What is the meaning of Altitude in EXIF of an image?

Question

Currently I have some photographs​ captured by DJI, and in their EXIF I saw that it contains: GPS Altitude, Absolute Altitude, Relative Altitude, GPS Latitude, GPS Longitude.

These images will be used for creating TIFF file by Pix4D. After creating TIFF file from those images, I use the GPS Lat/Long of one of those images and then extract the elevation from TIFF file and that elevation is different with the altitude from the image. I also use Google Elevation API and it also different from the altitude of the image (and the elevation from my raster, too).

For example, with this lat/lon (GPS value from EXIF) 15.99849675/108.246482777778 I get those 3 values​:

• Altitude (GPS and Absolute) in the EXIF: 62.9m Above Sea Level
• Relative Altitude: 49.80
• Elevation I get from TIFF: 21.9400272369
• Elevation I get from Google API: 4.45

So my questions are:

• What is the meaning of the altitude in EXIF of an image?
• The accuracy between the elevation extract from TIFF and the Google elevation API

Answer 1

The newest EXIF specification http://www.cipa.jp/std/documents/e/DC-008-Translation-2016-E.pdf knows only GPSAltitudeRe and GPSAltitude. They are defined as

GPSAltitudeRef Indicates the altitude used as the reference altitude. If the reference is sea level and the altitude is above sea level, 0 is given. If the altitude is below sea level, a value of 1 is given and the altitude is indicated as an absolute value in the GPSAltitude tag. The reference unit is meters. Note that this tag is BYTE type, unlike other reference tags.

GPSAltitude Indicates the altitude based on the reference in GPSAltitudeRef. Altitude is expressed as one RATIONAL value. The reference unit is meters.

Absolute and Relative altitudes seem to be some non-standard tags and you should ask DJI what they mean.