Topcon branded prisms all 0 offset

Turn 'er over.

Most prism holders are 0mm offset on one side and -30mm offset on the other side.

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Posted by: A Harris

Turn 'er over.

Most prism holders are 0mm offset on one side and -30mm offset on the other side.

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For some reason Topcon ones can't be flipped over to -30. They can only be used on 0.

Did some searching and I did find some Topcon branded prism holders that only have 0 offset.

I've found that with the prism in the 0mm offset the prism is too hanging out there and too vulnerable and the setup was not weight balanced and rather front end heavy.

My first mini prism purchase was one that slipped up and down a mini pole and it only had 0 offset. I use the poles for close monument stakeout in prismless mode. Have no use for the 0 offset holder because all my other targets are -30mm offset

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This is the typical Topcon prism holder

https://www.ebay.com/itm/NEW-10pcs-lot-topcon-Yellow-mini-Single-Prism-holder-female-thread-f-64mm-prism/162617162447?hash=item25dcbcfacf:g:JDcAAOSwal5YELxS

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I prefer this style as they come in a variety of colors to suit the individual.

https://www.ebay.com/itm/10pcs-Lot-All-Metal-PRISM-W-BAG-FOR-NIKON-SOKKIA-TOPCON-PENTAX-TOTAL-STATIONS/152654771858?hash=item238aeef692:g:bcQAAOSwsW9Y3Hhi

I am guessing that the offset is to match the constant of the Topcon 360 robotic prisms.

That being said, with the nearly universal standard of 0 and 30 (except for the Swiss), why worry about it? If the topcon doesn't do what you want, there are literally dozens of options out there that do.

Strange. We had Topcon TS's & prisms years ago, and they could be used with either -30 or zero offset.

On another note... I don't know how they are today, but Topcon used to make one outstanding construction TS IMO; they NEVER got out of adjustment. And we were tough on 'em...

These are the expensive traversing prism holders that mount on a tripod - they are not intended for mounting on a pole. The prism is not reversible since it does not mount on a plate.

The 0 offset disadvantages are negated by the peep sight that is used to accurately aim the prism back at the total station.

These were my favorite prism holders for traversing back when I had people instead of a robot...

@AHarris do you use those? I gather Chinese made.<br/>I'm tempted to buy a target with optical plummet to try but as cheap as they are it's that cheapness that puts me off. <br/>Others using these? Input appreciated thanks.?ÿ

I use Leitz and Sokkia tribrach with Leitz or Seco rotating disk and tilting target on top of a 0.3ft stub pole.

typical prism setup

I can easily swap TS and tribrach during traverse. My traverse methods change on small to large tracts and open to wooded tracts. It all depends upon the ability to get back and forth between the setups.

Posted by: Richard

@AHarris do you use those? I gather Chinese made.<br/>I'm tempted to buy a target with optical plummet to try but as cheap as they are it's that cheapness that puts me off. <br/>Others using these? Input appreciated thanks.?ÿ

Every cheap Chinese part I have used to date have been useless - tribrachs, adapters etc. don't waste your money.

Thanks. I've an aversion to most scientific/equipment ?ÿthings Chinese. <br/> Will keep my hands in my pocket, look elsewhere?ÿ

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"I've found that with the prism in the 0mm offset the prism is too hanging out there and too vulnerable and the setup was not weight balanced and rather front end heavy."

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yeah. How many of those 'economical' brands of adjustable target/prism holder have that worn spot in the normal operating range that gets worn after awhile. There's no aiming the prism assembly without it dropping to like a -30?ø vertical, and never stays aimed.

I have 5 Seco good quality tribrach with adapter disks that need a new home.

So we're down to laundering tribrachs now .. ? ?ÿ ??ÿ

Good surveyors puzzle game.<br/>align all at 72?ø rotation and adjust each bubble then readjust so all are in the mean range, then adjust all to centre.

Repeat, rotating 90?ø

Are you referring to the one with the locking at the back? It has a gun sight to help you sight the prism. I use them with tripods for my traverse.

Posted by: Richard

@AHarris do you use those? I gather Chinese made.<br/>I'm tempted to buy a target with optical plummet to try but as cheap as they are it's that cheapness that puts me off. <br/>Others using these? Input appreciated thanks.?ÿ

I have four of the Topcon style in the right photograph; I use them on tripods only, and have adjusted them to precisely the same height above the tribrach as the TS, so that when traversing, HR's and HI's all match. I like 0 offset because it's one less thing to have to remember to record, and because you can target the target, not the prism so that even if the prism is not aimed precisely at the TS, the sight is good.

Posted by: rfc

you can target the target, not the prism so that even if the prism is not aimed precisely at the TS, the sight is good.

The angle will be good when aiming at the target, but with zero offset the distance will be very slightly off when the prism isn't aimed at the instrument.?ÿ That is the purpose of the -30 mm offset - for the ~3 inch prisms that offset makes the distance measurement insensitive to aiming.

Posted by: Bill93

Posted by: rfc

you can target the target, not the prism so that even if the prism is not aimed precisely at the TS, the sight is good.

The angle will be good when aiming at the target, but with zero offset the distance will be very slightly off when the prism isn't aimed at the instrument.?ÿ That is the purpose of the -30 mm offset - for the ~3 inch prisms that offset makes the distance measurement insensitive to aiming.

I recall seeing a sketch someone here did (well over a year ago), that did that math, and if I recall it correctly, the error fell well below the standard error for a typical 3 or 5 second instrument (constant plus proportional parts); nearly an order of magnitude below, making it inconsequential to the analysis. I think the assumption was something like a typical pointing error of +/- 5 degrees or so (for the prism).

Your statement that the -30mm offset makes the distance measurement insensitive to aiming, but surely the same can not be said for the angle measurement, could it?