Photo Details

[mdasbach]

When opening the details of a photo, it has Altitude. The photo was taken from about 50 ft, but the Altitude in the details shows 241.133. Obviously this isn't feet. I thought it might be decimeters, but that converts to around 80 feet. Does anyone know what this value might be?

 

[Meta4]

When opening the details of a photo, it has Altitude. The photo was taken from about 50 ft, but the Altitude in the details shows 241.133. Obviously this isn't feet. I thought it might be decimeters, but that converts to around 80 feet. Does anyone know what this value might be?
View attachment 110473

It's metres and it's approximately metres above sea level.

 

[mdasbach]

Thanks Meta4, I thought it might be that, but 244 meters is about 800 feet and the ground elevation where it was taken is over 1,000 ft above sea level. Can the approximations be that far off?

 

[Kirby Johnson]

Thanks Meta4, I thought it might be that, but 244 meters is about 800 feet and the ground elevation where it was taken is over 1,000 ft above sea level. Can the approximations be that far off?

Yes. It is just a guesstimate based on barometric pressure. That's the only thing the little drone has to go on, as GPS altitude is even more inaccurate. It can't take into consideration local weather, like a high pressure or a low moving in, etc.

 

[Meta4]

Thanks Meta4, I thought it might be that, but 244 meters is about 800 feet and the ground elevation where it was taken is over 1,000 ft above sea level. Can the approximations be that far off?

Does this answer the question?

 

[NM_Quad]

As previously stated, the barometer is used to guestimate your home point elevation and flight altitude. Barometric pressure varies quite a bit based on the weather (the L and H weather things) over hours effecting the derived altitude. It's that barometer that knows the exact *relative* elevation of the home point. Ever wonder how the DJI drones can return to home and land so gently or hover so precisely? It's that onboard barometer. A barometer changes about 1in Hg for a 1,000 ft. change, so a barometer reading one day of 29.7 in. and 29.9 in. the next day would be a 200 ft. error in derived elevation above sea level. Temperature also changes the derived elevation. However, the barometer remains quite stable over your 20-25 minute flight for fairly accurate determination of altitude and elevation.

 

[sar104]

Yes. It is just a guesstimate based on barometric pressure. That's the only thing the little drone has to go on, as GPS altitude is even more inaccurate. It can't take into consideration local weather, like a high pressure or a low moving in, etc.

Actually GPS altitude is generally good to within 5 meters for airborne GPS/GLONASS receivers as you can easily confirm by looking at the GPS altitude data in the aircraft DAT files. It's unfortunate that it's not used in the EXIF data - even in the field labeled GPS altitude.

 

[Bad Andy]

I had noticed some time ago that there are two different altitudes recorded in two different places in the files' data blocks. One I think is in the EXIF data block and the other appears in the XMP data block. At least for JPEG files. I'm not sure about how the DNG files are constructed.

One is relative to take-off altitude and one is not. I recall some confusion as to whether the one that isn't relative to take-off altitude is GPS data or barometric but I'm guessing GPS.

There's a thread about it here someplace....

 

[sar104]

I had noticed some time ago that there are two different altitudes recorded in two different places in the files' data blocks. One I think is in the EXIF data block and the other appears in the XMP data block. At least for JPEG files. I'm not sure about how the DNG files are constructed.

One is relative to take-off altitude and one is not. I recall some confusion as to whether the one that isn't relative to take-off altitude is GPS data or barometric but I'm guessing GPS.

There's a thread about it here someplace....

It's not GPS data. See the discussion here.

There are 4 fields in the EXIF data referring to altitude:

1. Absolute Altitude: absolute barometric altitude based on standard atmosphere
2. Relative Altitude : barometric height relative to take off point
3. GPS Altitude : identical to (1) - barometric, not GPS despite the field name
4. GPS Altitude Ref : Above/Below Sea Level – refers to whether (1) is positive or negative. If negative, (3) will be set to zero on most firmware

It's neither consistent nor helpful, and I've discussed it with the DJI engineering group, who acknowledged the issue but said there were no immediate plans to fix it. If you want an accurate altitude then either add the ground elevation of the takeoff point to the relative altitude data or go the mobile device DAT file and extract the actual GPS altitude, which is corrected for geoid height and generally pretty good.