Getting Still Picture Altitude

[RickC]

Revisiting this topic, has anyone come up with an easy way to retrieve the bird AGL for a given still photo?

Since I need to do this all the time, easy is the relevant criteria.

I would even pay /././

- rick

 

[sar104]

Revisiting this topic, has anyone come up with an easy way to retrieve the bird AGL for a given still photo?

Since I need to do this all the time, easy is the relevant criteria.

I would even pay /././

- rick

The EXIF data include relative height (fairly accurate), absolute barometric height (accurate pressure but altitude accuracy varies with the weather) and GNSS altitude AMSL (usually fairly accurate, especially if WAAS is available). Any AGL estimate will require a DEM and calculation – it's not a simple retrievable value:

Height AGL = altitude AMSL – ground height AMSL

or,

Height AGL = HP height AMSL + relative height – ground height AMSL

 

[RickC]

thank you. but what does this refer to?

GPS Altitude :692.70m

 

[sar104]

thank you. but what does this refer to?

GPS Altitude :692.70m

Actually that should be GPS altitude above mean sea level but the field is mislabeled. It's actually altitude above mean sea level calculated from the barometric pressure. That assumes a standard atmosphere (temperature as a function of altitude) and so it is often significantly wrong.

 

[sar104]

Here's the explanation that I posted previously - what I wrote in post #2 is wrong. There is no GPS altitude data in the EXIF file - you have to get that from the DAT file.

From Incorrect EXIF GPS and Elevation

There are two altitudes listed in the EXIF data, in three fields:​

​

Absolute Altitude : +2849.93​

Relative Altitude : +73.00​

GPS Altitude Ref : Above Sea Level​

GPS Altitude : 2849.9 m Above Sea Level​

​

Note that "Relative altitude" is the barometric altitude relative to the takeoff point, and is the only one accurate enough to use. "Absolute altitude" and "GPS altitude" are the same quantity but, just to be confusing, are not derived from GPS data - they are absolute barometric altitudes above MSL based on a standard atmosphere. Those are not reliably accurate since the atmosphere is rarely standard.​

​

Also note that if the absolute barometric altitude is negative (pressure is higher than at sea level in a standard atmosphere) then "Absolute altitude" will read negative but "GPS Altitude Ref" will change to "Below Sea Level" and "GPS Altitude" will be set to zero:​

​

Absolute Altitude : -76.69​

Relative Altitude : +31.60​

GPS Altitude Ref : Below Sea Level​

GPS Altitude : 0 m Above Sea Level​

 

[RickC]

Thank you for the clarification.