altitude not showing right

[stargazing]

After turning on the Vision Plus, connection to the App and has 7 Sat linkup for some reason the APP shows that it is between 25 to 45' just sitting on the ground not even flying. I find this kind of frustrating as it is not giving me a true altitude from startup. is there anyway to get the APP to show 0 altitude before I start flying?

Thanks Paul

 

[Dirty Bird]

Your altitude isn't 0' unless you are at sea level. What is the altitude above sea level of your location?

 

[Meta4]

Your altitude isn't 0' unless you are at sea level. What is the altitude above sea level of your location?

But the Phantom has no knowledge of sea level - only relative altitude from the home point which is/should be zero.
I'm not sure what would cause or fix it for the OP.
Maybe a faulty barometric sensor?

 

[Dirty Bird]

This is an interesting question that I have never really considered. Does the Phantom start out assuming it is @ 0', or does it attempt to ascertain your actual altitude above sea level? I am unsure of the answer? My bird never shows 0' upon power up, and usually displays in the 3-15' range'. In my typical flight locations those would be expected & accurate ASL readings.

From a simplicity point-of-view, it makes sense to simply initialize the pressure sensor on power up, and display this point as 0'. The effect of variances in local atmospheric pressure would be minimal flight to flight, and no calibration would be necessary.

The bird cannot rely soley on the internal pressure sensor, as this is affected by atmospheric pressure conditions and requires calibration. Most devices with a barometer, a Garmin Vista for example, have a means for calibrating the barometer. No such means exists for the Phantom.

Using GPS-based altitude readings for determining initial altitude would result in wild variations in accuracy depending on the amount of clear sky surrounding the aircraft. If I recall correctly, GPS-based altitude readings suffer approximately 1.5x the error of horizontal positioning. The discrepancy can be quite high...75' or more!

I am curious about this now and will pay closer attention over my next flights. It will be difficult for me to judge as I am so close to actual sea level. I wonder what those who live at higher elevations see when initializing?

 

[Meta4]

Yes DB .. it's all related to home point.
I live at 1500 ft but my bird starts in the backyard at zero.
Yesterday I was flying from a coastal headland about 100 ft above the sea.
Chasing a sailboat my Phantom's altitude was showing as negative 50 ft because the bird was 50 ft below home point (but still 50 ft above the sea).
The Phantom uses a barometric altimeter for height because it's more accurate than GPS.
You see how sensitive it is when you hand catch and try to shut down.
It won't shut down until you can hold the Phantom completely still.
The sensor detects movements of just a few millimetres.
A Garmin needs calibration because it is concerned with real altitude but the Phantom doesn't because it only cares about relative altitude.

 

[Dirty Bird]

Thank you for clarifying. I don't usually hand catch but when I did I noticed you have to hold it really steady for the motors to stop automatically.

 

[morph000]

My understanding is that when you calibrate the compass it sets your point of origin to ZERO and so your distance and altitude are zero relative to your current position, not sea level or anything else, and that doesn't seem to be happening for folks.
Today (and recently) both have been out - distance at compass calibration point shows as around 10m and altitude minus 3 or 4m.
For the record, my smartphone satellite apps say I was 21m above sea level which I'd say is pretty right.
Not the end of the world but irritating if you want to know specific distances for a reason !
However...today I got a bit too close to a tree in very gusty wind, it hit the tree 2 foot above ground, flipped and landed on it's back.
No damage at all BUT when I put it back at the home point to go flying again, distance and altitude were both within 0.5m !!
Seems the knock did it good !
Any explanation ?
I'm thinking if the altimeter is mechanical,even the tightness of the mounting screws could impact on accuracy due to torquing of the device.
Dodgy IMU ? Where is it located ?
Maybe I'll try calibrating it upside down ! :mrgreen: