Hello. I have been trying to figure out how to do the following: I have aircraft flight data collected from an ADS-B receiver which contains lat, lon, and altitude, and I would like to convert the altitude to WGS84 (height above ellipsoid). After much google hunting, I have come to the conclusion that the altitude value sent over ADS-B is the same altitude that an aircraft would send to a traffic control tower (uncorrected pressure altitude, based on an altimeter): "transponders?ÿalways?ÿreport altitude in flight levels, that is, they report their altitude assuming the local sea-level pressure is 29.92 inHg (1013 hPa). It is up to the receiving station to correct this altitude according to the true local atmospheric pressure and thereby determine the aircraft's MSL altitude."
I got a pretty good explanation of how to "correct" the pressure?ÿhere and get a "true height" above the Mean Sea Level. I thought that the next step would be to run vDatum and set the input to "mean sea level" and the output to "WGS84", but it looks like it's more complicated than that, wanting a specific vertical reference called out, such as NGVD29.?ÿ
Can someone give me some guidance on how to complete the conversion? If getting a "perfect" answer is impossible, that is fine, but maybe show me the path that would get me the closest?
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This is interesting.?ÿ Why are you doing this?
I don't know the answer but photogrammetry is an ancient art by now, I'm sure that what you are trying to do is possible.
I guess we have to know how the ADS-B processes GPS signals for output.?ÿ What information do you have on that?
Be advised that your precision on this derived altitude will likely be very low, I'd expect +/- 50 feet at best
I'm not absolutely sure, but I think what aviators call "MSL" these days is NAVD88 elevation.?ÿ The term MSL was in common use when NGVD29 vertical datum was the geodetic model in use, but the term is no longer used with NAVD88 by geodesists and surveyors.?ÿ Both referred to a measure of height relative to a gravitational reference, a geoid model.?ÿ The geoid fluctuates with location, compared to an ellipsoid model of the earth, because gravity varies according to the composition of the earth and mountains.?ÿ?ÿ This difference between ellipsoid and geoid varies 180 ft in the continental US. For much of the US the geoid is below the ellipsoid.
At your likely accuracy, you can probably treat the corrected altitude as an approximation to NAVD88 and find the latest geoid to get a representative value for your region of interest.
If anyone knows better, please correct me.
At your likely accuracy, you can probably treat the corrected altitude as an approximation to NAVD88 and find the latest geoid to get a representative value for your region of interest.
I would say this is about as good as you're going to get. The pressure equation is an approximation, so a few centimeters of standard deviation in the geoid won't make much difference one way or the other.
VDatum will allow you to input MSL, apply a geoid and output an ellipsoid height:
Use the multipoint conversion option to load your point file.
Or, if you have access to a geomatics processing program such as Leica Infinity or Trimble Business Center, import the points with MSL ortho elevations to a project with a geoid applied, then export ellipsoid heights.
@bill93?ÿ Sorry Bill but your statement about?ÿ NAVD 29 and a geoid model is not correct. NAVD 29 had?ÿNO geoid.
There was not enough gravity data. Reductions that were made were to?ÿsea level by holding something like the mean of 26 tide stations. The wording that you read some place might have be something like "the datum (here we are talking about NAVD 29) for geodetic levels is mean sea-level, or the surface of the geoid, as found from tidal observations".?ÿ
JOHN NOLTON
@bill93?ÿ Sorry Bill but your statement about?ÿ NAVD 29 and a geoid model is not correct. NAVD 29 had?ÿNO geoid.
There was not enough gravity data. Reductions that were made were to?ÿsea level by holding something like the mean of 26 tide stations. The wording that you read some place might have be something like "the datum (here we are talking about NAVD 29) for geodetic levels is mean sea-level, or the surface of the geoid, as found from tidal observations".?ÿ
JOHN NOLTON
As I recall there was a "geoid map" relating to NAVD/NGVD-29 that John Hamilton has posted at some point. I ASSUME that it was based on whatever gravity data (probably pendulum derived), and deflection of the vertical (via astronomic observations). I don't think it was ever really used, and it was obviously rather crude by modern standards.?ÿ
Mean Sea Level was a misnomer at best, but the 26 tide gauges (21 in the US and 5 in Canada) served its purpose at the time.
Loyal
Have you checked your altitudes against EGM08? "True" WGS84 references EGM08 for heights.
https://earth-info.nga.mil/index.php?dir=wgs84&action=wgs84
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@loyal?ÿ ?ÿYou can see a contour of the geoid in C&GS Pub. "Figure of the earth and Isostasy from Measurements in the United States, 1909?ÿ By John F. Hayford". You can find that pub. online at NGS. I have not looked for it on line because I have the copy in my library.
Hope all is well with you Loyal.
JOHN NOLTON
Loyal I got up on NGS web site and checked to see if S.P. 82 by John F. Hayford was there and it was.?ÿWhen you get to the download you will want to look at page 180-183 and pg. 185.
This will save you time in your search.
JOHN NOLTON
@loyal?ÿ ?ÿ ?ÿ ?ÿ ?ÿ?ÿYES;?ÿ ?ÿbut I have not gone yet. I did go to another about one month back and had 1 cold one. Sure miss the old days. Hope covid-19 goes away soon so I can spread the wealth around; if you know what I mean.
JOHN
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NAVD 29 had NO geoid.
It implicitly assumed a geoid that depended only on latitude. See NOS NGS 34 under Orthometric Correction.
NAVD 29 had NO geoid.
It implicitly assumed a geoid that depended only on latitude. See NOS NGS 34 under Orthometric Correction.
Actually it assumed a normal gravity scenario where Latitude Difference AND Leveled DIFFERENCE along a given line defined the Orthometric Correction for that line. (at least that's how I remember it).?ÿ
Loyal
Thanks all for the help. I will definitely try the NAVD 88 method and see how it goes. The next step is to figure out the equivalent command line argument for vDatum.
