In total white-out conditions and with fresh, powdery snow covering everything, you will NEVER see a vertical drop before tumbling down. Everything around you is slightly greyish white. No features. No shadows. No landmarks or boundaries. It doesn't matter if visibility is 500 feet or 5 feet - anything covered with fresh snow will blend in with the other powdery snow, and that includes the lip of snow that overhangs the cliff. What stands out is those things not totally covered with snow, such as rocks or bushes. The only exception is if it is blowing, then the older, harder snow MIGHT stand out in those places exposed to the wind.
If you and I are traveling together in these cheery conditions, it is high time that we rope up and move very cautiously.
I have often speculated as to how useful a GPS would be in backtracking across a crevassed glacier, even with wands (markers) in place. This is a situation where even a three foot deviation can be lethal. We would need good equipment and all our skills to get down in good order, and we would need to employ them while we were still oriented. Realistically, that doesn't always happen..
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