Today, I had several flights where I maintained low altitude for most of the flight's duration.  I was on flat ground with no slope or change in altitude, taking off and landing from the same place.  I noticed about 3 minutes into the flight, after flying around about 3-4' AGL, the altimeter on the DJI Go4 app (latest firmare/software available for P4P+) was telling me the bird was 15' off of the ground, when in fact it was absolutely no more than 3' off of the ground.  It seemed like I was also getting some false positive readings from the downward and/or forward obstacle avoidance sensors.  More importantly, I noticed that the P4P shutters and shakes violently when landing and the altimeter is giving a reading of 6-8" AGL when in fact the drone is already on the ground.  I know that the shuttering/shaking problem is being discussed in the P4P+ firmware thread, but has anyone else experienced any of these symptoms, or come up with a fix?  The compass and IMU were calibrated not long ago.  I suppose I can re-calibrate, but all of the interference values are extremely low, and show no indication of needing a calibration.  I wanted to make sure this wasn't a bigger issue that needed a fix from DJI before I jumped in the rabbit hole looking for problems on my end.  In summary...

1. P4P is giving a bad altimeter reading when flown consistently 2-5' AGL. (Usually off by 10-15')
2. Low AGL flying seems to trigger obstacle avoidance (and bad altimeter reading) even when the area is free and clear.
3. Incorrect altimeter values seem to have negative effect on landing operation, causing violent shutter/shaking until motors are completely shut down.

Sidenote, after low AGL flying I took it up to about 200ft AGL (at least 100' above any land obstacles) and would get occasional obstacle avoidance alerts.  Very sunny day today, and wondering if it could have been sun glare causing the alerts...  is this a known issue?  

Attached is a screencap from my P4P+ controller, and a photo of the drone at the same time.  Huge discrepency in DJI Go4 reported altitude, and actual AGL altitude seen in the photo.  The bird is hovering right above where I took off from.  

Sun glare will definetly give faulse positive on the obstacle. Withe the altimeter, yoiu need to be above 10' to register the correct alt.

Flying High AZ Posted at 2017-2-15 15:30
Attached is a screencap from my P4P+ controller, and a photo of the drone at the same time.  Huge discrepency in DJI Go4 reported altitude, and actual AGL altitude seen in the photo.  The bird is hovering right above where I took off from.

The screen is reporting the VPS altitude as 1.6 feet, that looks pretty close to me.

'3. Incorrect altimeter values seem to have negative effect on landing operation, causing violent shutter/shaking until motors are completely shut down.'

The barometer reading has no effect on the shaking you see, it is an unrelated issue.

dronist Posted at 2017-2-15 20:02
Sun glare will definetly give faulse positive on the obstacle. Withe the altimeter, yoiu need to be above 10' to register the correct alt.

Thanks!  I'll check it at 10' and above.  Never noticed a bad alt reading until today.

Geebax Posted at 2017-2-15 20:11
The screen is reporting the VPS altitude as 1.6 feet, that looks pretty close to me.

'3. Incorrect altimeter values seem to have negative effect on landing operation, causing violent shutter/shaking until motors are completely shut down.'

Interesting... should I be using the VPS altitude when I'm below a certain altitude?  I wasn't even paying attention to that value, as I think I've trained my peripheral to see that data as obstacle avoidance notifications.  Any idea/explanation as to why the barometer is giving a false altitude reading?  Thanks for your insight.  

Flying High AZ Posted at 2017-2-15 20:52
Interesting... should I be using the VPS altitude when I'm below a certain altitude?  I wasn't even paying attention to that value, as I think I've trained my peripheral to see that data as obstacle avoidance notifications.  Any idea/explanation as to why the barometer is giving a false altitude reading?  Thanks for your insight.

The barometer reading is normal behaviour for such a sensor. Normal weather conditions cause changes in atmospheric pressure, and as the barometer is a sophisiticated pressure sensor, its reading will vary with the pressure changes. Even commercial aircraft have the same issues to contend with, except they can calibrate their altimeter with the known atmospheric pressure at a given location. This is also why the aircraft has an ultrasonic transducer to establish altitude accurately below 3 Metres.

edited for content...