Absolute altitude

[nwairphotos]

I'm ignoring it because I can't see your point.
DJI used to use barometer altitude relative to home point for exif info on stills.
It was useful for photogrammetry etc.
Over the time of a flight, the variation was trivial.
They replaced it with GPS altitude that potentially has so much error and the errors can change a lot over the time of a flight.
This means that it's 100% useless for any purpose for anyone.
You used to at least know how far above home point your photo was taken.
Now you know nothing - see the OP's example - according to exif info the photo was taken 43 metres below sea level.
How useful is that information?

Does this affect programs like map pilot and the accuracy of measuring volumes on their maps?

 

[sar104]

Yeah - sorry about that - I was including @SteveMann in the discussion.

No problem - it's still an interesting subject. Maybe the thread just needs a brief recap, since it started with the observation of a significant altitude discrepancy between the barometric altitude and GPS altitude during a flight on a beach.

That progressed to the question of why the EXIF data altitude field was changed at some point from barometric altitude relative to takeoff point to GPS altitude above MSL. It was agreed (I think) that if one is primarily interested in altitude above MSL then GPS, on average, will be more accurate than uncalibrated barometric altitude but, conversely, if one really wants altitude relative to takeoff point then barometric is going to be more accurate. But it is also worth remembering that both barometric relative altitude and GPS MSL altitude are both written to the image file EXIF data, in different sections.

Then the topic became an argument about just how big the random inaccuracies are in GPS vertical positioning, which further moved on to include systematic inaccuracies when considering the difference between the geo-ellipsoid and the actual shape of the earth. It really is important to keep in mind the difference between random and systematic error. The data I posted above are in relation to random error; that doesn't include systematic variation from local DEM data.

 

[sar104]

Does this affect programs like map pilot and the accuracy of measuring volumes on their maps?

I would expect that those programs would use the barometric data, for all the reasons mentioned previously.

 

[AlanTheBeast]

I'd just add that the baro altimeter in the P4P is not anywhere as accurate as an aviation baro altimeter. It may indeed have great repeatability (will come back to 0 at the home point) but that does not mean that when it says 400' that it is 400' above the home point. There may be a significant scale error as well.

 

[sar104]

I'd just add that the baro altimeter in the P4P is not anywhere as accurate as an aviation baro altimeter. It may indeed have great repeatability (will come back to 0 at the home point) but that does not mean that when it says 400' that it is 400' above the home point. There may be a significant scale error as well.

I looked up the specs and test results for these units some time ago, and they are actually quite impressive in terms of raw pressure measurement. I think the biggest sources of error are in compensating for the real pressure variations resulting from the airflow from the props. It's a much less clean environment than seen by an aircraft pitot tube.

 

[AlanTheBeast]

I looked up the specs and test results for these units some time ago, and they are actually quite impressive in terms of raw pressure measurement. I think the biggest sources of error are in compensating for the real pressure variations resulting from the airflow from the props. It's a much less clean environment than seen by an aircraft pitot tube.

You mean static pressure port.

The drones have the static port inside the shell. So prop action blowing down could create Bernoulli effects that reduce the pressure in the shell, the fan could increase the pressure (or decrease it if for exhaust), flight movement changing the pressure on the inlets/outlets ... ugh.

 

[sar104]

You mean static pressure port.

The drones have the static port inside the shell. So prop action blowing down could create Bernoulli effects that reduce the pressure in the shell, the fan could increase the pressure (or decrease it if for exhaust), flight movement changing the pressure on the inlets/outlets ... ugh.

Yes - they are static sensors. Not ideal at all, not that there is any viable alternative.

 

[Racklefratz]

All IFR approaches require a barometric altimeter because GPS altitude is notoriously inaccurate. The error is a mathematical fact and no amount of fiddling of the words will change that.

No "fiddling of the words" happening. Your assertion that "all IFR approaches require a barometric altimeter is wrong - apparently you're unfamiliar with WAAS - WAAS-equipped aircraft rely solely on GPS information for precision approaches.

The FAA is slowly decommissioning the VOR system for position information; the VOR's never had any altitude information.

Nor did I say they did.

 

[Meta4]

That's an incorrect statement, and you're mixing terms you don't understand.
Re-read the Garmin discussion you referenced. Nowhere does it mention the word "altitude". The discussion is about GPS accuracy of ELEVATIONS, not altitudes. Elevations refers to ground-level, as in the term "field elevation". Elevations and altitudes are two different things.
The dirty little secret is that GPS ALTITUDES are extremely accurate, so much so that, in real aviation, the FAA has been phasing out ground-based navigation facilities in favor of GPS navigation for decades, beginning with NDBs in the 90s.
Vertical positioning accuracy in instrument approaches requires extreme accuracy; a +/- 200 ft altitude error on one can kill you. GPS-based instrument approaches have been FAA-approved and published for decades. I personally owned an aircraft with a GPS installed which was legal for instrument approaches.
Don't confuse altitude with elevation. GPS altitudes are dead-on accurate as it gets.

I stick by what I said.
Whether you look at Altitude or elevation, data from GPS is woefully inaccurate.
In your part of the world WAAS helps take that error out but the OP is not in North America.
He's at sea level and his GPS is indicating that he's -42 metres.
That's for just one photo.
If he took another 15 minutes later it could well have indicated +30 metres.
I've seen similar numbers from the deck of a yacht.
When your GPS altitude or elevation is swinging +/- 200 feet over a 10 minute period, you are not looking at dead-on accuracy.

 

[SteveMann]

No "fiddling of the words" happening. Your assertion that "all IFR approaches require a barometric altimeter is wrong - apparently you're unfamiliar with WAAS - WAAS-equipped aircraft rely solely on GPS information for precision approaches.

The barometric altimeter is required instrumentation for all aircraft. If there is no barometric altimeter, the aircraft is unairworthy. Yes, a WAAS approach can be made without reference to the barometric altimeter, but it has a much higher DA or MAP, and higher weather minimums. The pressure altimeter is still required equipment. If you want to fly an approach to 50-ft, typical of many IFR approaches, your primary altitude instrument is the barometric altimeter.

 

[Richard R]

No problem - it's still an interesting subject. Maybe the thread just needs a brief recap, since it started with the observation of a significant altitude discrepancy between the barometric altitude and GPS altitude during a flight on a beach.

That progressed to the question of why the EXIF data altitude field was changed at some point from barometric altitude relative to takeoff point to GPS altitude above MSL. It was agreed (I think) that if one is primarily interested in altitude above MSL then GPS, on average, will be more accurate than uncalibrated barometric altitude but, conversely, if one really wants altitude relative to takeoff point then barometric is going to be more accurate. But it is also worth remembering that both barometric relative altitude and GPS MSL altitude are both written to the image file EXIF data, in different sections.

Then the topic became an argument about just how big the random inaccuracies are in GPS vertical positioning, which further moved on to include systematic inaccuracies when considering the difference between the geo-ellipsoid and the actual shape of the earth. It really is important to keep in mind the difference between random and systematic error. The data I posted above are in relation to random error; that doesn't include systematic variation from local DEM data.

Actually a while ago, DJI dropped the barometric height from the exif data. That's why Maps Made Easy missions start by taking a ground reference photo before taking off. The GPS height is subtracted from that recorded on other images in the mission to get determine the general height agl.

 

[sar104]

Actually a while ago, DJI dropped the barometric height from the exif data. That's why Maps Made Easy missions start by taking a ground reference photo before taking off. The GPS height is subtracted from that recorded on other images in the mission to get determine the general height agl.

It wasn't dropped - it's still there. Just not in the GPS altitude field which now displays GPS MSL altitude. It's in the XML location section, and not all applications will display it.

 

[Meta4]

It wasn't dropped - it's still there. Just not in the GPS altitude field which now displays GPS MSL altitude. It's in the XML location section, and not all applications will display it.

This issue has been puzzling me ever since DJI changed the altitude data shown in the Exif info.
Post #1 and #20 may have been pointing in the right direction.
Over in a drone mapping forum, someone has noted that what is shown as GPS data is always the same value that's shown for Absolute Altitude.
btw Exif Absolute Altitude is not what is usually meant by that term in aviation.

It appears that the number shown as GPS data doesn't come from the GPS unit at all but comes from raw barometer data.
To compute the barometer-based altitude the P4P seems to be using the standard equation:
p = 29.92 x (1 - 2.25577x10-5 x h)^5.25588 in units of "Hg, where h is the altitude above sea-level in meters.

GPS is not an accurate source of altitude data but when it comes to DJI Exif info, it seems that GPS Altitude is not GPS Altitude.

 

[sar104]

This issue has been puzzling me ever since DJI changed the altitude data shown in the Exif info.
Post #1 and #20 may have been pointing in the right direction.
Over in a drone mapping forum, someone has noted that what is shown as GPS data is always the same value that's shown for Absolute Altitude.
btw Exif Absolute Altitude is not what is usually meant by that term in aviation.

It appears that the number shown as GPS data doesn't come from the GPS unit at all but comes from raw barometer data.
To compute the barometer-based altitude the P4P seems to be using the standard equation:
p = 29.92 x (1 - 2.25577x10-5 x h)^5.25588 in units of "Hg, where h is the altitude above sea-level in meters.

GPS is not an accurate source of altitude data but when it comes to DJI Exif info, it seems that GPS Altitude is not GPS Altitude.

Interesting. That was an easy one to test. Below is a graph from a recent flight showing barometric and GPS altitude as reported in the DAT file.

The obvious first observation is that they are not reporting the same values. However, looking at the EXIF data for photos taken at the high point of this flight, the "GPS altitude" is reported as 3343 m and the relative altitude is reported as 60 m. Clearly the EXIF altitude data are indeed both from the barometric values, not the GPS values, as you stated. The explicit EXIF label "GPS altitude" is clearly incorrect.

One further interesting point to note, however, is that the actual MSL takeoff altitude was 3180 m - exactly as reported by the GPS altitude measurement. The absolute altitude derived from the barometric readings, and calculated assuming a standard atmosphere, was off by just over 100 m.

 

[skydriver]

As to the baro it's accurate-ish, but I suspect a proportional error with height from the t.o. point. So it may come back and say "0" but when it's at 400' there may be a 5% error (I don't know how much may be less, may be more). Where the GPS will rival any airline GPS receiver for accuracy, the baro altimeter in the drones isn't close by a long shot.

Curious--how do you know how accurate "airline" GPS altitude readouts are. Are you an airline pilot or avionics mechanic? Or is it just common knowledge that all normally working GPS altitude readouts at a particular position above or on the earth are very close to the same so that an expensive airline certified GPS would be no more accurate than a drone's? If you agree, then what is your basis for your statement? I am sure you know that aircraft do not use GPS altitudes as a basis for following flight rules and for aircraft separation. For example, all aircraft have to be using the same altitude data which is why at or above 18,000 feet (FL180 IFR only) all altimeters are set to a standard 29.92 inHg. Below FL 180, theoretically all aircraft in the same airspace should be using the correct baro setting for that area as given to them by a local WX briefing, tower, FAA center, etc. So when an aircraft is descending from, say, FL330 to 10,000 feet, the crew will set their altimeters to the correct local altimeter setting as they pass through FL180. Sorry if this got a little off the thread.