Seems ok, this altimeter smoothing is a little annoying as it takes an hours for the indicated altitude to stabilize. Does not make any sense on a glass cockpit anyway with an Air Data Computer. But even a conventional altimeter doesn’t respond this slowly to pressure changes.
I am probably missing something obvious here, but for MSFS, why not just use “GPS POSITION ALT” for altitudes below the Transition level, and effectively bypass all the complications of Pressure altitudes.
Using PRESSURE to determine altitude is such an ancient technology.
These days, with GPS, surely the future is GPS Positioning, and this would seem to solve many of the issues explained so clearly by Ross.
Edit: 6 months later. Learnt a lot in 6 months !! Embarrassing to read what I posted back here in my ignorance !!! (but I will resist the temptation to delete it – Lessons learnt !! )
That would not solve anything. It would be the same thing that is currently being done and have the exact same problems. “Plane Altitude” and “GPS Position Alt” both give the airplane’s true altitude above mean sea level; i.e., they are the same. “Plane altitude” is what is currently being used by VATSIM below the transition altitude, so using “GPS Position Alt” instead changes nothing.
Using pressure to determine altitude may be an “ancient technology,” but that is what is still being used in aviation in the real world. And that is what MSFS is trying to simulate.
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Because, all real altimeters and air data computers read only ambient pressure. In a real aircraft, even when flying below the flight levels, (with the altimeter synced to the closest sea level pressure reporting airport) indicated altitude does not necessarily equate to true geometric altitude above sea level. It depends on the air temperature.
In a real aircraft flying above the transition altitude, (with the altimeter set to STD pressure of 29.92) there is no direct correlation at all between indicated altitude and true altitude. When all aircraft in a certain area are flying at the same indicated altitude with STD pressure, they are all flying on a surface of constant atmospheric pressure, and the height of that pressure surface above sea level can vary greatly depending on air temperature and actual ambient pressure.
TRUE altitude is not used by aircraft or ATC in the real world. All aircraft performance charts are based on air pressure, air density, and indicated (calibrated) airspeed. At an indicated altitude of 25,000 feet, (with the altimeter set to STD). Ambient pressure will always be precisely 11.1035 in/hg (or 5.45355 p.s.i), and any performance charts for FL250 will be based on that. Air density at a given pressure altitude will vary with the temperature, and performance charts will always have corrections to things like engine thrust settings vs. airspeed and fuel burn at different temperatures - usually expressed as an offset above or below ISA.
Prior to the advent of GPS, the only way to directly determine TRUE MSL altitude in an aircraft would be to use a radar altimeter, and that would only be valid when flying over the ocean, and only the military (and some civilian research aircraft) have radar altimeters that will work above 2,500 feet.
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I did start my post with this comment – and it seems I was missing quite a few things !! 
Many thanks for the explanations – I am now a little older and a lot wiser.
If it works in the real world, I am sure that those involved, will come up with a way to accurately enough, simulate the real world, in the Sim.
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The only time the difference between indicated and true altitude becomes a factor is when terrain clearance is involved. In the US, all Victor airways depicted on IFR enroute charts will have a Minimum Enroute Altitude (MEA). The MEA is chosen based not only on the height of underlying terrain, but also includes a “fudge factor” based on the known fact that when the temperature is below ISA, indicated altitude will be less than indicated altitude (with an altimeter set to current pressure).
That is one reason that the transition altitude for the use of STD pressure in the US was set to be 18,000 feet. This was done before Alaska became a state. The highest elevation in the “lower 48” is Mount Whitney at 14,505 feet MSL, and a transition to STD at 18,000 would insure safe terrain clearance even when air temperatures are far below ISA.
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Such a wealth of Aviation Knowledge, and so wonderful the way the MSFS Community is willing to spend time, sharing with others.
As a clarification, he’s talking about the real world here, not the sim world. For networks like VATSIM that display airplanes from different sims relative to each other, and where MSFS is the only sim that attempts to provide an accurate representation of the difference between true and indicated altitude for non-standard pressure gradients, this difference becomes an important anytime you are below the transition altitude.
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Inaccurate altitude reporting remains an issue…
The solution to this is going to be interesting to see solved.
Trying to combine multiple Sims into the same world, that calculate altitude in different ways, is going to be a significant challenge to all involved.
While there is little hope for FSX to evolve and change the way it calculates altitude, maybe both P3D , and XP can , one day, adopt the more “Real World” methods now being used in MSFS. ?
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apparently still not solved. Yesterday while cruising at FL360 on VATSEYE, i was reported at 37,850ft 
Supposedly some of the mapping tools show a different altitude (possibly GPS altitude?) than what ATC sees (pressure altitude above transition altitude). Was it a mapping tool or what ATC told you that you were at?
This issue is fixed. If you are set standard the altitude reporting will be correct on the controllers scope. The vatsim map is most likely showing you without standard altimeter set. The controller will see what you indicate in the cockpit.
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Please read the previous messages. 3rd party apps like VATSPY use “plane altitude,” regardless of airplane altitude. This will not match your indicated altitude. except for below the transition altitude when you have the baro set per the sim’s QNH and the pressure gradient is the ISA standard pressure gradient.
Below the transition altitude, the controller will see you at the same altitude indicated by VATSPY, which will likely be different than your indicated altitude. (Below the transition altitude, VATSIM also uses the simulation variable “plane altitude.” ) Above the transition altitude, VATSIM uses the simulation variable “pressure altitude,” and in that case, the altitude they see you at should agree with your indicated altitude (assuming you set STD/29.92/1013.2 on your baro). Your airplane’s true altitude compared to airplanes from other sims will be different, however, for the same flight level.
VATSIM will be changing the way they do the altitudes as described in a post above. However, that work is on hold until a bug in how altitudes are reported through SimConnect is fixed.
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ATC actually didn’t correct my altitude at any time. So I guess you have a point there. It must have been mapping tool issue. Thanks for your help. In the meantime I have a lot of troubles either connecting to VATSIM, or staying tuned in. Sometimes, it would just kick me out during flight prep in the cockpit, and sometimes, upon arrival, or mid-flight… I can’t figure out if the problem is on my end, their end, or MSFS’s end
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thank you. Now only need to find out, my connection issues with Vpilot
I didn’t fly a lot recently but I had a flight on Monday and there I didn’t have any connection issues at all. I never had them with MSFS
Please keep this thread on topic. Your best bet is to post something in the vPilot forum on VATSIM.
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didn’t find a solution to my problem on the official website of Vatsim. but I will pay attention in the future to “stay on topic” 
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