Hello everyone this is my first post.

I've been a bodyman for a good long time. We use a Datum line when repairing frames unibody and such. It is a imaginary line usedfor measurments, this can be used for repair or manufacturing. For instance locating the lower controll arm mounting bolts. Also used for specific heights. Now change controll arms for landing gear strut mounting bolts, turbo prop mounts, wing spars and ribs while loaded or unloaded,etc. you get the idea.