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by Bart Arondson

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While the meaning of most of the parameters in the exif info is decipherable, I didn't understand what do these two mean? Also, it would be great if someone can explain how are they measured. For an example, check this out: http://www.flickr.com/photos/19224593@N04/5613491852/meta/

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This question is definitely on topic here, but there's also a Geographic Information Systems site. Some folks over there could probably go into way more depth about the whole geoid/ellipsoid thing. –  Evan Krall Apr 12 '11 at 23:35

3 Answers 3

up vote 2 down vote accepted

Check out the spec (pdf). I think Global Angle is GPSImgDirection (pp.50) and Global Altitude is GPSAltitude (pp.48).

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The altitude is the height above the geoid, and can be calculated by a GPS unit that has a good signal from at least four satellites. If the planet were a perfect ellipsoid, this would be the height above mean sea level.

The angle is the bearing in which the camera was facing at the time - this can either be measured with an electronic compass or interpolated from two GPS fixes.

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FWIW, a geoid and an ellipsoid differ. The latter is used for geodesy: it is an idealized mathematical model used to locate latitude, longitude, and elevation. The geoid is based on gravity and force; it approximates the hypothetical mean sea level. It is usually described as deviations relative to an ellipsoid. The deviations vary up and down, to about 100 m. If the altitude was recorded by a GPS unit, it could be referenced to a geoid (e.g., NMEA) or ellipsoid (e.g., WGS84) See gpspassion.com/forumsen/topic.asp?TOPIC_ID=10915 –  whuber Apr 12 '11 at 19:33

Here is a bit more in-depth Exif information:

The Exif property GPSAltitude holds ONLY the altitude in meters; it goes along with GPSAltitudeRef, which determines if whether it's above / below sea level. A negative value indicates it's below sea level (i.e. Holland or the Dead Sea).

The same is true for GPSImgDirection plus GPSImgDirectionRef, which specifies whether the direction is given as "true" north or "magnetic" north. iPhone 4 owners can see the difference with the Compass app, it allows you to switch between the two different references (I can see a difference of 13 degrees at my location). By default, GPS directions are given in "true" values.

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