Correct, altitude is still way off. So frustrating. I wish it could just go back to SU4. lol
Doesn’t matter if I’m using correct altimeter or going to 29.92, still off.
OK, so I did what @Viralwhite just did and my VATSIM controller shows me at FL400, which is correct – but now VATSPY and SimtoolkitPro show me at 41,000… but they haven’t updated anything? I don’t see a single update from either software source in the last month
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Are you noticing that it’s incorrect below FL180 for the controllers as well?
I can’t confirm that under FL180 from the controller - just could see it on SimtoolkitPro and VATSpy. It was over 1,000 ft difference around 16,000 ft in Dallas
No, that is not at all what I’m confirming. It is only guaranteed to be correct at field elevation at QNH. Any deviations from ISA pressures above field elevation will cause altimeter errors, just as they should.
If the client is using plane altitude, it is almost guaranteed to not be correct, increasingly so above the field elevation where QNH was calibrated.
@moxiejeff What you are seeing at 40K is correct. You are reading pressure altitude based on STD calibration and current local pressure, which may not match your geometric altitude (41K in this case).
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Got it (I think) - sorry I am slower than you guys on the “science” of all this. I guess my only other question, though, is why did I not see this big of a variance with SimtoolkitPro or VAT-Spy pre SU5?
Previously the altimeter did not at all take local ambient pressure into account. Any error was only based on the difference between sea level pressure and your present altimeter setting.
This works in ISA standard atmospheres and did in FSX, when the atmosphere was always ISA. However, it is not always ISA in MSFS, so how it was being calculated previously was incorrect (sometimes by a bunch) based on ambient pressure.
-Matt | Working Title
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So to @Viralwhite point earlier, @Bishop398 , it does seem the altitude is off for VATSIM controllers under FL180. Above 18000 feet, they had me at the correct altitude. Just FYI (this may just be the way it is)
Do we know for sure whether VATSIM’s ATC system uses the pressure altitude or the GPS altitude for under transition altitude? (Also, is VATSIM using FL180 as the worldwide transition altitude like MSFS, or does it use the real transition altitudes outside the US and Canada?)
If it’s using the pressure altitude, and pressure or temperature in the sim are off from what VATSIM controllers have, then the pressure altitude wouldn’t be corrected in their interface to the actual true altitude, but to a slightly different altitude. (Correction: wouldn’t be corrected to your indicated altitude.)
If I understand correctly?
However if VATSIM is using the GPS altitude, it should match your indicated altitude roughly, if you correctly set the barometer to the sim’s pressure and you’re at standard temperature, but would always be off by some amount if the temperature is not standard temperature. (?)
Might be useful to know how much it was off, in which direction, and at what temperature and pressure and weather settings.
Maybe it’s because I have the wrong assumption for what the MSFS Plane Altitude variable is. I assumed that it was height above mean sea level, which is what the altimeter will show with QNH aet for the local pressure.
All good questions, which I am not certain what the answer is. To your point, pre SU5 99.9% of the time, my altitude was within 200-300 ft of what it was in the sim. And I fly on VATSIM a lot. However, post SU5, I am having a 1,600 ft difference and at times only a 1,000 ft difference. So I have no idea what VATSIM uses (FPS or Pressure) but worth mentioning in their discord.
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Plane Altitude I believe is height above mean sea level, yes. However that’s not what the altimeter will show if temperature is not standard temperature unless you’re exactly at the altitude of the field for which the barometer calibration was given (from what I understand from Matt’s explanations about how it works now to be more correct to reality).
They are no longer trying to account for temperature (density) error anymore is what I was told previously.
Hmm, well if it’s not the temperature, could it be the local ambient pressure? (I’m not sure I understand what local ambient pressure is, sorry – does that mean the actual air pressure at the plane’s location and altitude?)
My understanding here, if I’ve got it correct, is that a static-pressure-based altimeter inherently is unable to produce an accurate reading unless the air pressure at the location and altitude of the plane is identical to the air pressure at the calibration location (the airport whose reading you calibrated to), even ignoring temperature?
Or am I just doubly confused… 
Not quite about what needs to be identical, but yes, if you are far enough away from where the QNH is derived for such that the local sea level pressure is different, then yes indicated altitude and plane altitude should differ (because QNH would differ).
I’m not certain whether Matt was referring to horizontal ‘local’ or vertical ‘local’ – eg, is the air pressure varying up and down with altitude with different weather conditions? Pressure should depend, in theory, on the weight of the air column above, and that’ll depend on density.
didn’t they just fix this 3 hours ago with the hotfix i just downloaded?
You tell us. 
How’s it work for you? The last two people who reported back said there’s still inconsistencies under transition altitude, but it’s unclear whether that’s actually correct behavior or not.
Nobody seems to know for sure what variables VATSIM is using or how it calculates the altitudes given to ATC controllers. Additionally, it’s unclear whether the temperature factor in the updated altitude calculations was removed or kept, and whether other differences still remain. (Update: see below, temp was removed but the new pressure variances remain.)
(I’m having trouble with my vpilot config for the microphone & push-to-talk, so I can’t get online on the radio myself at the moment.)
Ok re-reading Matt’s posts in the thread, I think they might have disabled the temperature variance, but definitely chose to keep the pressure-related variances. So any time the atmosphere is not at ISA conditions and the pressure is not exactly following the ISA lapse rate going up the air column, there will be differences between the indicated altitude and the actual altitude.
Separately, there’s … whatever VATSIM ATC controllers see below transition altitude, which we don’t know what it is. It might be pressure altitude corrected for a different atmospheric model than MSFS is using. Or it might be GPS altitude.
Going back in the thread, see Matt’s earlier rough description of the new altitude model:
If VATSIM is using the geometric altitude below transition altitude, then it won’t always match your altimeter now. If VATSIM is using the pressure altitude and adjusting it with the same wrong formula that MSFS used to use, then it won’t always match your altimeter now.
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This stuff is really hard to explain with no pictures! (Apologies if this explanation is something some of you already know)
You can think of an altimeter as a barometer, but it’s a special barometer. A special barometer where the pressure scale is actually marked in feet, and those feet correspond to specific pressures of the ISA standard atmosphere. The scale is such that when this “barometer” reads 29.92 inMg (1013.25mb), the feet read zero.
But these crazy barometers are full of all kinds of problems. One is: what if you’re on the ground and the reading is wrong? Well, you twist a little knob, and it moves the feet scale up or down. That’s the QNH. Another problem: what if you fly a bunch and the air pressure on the ground wasn’t what it was before? Well, that’s why ATC tells you how to twist the knob all the time to move the scale up and down.
But, what if the scale itself is wrong? What if you’re at 10000 ft and you totally twisted the knob to move the scale up based on the ground reading that ATC gives you, but you’re still 5mb too low? Well, the altimeter is just a dumb barometer, it doesn’t know that the atmosphere is dropping in pressure as you climb slightly more than usual. So, this dumb barometer is going to read high on the feet scale, because the pressure is lower locally than the scale was drawn for.
As mentioned before, we kept the local (vertical) pressure effects on the altimeter. So, your geometric altitude may not necessarily reflect your indicated altitude. We use the weather system data for this.
My sense is that clients must be using geometric altitude when they should not be. The pressure altitude simvar, though, will now match the altimeter when it is set to STD.
-Matt | Working Title
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As I understand it, VATSIM depends completely on the sim for the altitudes. There is no separate calculation within VATSIM. Below the transmission altitude, it uses the MSFS variable “plane altitude.” As I understand it, this is the airplane’s height above mean sea level, which would be based on the local ambient pressure and sea level ambient pressure.
The airplane’s altimeter would show the height above mean sea level for the local ambient pressure and the QNH set in the baro. Where the QNH differs from the local sea level pressure, either because of a location difference between where QNH is defined for and where the airplane is or a weather (pressure) difference between VATSIM and MSFS, the altimeter altitude will differ from the plane altitude seen by the VATSIM controller.
Above the QNH setting location, there should not be any difference in pressure lapse rate from standard since the temperature effect (which is what would cause this through a change in density) has been removed. Edit: According to Matt above, they are using a non-standard pressure lapse rate, it it exists, from the live weather system data even though this would be associated with a non-ISA temperature.
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