Coordinate systems in Panama


I'm working in the Republic of Panama.
Here they use "Nad 27 zone 17" . I put it in quotes because they actually do it in WGS84 Zone 17n and shift it 202.95m south and 14.87m west. Based on the coordinate conversion in Justin, the correct shift is 190.68m south and 19.81 m west. They do it different so that it syncs with the scaled set of aerial photos they have. I suspect it is because the scaled aerial photos were shifted by the difference by mistake back when they started using them. Anybody have another explanation?
Also it seems that locally the shift is constant. True?
But I suspect that outside of a certain range the shift is not constant? True?


btw: I think I have the forum structure figured out now.