Largest glacier calving ever filmed

[armando_m]

I guess they have a general idea, like say start of spring time

Right at the beginning of the video says : "Day 17 of glacier watching"

so yes there is hefty dose of patience needed to capture this event

[Akira]

I suspect that have a series of instruments that measure everything from temperature and rumblings and movement etc and therefor have a fairly accurate time when it will go down.

I think so.  Nowadays we can measure the ever-so-slight movement or the expansion of a mountain using some laser instrument.  The fine movement of such a famous glacier should have been constantly monitored.

Even though they had been watching it for 17 days, we could say that they "nailed" the moment.