Electronic Distance Measurement
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).
The Finer Points of EDM Measuring
Here a helicopter pilot (left) and two geologists discuss the finer points of making EDM measurements. The "gun", the source of the laser, is the orange device on the tripod between them. It gets lugged around in the orange box at left. The speed at which the laser travels the round-trip from gun to reflector and back is affected by the air temperature; the probe measurements are used to make corrections for this effect.
Single Corner Reflector
This is a single corner reflector set up on a piece of re-bar that has been pounded into the cinders. The painted board helps the EDM operator to find the reflector since he or she is often looking for it over distances of a few kilometers. This type of low-tech reflector is relatively expendable and is usually left out in areas where eruptions are possible. For more precise measurements, clusters of 3, 6, or 9 reflectors are used, but they are only installed while the measurements are being made.