OK..the manual for this gismo reads: "The prism constant value of Topcon is set to zero. When using prisms other than Topcon's, it is necessary to set the prism correction value of that specific prism." OK..then the gun is indeed set to zero (easily changed.) First question----I thought Topcon prisms were -30mm. We've got Leica prisms...-34.4 here. Going crazy trying to figure this out. I seem to be missing things on the SPCS by about 1 cm. (Pretty close to twice the 4.4 difference between the Leica's and what I thought was the Topcon value.) Yes, I'm taking scale and elevation factors into account. Going to get back to basics and lay down a short baseline with a steel tape, I've got a cheap -30 prism somewhere....just thought I'd get your feedback before I spent another few fruitless hours trying to get this straight. Thanks in advance.