GPS Altitude Offset?

[FlightDeck]

Your software (FSUIPC4) works perfectly with my Avmap EKP-IV GPS.

It's a great tool that allows me to learn and practice safely all the functionalities of my GPS before a real flight.

But the altitude displayed by my GPS is always 106 feet higher (+/- 32 meters)

that the altitude received by the $GPGGA sentence (example : 1000' = 1106', 1500' = 1606', 2000' = 2106', etc.) or the altitude display by FSX " shift Z " function.

That means that my GPS add +/- 32 meters (106 feet) to the altitude value received.

I have read on the Web that the altitude computed from GPS satellites signal are

corrected with a theoretical MSL (Mean Sea Level) offset of +/- 32 meters.

http://www.esri.com/news/arcuser/0703/geoid1of3.html

Does it means that FSX altitude should be decreased by 32 meters to emulate the signals broadcasted by GPS satellites?

If yes, what do you think about a selectable checkbox for this offset in a future revision of FSUIPC4 ?

If not, does it means that my GPS do not process altitude as others GPS ?

Best regards

[Pete Dowson]

But the altitude displayed by my GPS is always 106 feet higher (+/- 32 meters) that the altitude received by the $GPGGA sentence (example : 1000' = 1106', 1500' = 1606', 2000' = 2106', etc.) or the altitude display by FSX " shift Z " function.

How weird. Certainly the altitude provided by the GPSout facility is correct -- I use it together with Jeppesen FliteMap and it displays the correct altitude, just as if it were being fed from a real GPS. That is the whole idea.

That means that my GPS add +/- 32 meters (106 feet) to the altitude value received.

Yes, it does sound that way. It is not a normal thing for a GPS to do, I am sure.

I have read on the Web that the altitude computed from GPS satellites signal are corrected with a theoretical MSL (Mean Sea Level) offset of +/- 32 meters.

Really? Did it say why at all? If this is an adjustment made by all GPS's then programs displaying the altitude, like Jeppesen's own (FliteMap), and the Memory Map Navigator program I use (Ordnance Survey 1:50000 of the UK) will get the altitude wrong, as it is displayed correctly on those.

Does it means that FSX altitude should be decreased by 32 meters to emulate the signals broadcasted by GPS satellites?

Well, certainly not according to any of the programs I've tried, which are designed to show altitudes from a GPS.

Checking the reference you provided, it only actually says the difference in sea level datum is 32m in one place (Redlands California) -- 10 miles away it is already 0.5m less. Reading further it varies all over the place. What about this part:

"The maximum range of the Geoid99 undulations with respect to the WGS84 ellipsoid is 192 meters. The biggest anomaly was discovered southeast of India where the geoid is 105 meters below the ellipsoid, and its highest swelling was observed in eastern Indonesia."

Furthermore, it says

"A brief examination of elevation readings for ESRI headquarters in Redlands, California, demonstrates these differences. The campus elevation is shown on topographic quadrangle maps and high-resolution digital elevation models (DEMs) for the area as approximately 400 meters above MSL. However, a precise, nonadjusted GPS reading for the same location typically shows the elevation as 368 meters."

which explicitly states that the GPS reading at redlands would or does show 32 meters lower than the map. If this is the case, why is your GPS adding 32 meters? And what happens in other parts of the world where the difference is greater or less?

If yes, what do you think about a selectable checkbox for this offset in a future revision of FSUIPC4 ?

But to do what? If it subtracts 32 meters it will still only be correect at Redlands, won't it?

If not, does it means that my GPS do not process altitude as others GPS ?

I don't know. I have three GPSs and none of them have a function to take input from anything but their own GPS receivers. Really, GPSout is designed to feed computer programs which feed them output FROM a real GPS, and display the correct position and altitude. I don't know what your GPS is doing, but maybe there's another mode it can use which takes the altitude as given to it?

What model is it, by the way?

Regards

Pete

[FlightDeck]

My GPS is an « Avmap EKP-IV »

http://www.avmap.us

I will investigate my GPS and try to understand why the altitude displayed is always 106' higher. May be a setup? If not, I will contact Avmap. Navigation. This situation does not occur with the antenna.

Seems logic it is not your module because this kind of problem would have been identified long time ago on this forum!

Thank you very much for your answer and for your fantastic contribution to the sim and real world flight communities.

[dfournie]

This is a very interesting find. I had also previously noticed that the GPS altitude displayed on all of the GPS's (5 of them, one of them being the Avmap) I use with GPSout never exactly matches the altitude displayed by MSFS. I never really thought much about it because in real aircraft the GPS altitude displayed tends to always be off slightly from the barometric altitude because of the reasons given above (geoid, ellipse calculations, etc..) But this is being simulated. The Avmap and Garmin 396 for example have a built in terrain database that has elevations at approx 10m - 1km accuracy (depending on where your at). I suppose it's remotely possible that these untis are somehow processing the incoming MSL and performing a calculation on it. This really shouldn't be the case because the NMEA spec is very specific about what each string must contain. Pete has everything spot on. If the MSL altitude is being sent out by MSFS in the NMEA altitude string it should show that -exact- value on the GPS. I seem to recall that I did not see this problem when I ran GPSout in AV400 format out to my Garmin 396. I'll have to hook everything up and try to recreate it on that one.

[FlightDeck]

I did new tests with FSX

I notice that altitude offset changes according to my position on ground.

Exemple:

Airport: CSK3, Mascouche, Canada

Runway : 29

FSX Altitude: 78.7'

Avmap EKP-IV: 185'

Difference : 106.3'

Airport: CYUL, Montreal, Canada

Runway : 06L

FSX Altitude: 121.7'

Avmap EKP-IV: 229'

Difference : 107.3'

Airport: KJFK, New-York, USA

Runway : 04L

FSX Altitude: 16.7'

Avmap EKP-IV: 128

Difference : 111.3'

Airport: CYVR, Vancouver, Canada

Runway : 26L

FSX Altitude: 17.7'

Avmap EKP-IV: 76'

Difference : 58.3'

Airport: KEYW, Key West, Florida, USA

Runway : 9

FSX Altitude: 6.7'

Avmap EKP-IV: 86'

Difference : 79.3'

Airport: SPLP, Las Palmas, Lima, Peru

Runway : 29

FSX Altitude: 5087.7'

Avmap EKP-IV: 5055'

Difference : 33.7'

In some airport, my GPS even displays negative altitude!

That means my GPS apply an altitude offset in relation with the location.

May be it's a built in correction table? I did not found any setup for that.

My initial argument was based on tests made from one position (CSK3) in FSX.

This new test shows clearly that my GPS apply a different altitude offset in relation with the location on earth. Kind of satellite elevation correction table?

[Pete Dowson]

This new test shows clearly that my GPS apply a different altitude offset in relation with the location on earth. Kind of satellite elevation correction table?

Sounds like it has some sort of table or computation to deal with the "Geoid99 undulations with respect to the WGS84 ellipsoid". It may be interesting to try to find these places mentioned in your reference: the "biggest anomalysoutheast of India where the geoid is 105 meters below the ellipsoid, and its highest swellingeastern Indonesia."

In my opinion, whilst it may be making these corrections to its internal world model deliberately to correct it's own GPS reception, a very nice thing for it to do, it should not also do so for information fed to it from what is, as far as it is concerned, another GPS which may already be accurate. However, I don't know the designer's original purpose for the NMEA input facilities so I may be wrong. As I mentioned, none of my GPS's have such a facility in any case.

Have you checked to see if you can select a different world model (other than WGS84?) to which it won't apply these 'corrections'?

Regards

Pete

[FlightDeck]

Airport: VOMM, Chennai, India

FSX Altitude: 57.7'

Avmap EKP-IV: 339'

Difference : 281.3' ( 85,74 M )

Airport: VEBA, Behala, India

FSX Altitude: 8'

Avmap EKP-IV: 184'

Difference : 176' ( 53,64 M )

Airport: VCCT, Trincolalee, Srilanka

FSX Altitude: 10.7'

Avmap EKP-IV: 313'

Difference : 302.3' ( 92,14 M )

Airport: WAWW, Wolter Monginsidi, Indonesia

FSX Altitude: 167.7'

Avmap EKP-IV: -5'

Difference : -172.7' ( -52,64 M )

Amazing!

WGS84 seems the only mode available

But at least one thing is sure, it's not FSUIPC4

[Pete Dowson]

Amazing!

Yes. Looks like you got pretty close to the Maximum and Minimum the article mentioned!

But at least one thing is sure, it's not FSUIPC4

Sorry that there's no easy solution -- if you use the GPS for FS Navigation you'll need to treat the altitude with suspicion!

Regards

Pete

[dfournie]

AV400 format does not exhibit the same problem. Altitude is always right on.

[FlightDeck]

AV400 do not work with my Avmap EKP-IV.

This mode does not generate any parameter at all on the GPS.

The only FSUIPC4 selectable sentence that provide altitude in my case is $GPGGA.

Note that the altitude provided by FSUIPC4 sentence $GPGGA is exactly the same than the FSX altitude. I found that my GPS modifies the information displayed.

It seems to be a WGS84 ellipsoid Avmap built in feature.

Thank you for your support.

[Pete Dowson]

The only FSUIPC4 selectable sentence that provide altitude in my case is $GPGGA.

Currently the only outputs from GPSout giving altitude are GPGGA, PGRMZ (a Garmin proprietary one), AV400, RPY (non-standard, added for someone I can't remember now) and the Text output format.

Is there perhaps another sentence providing altitude you'd like me to add, to try? Or is GPGGA the only one accepted by the unit?

Regards

Pete

[FlightDeck]

Thank you for your offer. However, I believe that no matter the sentence used, my GPS will modify the altitude value.

I have to find a way to turn off this feature or to live with.

Best regards