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Offset of CST/berger universal prisms

Offset of CST/berger universal prisms

(OP)
We recently found that some new prisms we bought (CST/berger Universal Series Prisms, model 63-2010) don't have the same offset as another 0/-30 prism that we own (a mini-prism, also by CST/berger). The new prisms seem to have offsets of +7/-23 instead of 0/-30 (relative to the mini-prism - which I trust more based on comparison to another prism). I've tested 8 of the 63-2010's and get the same result with all 8.

I performed the test by setting a pole on a tripod about 12 m from our total station so that the total station and prism were about at the same height. I sighted in the mini-prism and set the 0 azimuth pointing directly at the prism. I then switched the total station into NEZ mode and recorded the position of the prism in both 0 and -30 offsets positions. I then carefully removed the prism from the pole so as not to move the tripod and replaced it with one of the 63-2010's, and measured the position at 0 and -30 offsets. I then swapped the mini-prism back on to make sure nothing had moved, then tested the next 63-2010, etc. There was a difference in height, as expected (the 63-2010's are taller than the mini-prism), but the N reading (the horizontal distance read from the total station to the prism) was consistently off by 6 to 7 mm for all of the 63-2010's.

Has anyone else observed this amount of difference in offset when comparing two different models of prism? Does anyone else own any of these 63-2010's who would be willing to verify this result? I've been in contact with CST/berger and they apparently checked some of their existing stock and didn't see a problem.

Any input would be appreciated. Thanks.

RE: Offset of CST/berger universal prisms

geophyz,
I don't have the model of prism that you mentioned, but I have a couple of dozen different prisms of different types made by different manufacturers. I've tested all of them by forced centering, and I found that they all agreed to within a hundredth of a foot.

Sounds odd too me.

Best regards,
Michael

www.sewellconstruction.com



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