Robert,
Further thought is that is predicating an approach on a barometric altimeter or ADC still valid? (Despite what Nav Canada would like us to beleive about Temperature Compensation). The majority of current Part 121 & 135 aircraft are going to use a radio altimeter below 2500 feet agl and, if an ILS is available, use a Decision Height (aka RADIO) not the BARO Minimum Descent Altitude.
Future aircraft with Wide Area Augementation System (WAAS) GPS will also not use a baro altimeter but derive it from the GPS for an LPV approach again with a Decision Height lower than the equivalent MDA.
BTW just talked to one of our vendors' engineers who's been flight testing their first WAAS-capable GPS. He said the most remarkable thing both he & the crew noticed was the absence of dither in position - their aircraft had a chart display with an own aircraft symbol and it was rock steady not the usual slight random movement.