GPSAltitude discrepancy

[Storhaug]

I am new to geotagging, and I am scratching my head about a mystery:
With some JPGs, I noticed a small discrepancy in the value of the GPSAltitude tag depending on whether I have the GPS-tags written into the JPGs by
- exiftool 8.15 on Fedora 13,  or
- GeoTagger on XP using exiftool 8.28.

To illustrate, here are some of the GPS Altitudes:

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GeoSetter with exiftool 8.28 XP:    exiftool 8.15 on Fedora 13:
587.4 m Above Sea Level             587.3 m Above Sea Level
585.5 m Above Sea Level             584.9 m Above Sea Level
590.1 m Above Sea Level             590 m Above Sea Level
589.7 m Above Sea Level             588 m Above Sea Level

There is no discrepancy in the values for Longitude and Latitude. So, I wonder how this discrepancy of GPSAltitude happens. Any idea is very welcome.

Attached are both outputs from exiftool for the GPS tags, and the GPX-file.

For now, I do not attach the photos in order to save some disk space. Upon request, I will attach the photos.

[Phil Harvey]

I don't understand the difference between the different versions if you are using the exact same GPX file, but this particular GPX file is problematic because of the "rtept" records that ExifTool isn't handling properly.  This was an oversight on my part since I missed these records in the GPX documentation and didn't have a sample like this for testing.  The result is that the altitude values are associated with the next fix (ie. wrong by 5 seconds).

I will apply a patch and this will be fixed in the next release (8.29).

Thanks for reporting this.

- Phil

[Storhaug]

Finally I ran the geotagging with exiftool version 8.29. In that particular case, the result was the same as with the previous exiftool version.

I assume, the fix may make a difference in other situations. I don't have much testing material because I do not own a GPS tracker myself (I want to buy a digicam with builtin GPS).

[Phil Harvey]

I tested this again with your gpx file and got these results:

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> Image-ExifTool-8.28/exiftool a.jpg -datetimeoriginal="2010:08:06 11:35:46Z" -n
    1 image files updated

> Image-ExifTool-8.28/exiftool a.jpg -geotag M-241_Start_20100806-105800_Finish_20100806-164740.gpx
    1 image files updated

> Image-ExifTool-8.28/exiftool a.jpg -exiftoolversion -gpsaltitude -n
ExifTool Version Number         : 8.28
GPS Altitude                    : 584.916

> Image-ExifTool-8.29/exiftool a.jpg -geotag M-241_Start_20100806-105800_Finish_20100806-164740.gpx
    1 image files updated

> Image-ExifTool-8.29/exiftool a.jpg -exiftoolversion -gpsaltitude -n
ExifTool Version Number         : 8.29
GPS Altitude                    : 585.516


I believe the value of 585.516 m (ExifTool 8.29) is correct.  There should be no platform dependency.  If you don't get these results then my guess is that you somehow aren't running version 8.29.  You can run these exact commands on a test file to check this.

- Phil