They no doubt had a pressure altimeter (pressure altitude), a radar altimeter ( altitude above ground return) and probably a gps altitude indication. I suspect they got distracted on approach and off the glideslope. Make no misunderstanding unless a down draft brought them lower or an autopilot system was misprogrammed/malfunctioned they were either making a poor visual approach, poor non precision instrument approach, or poor precision instrument approach. <img src="http://www.traveltalkonline.com/forums/images/graemlins/Yikes.gif" alt="" />

Definitely worth the company looking into that approach and what was going on. <img src="http://www.traveltalkonline.com/forums/images/graemlins/Yikes.gif" alt="" />

This is what you pay those two for.