Does anyone know offhand how to calculate convergence angles? I need to reproduce a line that was surveyed based on grid north NY-East in 1963, and I have 83spc's w/convergence angles from my software. If I could remember how to calculate the convergence angle for the 27spc's I could go from grid83 to geodetic to grid 27. I tried the NGS toolbox, but didn't see anything that would do it. I suppose I could convert a pair of 83 coordinates to 27 and compare inverses between the two pairs. I've heard that converting coordinates between 83 and 27 is not that reliable, but does that go for alignment as well? I don't need the coordinates.