism offset in the wrong place? If you are using a prism that is -30 then the prism offset in the topcon gun should be -30 (nikon +30) unless you have the controller(dc) offset the difference. If in doubt as suggested above tape distance between 2 points on flat surface such as a parking lot and check it out.

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