GPS Elevation Datum

I thought the GPS ignored that and did a round circle hence when you read the nmea string you are quite often under water in some parts of the world.
Hence the use of local datums for all subsea work.

I can’t see that – the altitude quoted accuracy is +/- 100 feet 95% of the time. I don’t think the 5% means any time you’re near the equator.

Raw GPS data uses a ellipsoid for altitude. The Geoid is mean sea level.

Google tells me that the altitude of any point on the surface is called orthometric height which can be estimated using a model, the undulation @Urs mentioned in his post. Also an orthometric height can be measured directly using a GPS if I understand correctly.

1 Like

For navigational use at sea, there is no need to display the altitude; receivers for inland use can do it.

Once, I could help a person lost in his installation. He used the output of the GPS-receiver to fuel some statistical charts. Always he was some 40 meter off the altitude on geographical maps, even after changing the receiver.

I discovered that his code used the wrong string in the GPS-data, the theoretical ellipsoid altitude, instead of the undulation-corrected one (both strings are given).

1 Like

I have the nmea threads displaying on my phone and IMHO is not related to sea levels.
It is repeatable in the same locations and its above and below either side of the tide depending where you are by about 30feet or so.

I know if you run Veripos Verify it gives you very accurate tide heights from your vessel.
Never compared my phone with with Verify though ( should have…DOH)

Just a thought. Is this 60’ overall altitude error what translates to a 3 meter position error at the surface?

It was probably 10/15 years ago and the forum where I helped does not exist anymore.

I did not know how he extracted the data strings, I was just intrigued by this permanent altitude error of 40 meters in a small region; and I knew something about undulation…

He showed me his code, and knowing where he worked, I could see the undulation, there, being just about 40 meter. He had two altitudes to extract, the theoretical and the real one, he opted for the false one.

I wondered who could use this ellipsoid altitude…

every gps chip including your phone works on a std that pumps out several nmea strings.
You just need to get to them.
Proprietary gear as used in the offshore industry pulls raw sat data and then does it own calcs.
When its plugged into a DP system it must send a few std nmea strings.
When you plug in the vendors software into their box, it can show lots as that is using data not part of the nmea std.

not sure, never read about height error versus HDOP error?
I will call one of my gps boffins to ask…

what I meant was I have looked at the gga string ( you can view that on any DP system)

545.4,M Altitude, Meters, above mean sea level
46.9,M Height of geoid (mean sea level) above WGS84

Seen the height of geoid negative by 30-40metres and wondered why?


Another error prone altitude difference, not really GPS related:

Inland, each country uses its ‘Standard Elevation Zero’, related to a nearby seaport, at mean sea level.
Germany uses as Zero the North Sea port of Amsterdam, Switzerland the Mediterranean port of Marseille; the difference is 27 centimeter.

In 2004, they built a bridge over the Rhine River, between the German city of Laufenburg and the Swiss city of Laufenburg. Indeed, the engineers were aware of this altitude difference.

When the works advanced from both banks, someone had seen that the two parts would not join at the same level! The difference in altitude was 54 centimeter.
A Swiss engineer had inverted the sign before the 27 centimeter.

With some adaptations, they could save the work already executed…