How do Altimeters measure gradient? Do they assess gradient as units of time pass ( say every 10 or 20 seconds ) , or as distance is travelled ( say every 10 or 20 metres ), or as metres are climbed vertically ( say every 2 or 5 metres climbed ).

Specifically -- Has anyone experienced the VDO MC 1.0+ Altimeter/Computer (analog).
How reliable is it? Has anyone used the VDO Z1 ( new, digital ).
How do analog & digital speed functions differ in terms of reliability with wireless
cycling computers?