I need a way to mark the same position at both sides of a metal sheet (in my case I need to make a hole in the passenger compartment of my car and end up in the right place both in and out, that is not hit any cables or other vital parts of the car) when it's not possible to measure. If it was just a flat sheet of metal, you could easily measure out the same location on both sides to see where the hole will end up, but on a curved sheet (like in my case) or on a large wall where it's not practical to find the exact point on both sides by measuring you have to use another method.
Another example is if you need to make a hole in a double layer insulated metal sheet wall, and it's not practical to measure to find the same point at the other side of the wall -- still you want to make sure that the hole will end up in an acceptable location on both sides of the wall (i e not hit a cable on the outside, and not hit a beam on the inside) -- you have to know where the hole will end up on both sides of the wall before you drill.
- If the sheet would have been transparent I could have seen where I was, but metal isn't transparent ...
- I could knock on the sheet and listen on the other side, but this is not accurate enough.
- Maybe use a magnet in some way?
Since this is a common problem I guess there must be people out there that has a great solution! Please help :)