Different calculation of GPS-data?

[Archive]

[Originally posted by linuxuser on 2007-04-15 21:41:28-07]

I write GPS-Data with a script and variables, so I am sure to write the same values.

Code:

curl http://farm1.static.flickr.com/193/460424306_9ff41d3697_o.jpg | /usr/local/bin/Image-ExifTool-new/exiftool -G -H -a -n -  | grep -i GPS
  % Total    % Received % Xferd  Average Speed   Time    Time     Time  Current
                                 Dload  Upload   Total   Spent    Left  Speed
100  141k  100  141k    0     0  94805      0  0:00:01  0:00:01 --:--:--  128k
[EXIF]          0x0000 GPS Version ID                  : 2 2 0 0
[EXIF]          0x0001 GPS Latitude Ref                : N
[EXIF]          0x0002 GPS Latitude                    : 47.8976694444444
[EXIF]          0x0003 GPS Longitude Ref               : E
[EXIF]          0x0004 GPS Longitude                   : 12.8522805555556
[EXIF]          0x0005 GPS Altitude Ref                : 0
[EXIF]          0x0006 GPS Altitude                    : 451
[EXIF]          0x000a GPS Measure Mode                : 3
[EXIF]          0x0012 GPS Map Datum                   : WGS-84
[XMP]                - GPS Altitude                    : 451
[XMP]                - GPS Altitude Ref                : 0
[XMP]                - GPS Latitude                    : 47.8976666666667
[XMP]                - GPS Longitude                   : 12.8523333333333
[Composite]          - GPS Latitude                    : 47.8976694444444
[Composite]          - GPS Latitude Ref                : N
[Composite]          - GPS Longitude                   : 12.8522805555556
[Composite]          - GPS Longitude Ref               : E
[Composite]          - GPS Position                    : 47.8976694444444 12.8522805555556

Obviously latitude and longitude are different with exif and xmp. Why this? I use version 6.86. Please note, probably I have to remove this image soon, after I got a response.

[Archive]

[Originally posted by exiftool on 2007-04-15 22:29:51-07]

I'm sorry.  I don't understand what you are asking.

- Phil

[Archive]

[Originally posted by linuxuser on 2007-04-15 22:43:06-07]

First I have to say sorry, but I needed to test the gps-data with flickr and the image is now at farm1.static.flickr.com/213/460614985_5b35c03581_o.jpg

Please compare

[EXIF]          0x0002 GPS Latitude                    : 47.8976694444444
[XMP]                - GPS Latitude                    : 47.8976666666667

The values are _not_ identical, although I used these values from a textfile: 47.89767|N|12.85228|E| when I wrote the tags.

[Archive]

[Originally posted by exiftool on 2007-04-16 13:48:16-07]

I see now, thanks for the clarification.

There are always round-off errors due to the way the values are stored.
With EXIF, the precision is fixed by the precision of the 64-bit rational
values used, which is very high and much better than the accuracy of
a normal GPS measurement.  With XMP however, the format is ASCII so
the precision is arbitrary and depends on how many decimal points are
written.

Currently, ExifTool writes XMP GPS coordinates with a resolution of 0.01
minutes, which corresponds to an accuracy of a bit better than 20 meters.
Thinking about this now I could see where people may want higher
precision, so in the next ExifTool release (6.87) I will increase the number
of digits to give a resolution of 0.000001 minutes, or about 2mm -- this
should be good enough for just about anyone.

With the update there will still be round-off errors, but they should be
insignificant compared to the accuracy of a GPS measurement.

Thanks for pointing this out.

- Phil