Because in the Phantom,  altitude can be better described as height above home point.  And that is determined by only 1 thing,  the barometric altimeter.  The Phantom does not use GPS for altitude data.  So,  yes,  there can be large variations in altitude,  especially on waypoint missions and even in spite of VPS being active.  Plan accordingly is good advice.

I think it is a change in altitude relative to where it took off. It is not absolute. They might use barometric sensor readings. Maybe relay that with some GPS information. But ultimately they display the altitude as a change from home point.

Try lifting your phantom from the ground and place it on the table.

You can see altitude change. Then when it settles on the table it goes back to zero.

GPS its self only fixes position all other information is calculated

pi-inthesky@hot Posted at 2016-4-1 14:36
GPS its self only fixes position all other information is calculated

Altitude is also calculated and interpolated when using a barometric sensor too. You can place a barometric sensor inside a pressurized chamber and 'fool' the altimeter reading.

Actually GPS does provide your altitude. Why would it not? It works by triangulation.

http://gpsinformation.net/main/altitude.htm

So it relies on a geodesic model and knowledge of the satellites orbit to calculate your altitude. Similar method to how GPS is used to determine your geographical positioning location.

Just like how a barometric sensor also relies on a model to predict your altitude based on pressure change. It is no different. Both sensors has interpolation and assumptions. They are not a direct measurement of AGL.

The only true method to measure above ground level is to ping the ground with sonar/lidar/radar to measure time of flight of reflections, or maybe using an optical or radio interferometer to measure distances. And even with these methods, changes in air density and refractive indexes will skew your measurement. Again, requring assumptions.

quaddron3 Posted at 2016-4-2 05:45
Altitude is also calculated and interpolated when using a barometric sensor too. You can place a ba ...

"Actually GPS does provide your altitude. Why would it not? It works by triangulation."


Here's why not.
Altitude data from GPS is woefully inaccurate and commonly out by +/- 200 feet or more and changing.

Here's what Garmin tell you about GPS altitude accuracy:
        How accurate is the GPS elevation reading?

GPS heights are based on an ellipsoid (a mathematical representation of the earth's shape), while USGS map elevations are based on a vertical datum tied to the geoid (or what is commonly called mean sea level). Basically, these are two different systems, although they have a relationship that has been modeled.

The main source of error has to do with the arrangement of the satellite configurations during fix determinations. The earth blocks out satellites needed to get a good quality vertical measurement. Once the vertical datum is taken into account, the accuracy permitted by geometry considerations remains less than that of horizontal positions. It is not uncommon for satellite heights to be off from map elevations by +/- 400 ft. Use these values with caution when navigating.

Correct, DJI does not use GPS data for altitude. When in VPS range it uses the sonar and out of VPS range it uses the barometer.

labroides@yahoo Posted at 2016-4-1 23:58
"Actually GPS does provide your altitude. Why would it not? It works by triangulation."

Thanks for putting my point so eloquently  

pgrover516 - Where did you get this information?  What app?

I have a question.
I am creating a mission to photograph a property on the edge of a ravine. My first way point is set for 40 ft. Is that 40 feet above the home point or 40 feet above the 1st way point? The way point is over the ravine. My concern is if its above the way point I'd be 20 feet below the edge of a 60 foot ravine. THAT WON'T WORK. Especially when it trys to go to the next way point... get my drift?
Any thoughts or comments will be appreciated.

icmalz@gmail.co Posted at 2016-6-5 22:18
I have a question.
I am creating a mission to photograph a property on the edge of a ravine. My firs ...

Hey Icmalz,

I have done a various altitudes flight with the App, and Phantom hit the spots perfectly well. I have no idea how etc but I can assure you did. On one of my flights, the distance was around close to 3km and I lost RC signal but it came back with no issues. So, you can trust the Phantom and the App. Just make sure nothing on its path and keep the altitude a some degree higher to avoid any issues. ;)