Prism constant for Nikon mini-prism in corner

[MalcolmStewart]

I'm new here, and a retired avionics / radar engineer with an interest in surveying archaeological sites such as stone circles in Wales.

In the past I've used a Wild T2 theodolite with a Disto Basic mounted above and parallel to its telescope, and measuring angles and ranges to a custom designed detail target. This although slow and non-automated has worked well with good accuracy. (3mm or better)

I've now bought a Nikon mini-prism kit as in the link, and am totally baffled with the concept of prism constant with this particular device. The prism constant is 0 according to the Nikon information, so does this mean that the Total Station measures correctly to the mechanical axis of the prism holder? If so, I don't understand the purpose of the accessory holder designed to support the prism in a corner of a building. The corner holder accessory is designed to allow the rear apex of the prism to get into the corner, and it does get in to within 2mm. BUT, this is some 15mm away from the plane of the prism which aligns with its other mounting device.

I've downloaded the manual for a Nikon Total Station. but whilst comprehensive, it's silent on the above point.
(I fully appreciate that I can't use a prism with my Disto Basic.)

Whilst I'm asking about this, is a "0" prism constant the same for different makes of Total Station?

Many thanks - from England.

 

[cherrypicker]

Malcolm

This depends on the matching of a prism to an instrument. The Leica standard prism, for example has a constant of 0mm when used with a Leica instrument but 34mm when used with other instruments. The Nikon would encounter similar issues when using prisms from different manufacturers.

I tend to check other prisms againts a known base line (on both faces) and use the difference in measurement as the constant. A prism is only a mechanism for refecting a pulsed light beam back to its source. The optical centre (or offset) for each prism has to be determined so the easiest method is to match the manufacturer of prism and instrument.

Leica, unfortunately manufactures different prisms (with different constants) for use with its instruments (Standard Circular Prism - 0mm, 360 Prism - 23.1mm, Mini Prism - 0mm OR 17mm, Mini 360 Prism - 30mm, Reflective Tape - 34mm etc.). Leica does help by publishing the offset for use

 

[Apakrat]

You might want to visit with some local survey firm about possibility of two monuments separated by an acceptable accurate known distance. Some where there is such an arrangement where You can physically check out the accuracy of Your equipment.

At 74th year working on IR-One2 PhD from UHK - - -

 

[underhill]

".. visit with some local survey firm about possibility of two monuments .."

Or even build your own baseline on level ground with a good tape measure. Repeated observations along same should allow you to calibrate your measurement system.