Prism constant question
> > I know they are both 0mm constant, so should be okay, but I am not 100% sure.
> They're not the same. I put together this graphic to help me manage my prisms:
used another company's GMP101 the other day and found prism constant errors of about 2mm.
we were using it on 10no. spigots mounted on the walls of a tunnel. When I free-stationed from a different position, the prism positions that were being observed from the opposite side to the original observation gave errors 3-4mm. (basically the same test as described above). I was using a constant of -17 on a trimble instrument. (17.5-34.4).
Swapped back to using leica circular prisms and no problems - tight freestations from all positions.
As the GMP101 is meant to be the most accurate mini prism in the range I was surprised and wonder what I was doing wrong.
> I suspect the mini-prism setting on your instrument may match the second type.
We already checked that on TPS1200 and confirmed in LGO raw data. The correct constant was used. My unresolved question was more in regards of the ATR and how different sizes prisms affect its automation.
Unlike the Leica round prism the GMP111-0 isn't a nodal point prism so you would get small pointing errors if the miniprism wasn't perfectly facing the instrument. At steep vertical angles there will be a (tiny) level error with prisms that aren't nodal even if they are pointed correctly.
I don't see that it could affect the ATR centring.
> In this particular case, the mini prism used was a GMP111-0.
I wasn't aware of that model. Good to know.
I never did care for Leica's decision to hide the true offset from the user. It's been a source of confusion for anyone who steps outside of Leica's walled garden.
I was thinking along the line of the ATR using the radius of the prism in its calculation to adjust to true center. If the instrument is configured for a large prism / large radius and a small one is used, any effects on the computation?
Anyway, mm stuff which will not create a problem on this here job, just good to know for future reference.