I have a serious problem. Shooting forsights and baksights for my fellow theodolight user on gridlines, I set a nail on that same gridline. When I attempted to interline on that nail, I discovered my foresight and backsite were way off. I was resecting all over the site shooting gridlines using verified verified 5 different backsite. All residuals were within 3 mm. How is this possible? Previously my points, prism and prismless were within two mm of two different top end Leica's, two different professional surveyors. It's a new top me total station running an old version of SurveyPro. Please please help me solve this. I have tonnes of respect for you all, I'm no where near as experienced as most of you and desperately need/request your input on this. Thanks to those whom help.