So i was just out flying a few minutes ago.  Ive been flying to a couple weeks now so im kind of seasoned and im very careful.  I was doing a pretty tight shot over a fence between two trees but before the fence the ground dips down a little bit.  as i was creeping up to the pass through my drone just decided to lose elevation enough where the legs caught the ground and it flipped forward released the terrible screaming agony of the motor (yes i know they are okay and they will shut off after a second so it doesnt burn out)  So i say a few swear words and walk over and pick the drone up.  the Gimbal is just loose and hanging but after i prop the drone back up the gimbal picks up and works fine.  I place the drone down and start the motors.  I take off and the drone just reems to the right and crashes again.  Of course now im thinking how did a little crash fk my drone up when my little toy RC has crashed a dozen times and takes it like a champ.  Well low and behold i had a chip in one of my carbon fiber blades. No biggie.  I thought I had checked the props before the second take off but not good enough.,

I put the white props back on to test a third time and the drone worked perfectly, i know, the second half of my story isnt neccessary but thats just a lesson to make sure you check your props THOROUGHLY after a crash.  But as the first part of my story.  Can the little height detection on the bottom of the drone make the drone lose or gain alitude by itself?  I can see forcing the drone to gain alitude but why did my drone just decide to drop down to the ground.  The blades didnt stop spinning so the drone didnt fail

Altitude accuracy primarily comes from a barometer sensor, then gps and compass.  Still, when hovering really close to the ground, don't expect only a couple inches of drift.  Learn, grasshopper, learn.  Oh, and carbon blades....  why?

If I read your story correctly, it sort of suggests you were very close to the ground when shooting the shot. I have tried doing this a few times, in doing low passes along a subject (bridge) and found the drone does not reliably maintain height that well. I have now decided, through experimentation, to disable the VPS system when doing this, and fly no lower than about 6 feet of altitude. So far I have not had a problem, fingers crossed.

boxerman Posted at 2016-3-6 10:11
Altitude accuracy primarily comes from a barometer sensor, then gps and compass.  Still, when hoveri ...

Altitude only comes from the barometer.
The Phantom does not get any altitude data from GPS at all and the compass can only supply orientation data.  It is unrelated to altitude.

Awesome geebax, both love and hate the fact that I'm not alone and this has happened to others.  Ill make sure to turn that off next time I'm doing the shot.  I rewatched the video and thank goodness for my quick pull back because as the drone was losing altitude it began heading right into the fence.  and I pulled back quite hard.  I'm now wondering if when  pulled back is when the drones landing gear made contact with the ground and cause it to flip.  Still, much rather lose a prop than nail the gimbal and face of the drone into a fence.  The shot would be been great had it just kept altitude as expected.  

Also,  I'm a sucker when I get a new hobby.  The descriptions made it sound like the CF props were useful and a few youtube videos voted in favor of the CF props due to stability..(/irony)

labroides@yahoo Posted at 2016-3-6 08:00
Altitude only comes from the barometer.
The Phantom does not get any altitude data from GPS at all ...

Right about compass, wrong about gps.

boxerman Posted at 2016-3-7 08:22
Right about compass, wrong about gps.

No .. I'm not wrong about GPS and altitude.
GPS is great for position accuracy but woefully inaccurate for altitude.
It can easily be +/- 200 feet out and that sort of inaccuracy is unacceptable for altitude and that's why the Phantom does not get any altitude data from GPS.

Since you don't believe me, here's what Garmin have to say on 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.

boxerman Posted at 2016-3-6 21:22
Right about compass, wrong about gps.

GPS is really bad for altitude without a cross reference to map data, which the P3 doesn't have.

I never said that the GPS altitude sensing was accurate.  But the Phantom does gather the calculated three-space values from the conglomerate GPS signals.  If you go back to the OP and my initial reply and read it slowly....  you'll understand that I understand what you are saying.  I was just telling the OP my thoughts about relying on altitude accuracy while flying in close proximity over something, anything - earth, water, etc.  Get it?  Got it.

From what others are saying, does this mean do not fly low (below 6 feet).

I normally never have even tried anything lower then 30 feet up when buzzing around.

microcyb Posted at 2016-3-8 06:14
From what others are saying, does this mean do not fly low (below 6 feet).

I normally never have ev ...

You should be OK at about 6 feet, but just watch if the height varies and be ready to correct. The VPS system should keep you at a constant height.

I wonder if the VPS system is ignored when in the intelligent flight modes, particularly flying waypoints, as every time I use this mode I get warnings about being too low. Even despite the warnings, it still flies the mission.

Geebax Posted at 2016-3-7 17:17
You should be OK at about 6 feet, but just watch if the height varies and be ready to correct. The ...

Good to know.  I am going to stay "high" just in case..

I get strange behaviour when I have VPS enabled.., and the drone is close to the ground <0.5m.

One time it even decided to land from about 30cm off the ground, and landed hard.
I worry about this when flying/hovering over trees. It may think it's the ground, and try to land.

It is much easier to land without VPS on.., because you aren't constantly fighting the software.