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What is the meaning of GPSInfoIFDPointer in an image's EXIF meta data?

Just found a "spec-type" description about "a set of tags ...", but have no idea what that is supposed to mean.

Especially the value 5438 I would like to understand.

EDIT: It seems that the GPS information itself had been removed from the pic. That was what caused the confusion. But still thanks for the clarifications!

That's the picture meta data I am referring to:

Date Time Original: 2011:12:06 09:48:52
Exposure Time: 1/200
F Number: f / 2.80
Exposure Program: Manual
ISO Speed Ratings: 800
Metering Mode: Pattern
Flash: Flash did not fire
Focal Length: 195mm
White Balance: Manual white balance
Make: NIKON CORPORATION
Model: NIKON D3S
Exif Version: 
Flashpix Version: 
Pixel X Dimension: 3661
Pixel Y Dimension: 2432
CompressedBitsPerPixel: 4
Date Time Digitized: 2011:12:06 09:48:52
Subsec Time: 08
Subsec Time Original: 08
Subsec Time Digitized: 08
Max Aperture Value: 3
Light Source: Unknown
Sensing Method: One-chip color area sensor
File Source: DSC
Scene Type: Directly photographed
CFA Pattern: 5420
Custom Rendered: Normal process
Exposure Mode: 1
Digital Zoom Ration: 1
Focal Length In 35mm Film: 195mm
Scene Capture Type: Standard
Gain Control: Low gain up
Contrast: Normal
Saturation: Normal
Sharpness: Normal
Subject Distance Range: Unknown
InteroperabilityIFDPointer: 5400
ExifIFDPointer: 2790
GPSInfoIFDPointer: 5438
Orientation: 1
YCbCr Positioning: 2
Date Time: 2011:12:06 11:18:21
Image Description: REPORT - Der ehemalige Bundesfinanzminister Peer Steinbrück (SPD) sitzt am Dienstag (06.12.11) auf dem Bundesparteitag der SPD in Berlin im Plenum. Unter dem Slogan "Unser Kapital: Demokratie und Gerechtigkeit" berieten die Delegierten der Sozialdemokraten drei Tage lang über die Ausrichtung der Partei vor den Bundestagswahlen 2013.  Foto: Sebastian Kahnert    dpa/lbn
Software: Adobe Photoshop CS4 Windows
Artist: A4363 Sebastian Kahnert
Copyright: Verwendung weltweit, usage worldwide
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3 Answers 3

up vote 2 down vote accepted

Header

From here is a diagram of a basic structure of information embedded in an image file. A pointer is just that, it points to a value/offset/address/place (correct term is offset) within a group of data. All that value does

GPSInfoIFDPointer: 5438

is points to the GPS (Global Positioning System) IFD, or Info Tag. I don't know what 5438 means specifically, but it will be different depending on the amount of data in the stuff before it. So if the Exif IFD takes up little room, the pointer will be a smaller value. Likewise if the information before the GPS IFD takes up more data, it will have to point to a farther value.

Nothing that important. :)

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It is the offset where the GPS Info is. You have to jump to that offset to read GPS info.

EXIF info works the same way. See on the previous line in your example:

ExifIFDPointer: 2790
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thanks for the quick reply, though I don't understand your answer. you mean the offset within the picture-file? I opened it with a hex-editor but didn't find anything useful there. Also converted the number to hex-base and had a look there ... ? –  Raffael Oct 3 '12 at 20:50
    
IFD is Image File Directory. The value is an offset to the specific directory. I am not sure which format your tool is displaying it as. Could be decimal or hex I guess. –  Itai Oct 3 '12 at 21:12

EXIF data in TIFF (and other files based on TIFF, like Nikon RAW and many others) is tree structured. Actually TIFF data is tree structured, with EXIF just being one node in the tree. For example, here is the meta-data dump from one of the RAW files directly from my camera:

TIFF image file directory 1, 27 entries:
  New subfile type: 1 (reduced)
  image_width = 160
  image_length = 120
  bits_per_sample, count 3:
    8 8 8
  Compression: 1 (none)
  Photometric interpetation: 2 (RGB)
  make = "NIKON CORPORATION"
  model = "NIKON D3S"
  strip_offsets = 118478
  orientation = 1
  samples_per_pixel = 3
  rows_per_strip = 120
  strip_byte_counts = 57600
  x_resolution = 300.00
  y_resolution = 300.00
  Planar configuration: 1 (whole pixels together)
  Resolution unit: 2 (inch)
  software = "Ver.1.01"
  date_time = "2012:08:12 11:43:03"
  artist = ""
  subIFD, count 2:
    176320 176440
  reference_black_white, count 6:
    0.0000 255.00 0.0000 255.00 0.0000 255.00
  Copyright: ""
  EXIF IFD = 600
    EXIF exposure time: 4.00 ms, 1/250 s
    EXIF aperture: f/8.00
    EXIF exposure program ID: 3 (Aperture priority)
    EXIF film speed iso: 400
    EXIF date/time original: 2012:08:12 11:43:03
    EXIF date/time digitized: 2012:08:12 11:43:03
    EXIF APEX exposure bias: 0.00
    EXIF APEX max lens aperture: f/4.00
    EXIF metering mode ID: 5 (Pattern)
    EXIF light source type ID: 0 (Unknown)
    EXIF flash status: 0000h
      Did not fire
      No strobe return detetction function
      Flash mode unknown
      Flash function present
      No red-eye reduction mode or unknown
    EXIF lens physical focal length: 40.0 mm
    EXIF manufacturer info, length 117346:
      78 105 107 111 110 0 2 16 ...
    EXIF user comment, character set ASCII:
    EXIF sec fraction date/time: 0.140
    EXIF sec fraction date/time original: 0.140
    EXIF sec fraction date/time digitized: 0.140
    EXIF sensing method ID: 2 (One-chip color area sensor)
    EXIF file source equipment type ID: 3 (Digital still camera)
    EXIF scene type ID: 1 (Directly photographed)
    EXIF sensor color pattern, 2 x 2:
      red grn
      grn blu
    EXIF custom rendering ID: 0 (Normal)
    EXIF exposure mode ID: 0 (Auto)
    EXIF white ballance ID: 0 (Auto)
    EXIF digital zoom ratio: 1.00
    EXIF 35mm equivalent focal length: 40 mm
    EXIF scene capture type ID: 0 (Standard)
    EXIF gain control ID: 1 (Low gain up)
    EXIF applied contrast ID: 0 (Normal)
    EXIF applied saturation ID: 0 (Normal)
    EXIF applied sharpness ID: 0 (Normal)
    EXIF distance range ID: 0 (Unknown)
  GPS info IFD = 176078
    GPS version: 2,2,0,0
    GPS latitude reference: N
    GPS latitude:  33.00000 deg 22.7547 min  0.0000 sec, =  33.37924
    GPS longitude reference: W
    GPS longitude: 113.00000 deg 30.9130 min  0.0000 sec, =-113.51522
    GPS altitude reference: 0
    GPS altitude:   412.0 meters (1351 feet)
    GPS time:  18.00 43.00  3.00 (18:43:03.000 UTC)
    GPS sat: 08
    GPS map datum: 
    GPS date: 2012:08:12
  Original creation date/time: 2012:08:12 11:43:03
  TIFF/EP version ID: 1.0.0.0
Reading subdirectory at 176320, 8 entries:
  New subfile type: 1 (reduced)
  Compression: 6
  x_resolution = 300.00
  y_resolution = 300.00
  Resolution unit: 2 (inch)
  jpeg_interchange_fmt = 179968
  jpeg_interchange_fmt_len = 1545395
  ycbcr_positioning = 2
Reading subdirectory at 176440, 17 entries:
  New subfile type: 0 (normal image)
  image_width = 4288
  image_length = 2844
  bits_per_sample = 14
  Compression: 1 (none)
  Photometric interpetation: 32803 (CFA, color filter array)
  strip_offsets, count 407:
    1725376 1785408 1845440 1905472 1965504 2025536 2085568 2145600 ...
  samples_per_pixel = 1
  rows_per_strip = 7
  strip_byte_counts, count 407:
    60032 60032 60032 60032 60032 60032 60032 60032 ...
  x_resolution = 300.00
  y_resolution = 300.00
  Planar configuration: 1 (whole pixels together)
  Resolution unit: 2 (inch)
  CFA repeating pattern size: 2 x 2
  CFA pattern, count 4:
    red grn
    grn blu
  Sensor layout type: 2 (1-chip color area)

TIFF data is stored in "directories" or "IFDs", which can be tree structured. My program that dumps this information uses indentation to show the tree structure. Note how all the EXIF data is in one directory (IFD), and the GPS info added because I had a GPS receiver attached when the picture was taken in another separte IFD.

The TIFF and EXIF tags are standard and documented out there. Sometimes manufacturers add their own private information using private tags. Notice the "EXIF manufacturer info" tag. That is Nikon proprietary, and contains some color ballance information apparently.

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