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EDM stands for Electronic Distance Measurement. This is the view from
behind a cluster of EDM reflectors looking back towards the EDM "gun". It
isn't really a gun, but rather a sophisticated laser that can measure very
precisely the time it takes for a laser beam to make the round-trip from
the "gun" to the reflectors, and back. Using this time, the known speed
that the laser travels (the speed of light), and correcting for air
temperature and pressure, the distance can be determined to a precision of
1 part per million (i.e. 1 mm over a distance of 1 km).
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