Final Glides: GPS or Pressure Altitude?

Agree with P3. The question is: given one indicated altitude (based on pressure, whether from a mechanical altimeter or modern device employing a pressure transducer) that is 1,000 feet lower than another indicated altitude (GPS, which is arguably a lot closer to the actual geometric altitude), which one should I use for an aggressive, 50+ mile final glide into the blue where I may not get another chance to climb?

Thanks to Craig Funston for the Mike Borgelt article. Mike's RAS postings are actually one of the sources I'd consulted before I posted but this article is pretty definitive: "your glider cares about GEOMETRIC [edit: GPS] altitude when it comes to the distance you can glide at a certain glide angle."

So...assuming that Mike is correct (and I have a lot of respect for his views) I must assume that the glider will still achieve the same glide ratio out west on a hot day at higher altitude (than back east in cooler temps at lower altitudes)...but at a higher true airspeed accounting for the much lower density of the air.

So it's OK to set off with the GPS altitude in hand and fly aggressively (guided by indicated airspeed) even though my trusted pressure-altitude-based glide computer says I'm 1,000 ft. lower when I start.

Does that make sense?

Chip Bearden
ASW 24 "JB"

On Thursday, July 7, 2016 at 8:41:54 AM UTC-7, wrote:

So...assuming that Mike is correct (and I have a lot of respect for his views) I must assume that the glider will still achieve the same glide ratio out west on a hot day at higher altitude (than back east in cooler temps at lower altitudes)...but at a higher true airspeed accounting for the much lower density of the air.

So it's OK to set off with the GPS altitude in hand and fly aggressively (guided by indicated airspeed) even though my trusted pressure-altitude-based glide computer says I'm 1,000 ft. lower when I start.

Does that make sense?

Yes, that is correct. Old-school glide computers used pressure altitude because that's all they had. My homebrew glide software always used GPS altitude, and it was as accurate as could be expected (allowing for localized lift/sink) in western US soaring conditions. With a proper polar, it nailed more than one 75+ mile final glide into still air.

Just to reiterate, GPS altitude is a short term noisy, long term stable measure of geometric altitude (which is what you want). Pressure altitude is a long term noisy measure of pressure altitude (which you don't want). My software actually blends the two, essentially continuously recalibrating pressure altitude against a 2 minute moving average of GPS altitude, producing a stable short term measure of geometric altitude, which (to me) is the ideal. I assume most modern glide software and computers do something similar.

Marc