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Your altimeter displays one altitude while your GPS displays another. Under VFR, which do you use/when?


From Pilot Tips / Workshops

"Under VFR, you should be flying at a cardinal altitude (odd or even thousand-foot altitudes e.g. 4,000, 5,000, 6,000, 7,000, etc.) plus 500 feet. This provides separation from IFR aircraft and other VFR aircraft on crossing courses.

GPS AltitudeYour altimeter displays Indicated Altitude and that is what you should always use (for traffic separation). Indicated altitude is pressure altitude corrected for local atmospheric conditions. The correction is done by entering the altimeter setting given by an Air Traffic Controller or an AWOS.

All aircraft in a given area should be on the same altimeter setting so relative (altitude) separation is maintained.

A GPS, on the other hand, measures your Absolute Altitude as it trilaterates between multiple satellites. While more accurate than pressure altitude, it does not provide the same relative separation from other aircraft."


  • XC Triker

    That's straight forward, but, if you are in the middle of nowhere and cannot get the local altimeter setting before taking off, is it OK / sufficient to use the GPS altitude to set your altimeter prior to takeoff?

  • Ken

    I think you probably have to. And for that matter how do they handle aircraft that are flying over areas where settings aren't reported? I don't think the altimeter setting would remain the same for the entire trip from El Mirage to Boulder City that I just did, and if these ASOS/AWOS maps are correct the last available reporting stations are Barstow coming from the west, and Henderson coming from the East. Do we really think a plane departing Barstow, and another departing Henderson will remain separated somewhere over Mountain Pass if they keep their departure settings?

  • XC Triker

    I found THIS WEBSITE which is GREAT on altimeter discussion, stories of near mid airs / crashes due to mistaken altimeter settings, "minor" misreads in Altimeter resulting in 1000' errors.

    Large portions of the Canadian provinces and territories are remote, making aircraft an ideal form of transportation to and from these far-off areas. However, flying in such remote locations is not without some hazards. Two incident reports from Canada provide graphic examples of why an accurate altimeter setting can be critical:

    • "The pilot stated that he was lined up [on approach] for the runway and that the altimeter was reading 300 feet when the nose wheel struck the ice. The pilot applied full power...and flew back to [his departure airport over 100 nm south-Ed.], where the landing was uneventful. The pressure in the area was lower than the point of departure, sufficient to make the altimeter read 250-300 feet high if not properly reset." (A80W0001)
    • "On a night VFR flight, the pilot encountered deteriorating weather as he approached his destination. He received an IFR clearance...During a procedure turn, the aircraft started to strike the tree tops. The aircraft stalled and crashed into the trees. Because the airport had been closed for the night, no altimeter setting was available. The FSS operator gave the pilot the setting for XYZ (29.68) and for ABC (29.87) [approximately 90 nm south and 90 nm east, respectively-Ed.]. The aircraft altimeter was set at 29.94. Both pilots had been without proper rest for approximately 20 hours." (A80C0079)

    "This was the last leg of a long 3-day trip...Inbound...we ran the 'preliminary checklist,' cross-checking altimeters at 30.22. This seemed a little odd to me at the time as the area had a low front moving through, but we were busy and I did not press the issue. Once on approach, everything was normal until just before the final approach fix when we broke out of the clouds and a ridge was looking very close. Also the GPWS went off as we passed over the ridge. I checked our altitude and we were right on profile. I had the Captain check the altimeter with Tower. Altimeter was actually 29.22, not 30.22, putting us approximately 1,000 feet too low on approach."

    FAR 91.121 states that, when operating an aircraft below 18,000 feet MSL, pilots will maintain altitude by reference to an altimeter that is set to: 1) the current reported altimeter setting of a station along the route of flight and within 100 nautical miles of the aircraft; 2) the current reported altimeter setting of an appropriate available station; or, 3) in aircraft without a radio, the elevation of the departure airport or an appropriate setting available before departure.  At or above FL18, all aircraft set the altimeter to 29.92.

    You are right Ken.  I knew that once up in the air & able to receive transmissions you should set the altimeter to the nearest altimeter station within 100 miles, but I wasn't sure the GPS was accurate enough on the ground to set by if no other data.

    @Firstlight taught me ATC procedures-  before that I didn't know you had to read back Altimeter settings.  For example, suppose you're just crossing above a Class C or D.  You're not actually "IN" their airspace but you want to be polite, so you radio to them, "Big Town Airport, ELS N1234 is 2 miles to the West at 5,500 feet transitioning above your airspace West to East."     They radio back, "LS 1234, Big Town Airport, proceed as planed, maintain VFR, Altimeter setting 29.95"     I figured you could just reply, "Thank you, LS1234."  However, you should always read back the altimeter setting they gave you, "29.95, Thank you, LS1234."

    Maybe @Hostman (our honorable retried ATC member) can comment further?

    "The helicopter was being used to transport personnel to work sites across a large frozen lake. An approaching low pressure area with snow and high winds...reduced visibility to near zero in some areas. The pilot most certainly encountered adverse conditions and altered course to circumvent the worst areas. The aircraft was later found...wreckage was widely scattered. The altimeter showed a setting on impact of 30.05; the correct setting would be about 29.22, causing the altimeter to read about 800-850 feet high. The altimeter had obviously been set two days previously [apparently during a time of high barometric pressure-Ed.]."

    Crews can avoid a Barometer Surprise by listening carefully to ATIS and ATC broadcasts, especially before, during and after significant weather, when the altimeter setting may be an unusual number.

  • XC Triker

    Hey, this is interesting for the International pilots here, not all countries / continents use FL18 (18,000) feet as the transition to set the altimeter to 29.92.  For example, in Argentina, if you are only 3,000 feet MSL, you have to switch to 29.92 !!!!


    Beyond the North American continent, the pressure altitude/indicated altitude transition level is variable. In South America, Buenos Aires, Argentina is at the low end at 3,000 feet; the high end is 18,000 feet in La Paz, Bolivia. Most of Europe uses 4,000-6,000 feet; much of India also uses 4,000-5,000 feet. The transition level in Tel Aviv, Israel is 10,500 feet, but Jerusalem's transition altitude is changed by ATC as required. Cape Town, South Africa uses 7,500 feet, and further north, Cairo, Egypt uses 4,500 feet. To the East, in Riyadh, Saudi Arabia, the transition level is 13,000 feet. Australia uses 10,000 feet; Japan uses 14,000 feet; much of the rest of the Far East uses 11,000 feet. Above these transition levels, altitude is expressed as "Flight Level" (FL), and altimeters will be set to QNE-the standard pressure setting of 29.92 inches of mercury, or 1013.25 hectopascal.

    In the following report from a flight crew on a European flight, the unfamiliar, non-standard transition altitude simply added to the distractions of the departure workload.

    • "Climbing to FL60 (transition altitude 4,500 feet)...We were task saturated flying the Standard Instrument Departure, reconfiguring flaps and slats, resetting navigation receivers and course settings, resetting engine anti-ice, etc. The crew missed resetting the Kohlsman window to 29.92 at 4,500 feet MSL, and leveled off at FL60 indicated altitude with a Kohlsman setting of 28.88 inches. Departure informed us of our error." (#206218)

    It would have been easy for this three-person crew to unconsciously think, "We'll get all this other stuff taken care of, then change the altimeter at FL180." Again, it was ATC to the rescue, bringing the problem to the crew's attention before the error became critical.

    Hey @Yarrawonga, waht do they do in Oz?

  • Mark

    I'm using WingX which shows your current AGL based on your GPS and it's terrain info. I tend to trust it more than the altimeter which never seems to agree with field elevation after I land. WingX tends to show 0 AGL when I'm on the ground. It also seems that the barometric pressure based on the field elevation is never the same as the AWOS pressure from the airfield 17nm away.

  • white eagle

    Mark would you post a little more info on the wingx system. I am always a little embarassed when it comes to different inst systems cameras radios.i flew hang gliders for many years pretty carefree.but now iam flying trikes it seems that there is so much tech data to learn and i have always been gadget illiterate.i should have been born a bird.what gadgets do they use.

  • Firstlight

    I see a problem with poor pressure transducer performance in just about all of the non-certified glass cockpit equipment I have flown (The same can be said for inexpensive analog altimeters, but they are becoming less common).  In general, the indicated altitude will be outside of the acceptable tolerances when the correct altimeter setting is keyed into the unit. Their may be a way to re-calibrate (or at least set a new zero-point) the units, but I have never encountered it. Your equipment ought to indicate within 70' of field elevation when using the correct altimeter setting.  My current unit never does.

    As a species we tend to "fudge" numbers in a way that we feel works in our favor. I see a tendency for pilots to fly at 5750 feet MSL, when the hemispheric rule would suggest that 5500' is the correct altitude to be at. I've lost count of the number of times pilots have told me, "everybody else is at 5500', so I'm going to be at 5750' to be safe". The problem, of course, is that IFR traffic is using 5,000' and 6,000' as their altitudes, and the "fudging" habit actually increases risk by cutting the safety margin in half. If you roll that extra altitude up with poor pressure transducer performance, you may well be cruising directly in the path of IFR traffic. Even on a good day in perfect sight-seeing weather, IFR pilots are often head-down in the cockpit, managing charts, plates and radios for whatever task is coming up next. The two types of traffic really do not mix! Flying at higher altitudes with GPS derived altitude is a total crap-shoot: you just have no idea where you are relative to where you are supposed to be.

    I'd love to know if there is a recalibration procedure I can apply to my Enigma. The degree of error drives me nuts. It is not an big operational problem for me these days, as my trike is so slow that I rarely go anywhere far enough away that I have to worry about altimeter settings (my local field elevation is perfectly fine), but it makes me crazy to fly around with obviously incorrect info staring me in the face. Since my transponder uses the Enigma as the encoder, I have stopped using it. For what it might be worth, I have tossed out three analog altimeters once they drifted out of range, too.



  • XC Triker

    @Firstlight I think there is an altimeter correction/calibration procedure for the Enigma.  Let's see if we can get Matt from MGL to comment.

  • Jozinko

    Im verry sorry but I cant to give you no info about it. Why? Because all standard air pressure procedures showed in HPa... My instruments are old Russian and altimeter works in TORRs. Then I didnt to know what pressure is if tower told me QNH 1013. In our country is the transition level 5000-5500ft to the standard pressure and back. I never flew higher than G class then I never need resetting my altimeter. There are more TMA,MTMA or other areas in our country, then we usually use free G - max 1000ft AGL. I could to learn to use it, but the TORR knows the military airport only... Yes, I should to buy new standard altimeter... but...its too expensive :)

  • XC Triker

    Jozinko (@Sajan), I had an altimeter at one time that would only read in mBar, so I printed a small set of conversions on a strip of paper and stuck that below my altimeter and used it when I needed to set the Altimeter to inches Mercury (Hg).  It looked something like this:

                           Altimeter:   1013 mBar = 29.92 inHg

            28.0 = 948    28.25 = 957     28.5 = 965      28.75 = 973     29.0 = 980     29.25 = 991

            29.5 = 1000  29.75 = 1007   30.0 = 1016    30.25 = 1024   30.5 = 1033   30.75 = 1041


    It involved some interpolation and error, but it was better than not trying at all.

  • Jozinko

    Hahaha, Thank you David. I could made the conversion like your. But I neednt it here :) My personal record for high flight was 2750m ALT when I flew over the Velka Fatra mountains. But still in G class.

  • Mark

    WingX is a moving map application that I use on my iPad. http://www.hiltonsoftware.com/

  • Mark

    Getting back to the original question about which altitude would I use. Under 3000 AGL I will watch my gps, over 3000 AGL I will use my altimeter on the hopes that everyone else's altimeter is set to the same pressure and reading some what the same.

  • Matt Liknaitzky

    Hi Firstlight, you can calibrate your Enigma altimeter in MENU > MENU > SYSTEM SETUP MENU > FLIGHT INSTRUMENTS SETUP > ALTIMETER SETUP.

    When calibrating any instrument a good reference is required. The ATIS is not a good enough reference. A pitot/static testing kit is the way to go (that you need during your transponder check). I don't know of any Enigma that has failed a VFR altimeter test. You can always get it to read correctly (within tolerance) throughout its full range.

    Bottom line is that you will be able to calibrate it to read within tolerance if it is not already doing so with the factory calibs.



  • Firstlight

    Hi Matt,

    Thanks for the details. I'll give things a whirl and report back. Cheers!


  • XC Triker

    Thank you Matt!!!  Really great that you came on to explain things.

    Mark (@Mpansing) I can see why you're thinking that, but 2 things come to mind:

    1. Is the WingX GPS really more accurate for altitude than the pressure altimeter?  Or is the programming behind it just telling it to display "zero" AGL when near the airport and within XX feet or so of surface.  What happens as you taxi along the runway does your "altitude" (MSL) change very accurately with the slope of the airport (the AF/D or taxi diagram defines where the GPS reference is and the true difference in heights either end of the Rwy), or does it stay the same the whole way no matter where on the airport-  if it's the latter, it's just software making itself look good, not true accuracy.
    2. Like Firstlight says isn't it more important (whether above 3K or not) to be off by the same amount as everyone else (altimeter set to common reference), than to be perfectly right but on a different standard?  I'd say it's better to be wrong by the same amount as everyone else-- but this gets back to Firstlight's issue of accuracy of the barometric altimeter in the first place-- kinda a catch 22 if your barometric altimeter is not accurate--  So, thanks again for the update Matt.  Because of all this convo, I was paying closer attention lately as well and will try and super-calibrate my Enigma and keep an eye on it to see if/how it deviates.  I'll report back later ...


    Mark said: "Getting back to the original question about which altitude would I use. Under 3000 AGL I will watch my gps, over 3000 AGL I will use my altimeter on the hopes that everyone else's altimeter is set to the same pressure and reading some what the same."

  • Mark

    Most of my handheld gps seem to wander a bit, however the WingX seems to do better. Not only does the AGL seem to be correct but the actual reported altitude seems to be within 20' or so of what I believe the actual altitude is at that spot based on info from some online maps. I generally use the WingX GPS AGL because I'm usually flying under 3000' AGL in and around some terrain. So im not so much worried about what altitude someone else might be flying at as I am about the upcoming terrain.  If I'm doing something like flying a pattern then I will use my altimeter. 

  • XC Triker

    Mark ( @mpansing ), since your WingX GPS seems to be accurate (doesn't that also depend on the external GPS antenna a person uses?  Or is your tablet using an internal antenna-  which tablet you using? iPad? Generation?) ...   since your WingX GPS appears to be accurate, the question then is what is safer? 

    1. To be right (using an accurate GPS) BUT on a different standard than everyone else?   Or ....
    2. To be on the same standard as all other pilots (pressure altimeter) but using an inaccurate pressure altimeter in your aircraft?

    I don't really know the answer to that question, except that personally, my next step is to look at my pressure altimeter closer.  I use an Enigma.  Using MGL's calibration procedure (see Matt Liknaitsky @mattskyflyer above), I'm going to recheck to make sure my altimeter is calibrated to read correctly on the ground.  Matt told me that the main problem he sees when people say the altimeter is not accurate is that the "calibrator" (owner) is not using the correct reference point.  To know what the correct ground altitude is, I need a documented reference point on the airport.  To find that, I went to AirNav.com and found the link to my runway diagram:


    Field Elevation

    I found that although the listed Field Elevation for the airport is 261 feet, that is only valid at the approach end of one of the 4 main runways.  The elevation at the other approach ends can be as much as 51 feet off, and since my hangar is nearer to a different approach end, my elevation there may be around 34' off (or more depending on grading for drainage / etc at my hangar--  I suspect that the runway is probably higher than the surrounding- so my hangar should be even lower than that nearby approach end).

    OK, so ....  (since I'm at a towered airport) I then need to get permission to taxi to near one of the approach ends (maybe the rarely used one near my hangar).  Spend a couple moments calibrating the altimeter to agree with the documented elevation there.  Then taxi to the approach end of the Main runway and see if it stays accurate.  Then take off from that end fly around some, land and taxi back to the main approach end again and see if it's still accurate after going up & down a few thousand feet while flying and bouncing onto the runway.  If that is true, then taxi back to my hangar (verify the ATIS reported setting has not changed in the last couple seconds), write down the altimeter reading while right outside my hangar and post that on the wall of my hangar to refer to in the future when calibrating the altimeter.

    Then over the next few months, keep an eye on the altimeter reading when set to the ATIS barometer and see if my pressure altimeter is accurate.

    If my pressure altimeter is accurate, then I will continue to always refer to my pressure altimeter (even if my GPS (however accurate) disagrees) when setting / reporting my altitudes.

    At least that's how I see it?  Does that seem correct?