Ah, kinjalp,
Yes i think you've sussed it.
After re-reading his posts, i think i have it now.
This is how i now think it goes:
i think he wants to send signals
which represent the two co-ordinates,
these signals are to be reproduced
at the electronics lab on a scope,
presumably as a dot of light
on a screen,
with a map drawn on the screen,
(presumably a removeable tracing)
The way i now read his posts makes a little more sense,
He wants to make an assembly
that would generate these two outputs,
from -as you say- by moving an arm,
probably over a similar picture.
The moving arm having two outputs,
one vertical and one horizontal.
Before diving in to the various ways of making and
sending such signals,
Does that sound like its a correct interpretation ?
Regards, John