How to Work With Image Metadata

Updated on December 17, 2018
Image and Metadata Shown in Windows Photo Gallery
Image and Metadata Shown in Windows Photo Gallery | Source

What is metadata?

Metadata is news these days: new data retention laws make a stab at defining what it is in the context of tracking phone and message traffic. The non-veracity of image or video data can sometimes be detected by inconsistencies in the metadata. Whilst digital manipulation can create images or videos of non-existent scenes for nefarious purposes, image and video metadata is so complex that efforts to hide the manipulation are frequently unsuccessful if they are attempted at all.

A concise definition of metadata as ‘data about data’ captures it pretty well. Metadata exists as a group of fields, each of which stores some aspect of the data. All digital images on any device exist as files – regions of binary storage that contain bytes describing a scene. Files can only be easily accessed via a filesystem, which provides a framework for storing and accessing files. Metadata can occupy any number of bytes – decoding it relies on applications distinguishing the bytes used for metadata from those used for describing the image scene.

Image Metadata

Digital images contain a large number of metadata fields describing its attributes as a file. The most conspicuous field is the file name. For digital cameras, this field is defined automatically to be unique for each photograph, usually by including an ascending number in the file name. When files are transferred between devices (such as from a camera or mobile phone to a computer), metadata fields common to the source and destination filesystems are transferred. However, the fields describing the same data attribute may have different names in the source and destination fields, so even at this most basic level, the file metadata appearing on a different device from the one on which the image was taken will be different. Date metadata is particularly problematic – should the original Created date of a file be preserved when copying to another device or should it be set to the date of copying? There is no right answer here – the result is a matter of convention, and conventions differ between filesystems.

Names of metadata fields are not included in the data. Metadata format definitions typically specify names, indexes and data types for fields, but applications may use different names for fields which they display. Software components for accessing metadata used by applications may use different names from those specified in the standard and by the application. The variety of names for the same metadata field in different applications adds to the confusion.

For images, data about the image and the settings of the device used to record it form another domain of metadata. There is a standard (EXchangeable Image Format, or EXIF) for storing image and audio file metadata as follows:

  • Date and time information. Digital cameras will record the current date and time and save this in the metadata. Mobile phone cameras usually include Global Positioning System (GPS) data from a built-in receiver including latitude and longitude in decimal degrees.
  • Camera settings. This includes static information such as the camera model and make, and information that varies with each image such as orientation (rotation), aperture, shutter speed, focal length, metering mode, and ISO speed information.
  • A thumbnail for previewing the picture on the camera's LCD screen, in file managers, or in photo manipulation software.
  • Descriptions
  • Copyright information

Nearly all phone and digital camera manufacturers support this standard for JPEG files which they create.

There are many other groups of metadata fields as well as EXIF which may be found in JPEG files.

Metadata Groups

Metadata Group
Description
JFIF
JFIF stands for JPEG Interchange Format. Thus, the JFIF version can be found in JPG or JPEG files only. It is a standard to define JPEG image files. It contains information that would otherwise be missing from the original JPEG Part 1 image format standard.
XMP
The Extensible Metadata Platform (XMP) is an ISO standard, originally created by Adobe Systems Inc., for the creation, processing and interchange of standardized and custom metadata for digital documents and data sets.
IPTC
The Information Interchange Model (IIM) is a file structure and set of metadata attributes that can be applied to text, images and other media types. It was developed in the early 1990s by the International Press Telecommunications Council (IPTC) to expedite the international exchange of news among newspapers and news agencies.
ICC
Colour Space Profile data.

The Wikipedia page on EXIF provides a lot more detail on this standard, its implementation, and its limitations. Many image processing programs (such as Adobe Photoshop) store a record of their use as a separate group in metadata when the file is saved in a format supporting EXIF metadata. However, image editing software may not always preserve all existing metadata when saving a file and it may add some metadata of its own.

Why Should I Worry About Image Metadata?

Most of the time, you needn’t do so, unless you’re a professional photographer. Then you might be interested in adding a wide range of metadata in a robust way that’s always attached to the image, whatever platform it is stored on.

You may have an interest in setting date and location metadata on scanned images of old slides and photos so that they can be integrated into presentations including images from digital cameras. Tablets and phones work excellently at displaying images which they have recorded, using data recorded at image capture, but if you upload external images to them you may find that they appear out of time order, with incorrect times, or with incorrect or missing location data. One tablet will only display images in filename order using the native image viewer. If this happens, you’ll have to delve into setting image metadata, which involves a few technical hurdles.

Dates and Times

There is no one answer to the question “What is the image date/time? ” in the metadata. The listing of metadata fields for a mobile phone image copied to a desktop contains 8 date/time fields that might be used as shown below:

ExifTool Name
ExifTool Value
Notes
File Modification Date/Time
2018:09:10 12:36:57+10:00
Filesystem field - will change on editing. Local time includes GMT offset at time of setting.
File Access Date/Time
2018:11:07 14:53:58+11:00
Filesystem field - will change on access. Local time includes GMT offset at time of setting
File Creation Date/Time
2018:09:06 17:12:00+10:00
Filesystem field - may change on copying. Local time includes GMT offset at time of setting.
Modify Date
2018:08:27 12:58:05
EXIF field - not changed by editing, copying or access. Local time.
Date/Time Original
2018:08:27 12:58:05
EXIF field - not changed by editing, copying or access. Local time.
Create Date
2018:08:27 12:58:05
EXIF field - not changed by editing, copying or access. Local time.
GPS Date Stamp
2018:08:27
GPS field - not changed by editing, copying or access. GMT
GPS Time Stamp
02:58:04.34
GPS field - not changed by editing, copying or access. GMT.

5 different dates are present in these metadata fields. The GPS and EXIF dates differ only from the use of GMT rather than local time, but the range of filesystem dates reflect dates of copying and modification of the file.

Images from digital cameras (not phones) will seldom have GPS data included and scanned JPEG images will have not have EXIF or GPS data.

The choice of which field to use for the image date is up to the application designer. Filesystem date fields are always present but may be modified by processing. The image date may sometimes be conditional on the presence of particular fields or use the field has the earliest date. Most commonly, one of the EXIF fields is used. In many cases, the actual time is less important than the ordering of the images, but if you want to avoid showing photos of yourself in full daylight with a time of 3 am you may need to do some adjustment.

Geotagging Software

The location information presented as text in phone and tablet photo display apps is derived from the GPS latitude and longitude using a reverse geolocation web service, which is why the location text takes some time to appear after the photo is taken. After reverse geolocation, the location structure is stored in metadata for rapid access.

The free Windows desktop application GeoSetter can set location(Lat/Long and text fields) and date information, but the interface is somewhat daunting. It needs careful configuration as described here in order to reproduce the functionality of Photo Gallery. Locations are set by clicking on a map. Other geotagging applications (GPicSync, GeoTag Photos Pro) are specifically designed for geotagging born-digital photos by correlating dates with locations. It’s not clear if these would work efficiently with scanned images. The Google free geolocation service of providing a latitude/longitude from a location name no longer seems to be available. There are a number of web applications offering geotagging of images (eg GeoTagMyPhoto, GeoImgr) but not all of them appear to work.

Using Metadata

When images are viewed on phones and tablets, only the image pixels are visible using native viewing/editing applications. The only metadata which can be displayed or used for ordering is date and location. The timeline (date ordered) view is particularly popular. Images can be grouped into albums, and text can be written on top of the image pixels. Information stored in other metadata (including the file name) that desktop users might expect to see, is not shown. If you want to provide text information (such as the names of people) in the image without writing over existing pixels, a review article here looks at some software you might use for this. Some software will automatically map text from existing metadata fields onto image pixels and provide facilities for setting dates. Ken Watson’s excellent website Digital Photos gives an overview of many aspects of dealing with digital images for archival purposes, where metadata is crucial.

Viewing and Editing Image Metadata

Most image viewing software shows some metadata, most commonly the pixel dimensions of the image and a date, but no image viewing applications show all of it, probably because there are such a large number of different fields which may contain data. (See Exiftool section for an example of the 141 metadata fields stored in an iPhone image.) Applications may also choose their own names for metadata fields when displaying them. In Windows, some metadata fields can be read (and set) via the Details tab of file properties, accessed via a right-click on the file. However, the names shown do not correspond to metadata field names. Metadata may be processed before display – the Location data shown by iPhone Photos app is a text string derived from the latitude and longitude stored by the built-in GPS receiver. Many applications use metadata for their own purposes.

Microsoft Photo Gallery

Microsoft’s now-unsupported Photo Gallery, which was included in Windows Essentials, offered editing and storage of metadata for People, a Geotag, a Caption, and Descriptive Tags within image file metadata as listed below. It included face recognition to identify named faces in loaded photos in the same way as Google Photos. Although no longer supported as part of Windows Essentials (and not available for download from Microsoft), downloads are available from other sources and the code will run under Windows 10. Geo-tagging and facet recognition (to identify named people in other photos) are no longer available. Many fields accessible from Photo Gallery are stored in more than one metadata field as shown below:

Name
Single/Multiple
Storage (Exiftool group and field names)
People
Multiple
XMP - Region Person Display Name
Geotag
Single
XMP - Location (also read from XMP - State, City & Country)
Caption
Single
EXIF - Image Description, EXIF- XP Title, XMP - Title, XMP - Description,
Descriptive Tags
Multiple
EXIF - XP Keywords, XMP - Subject , XMP - Last Keyword XMP

For users who have processed images with Photo Gallery, there is a blog post on how to move on. Windows desktop application Caption Pro also provides inbuilt functionality for transferring Photo Gallery metadata to image pixels, as well as IPTC and XMP metadata added by professional photographic software.

ExifTool

Metadata handling is very much the domain of the enthusiast and anyone with an interest in the area owes a debt to Canadian physicist-photographer-programmer Phil Harvey, creator of the multi-platform ExifTool freeware, which can be used for viewing and editing image metadata. Under development for 15 years, ExifTool is a command-line utility that can list and edit a vast range of image metadata. An example of 141 metadata fields extracted from a mobile phone camera image by ExifTool is shown below. If you don’t know what most of them describe, you’re not alone.

Metadata Fields from a Digital Camera Image

Field
Value
ExifTool Version Number
10.61
---- File ----
 
File Name
IMG_2256.jpg
Directory
.
File Size
674 kB
File Modification Date/Time
2018:09:10 12:36:57+10:00
File Access Date/Time
2018:11:07 14:53:58+11:00
File Creation Date/Time
2018:09:06 17:12:00+10:00
File Permissions
rw-rw-rw-
File Type
JPEG
File Type Extension
jpg
MIME Type
image/jpeg
Exif Byte Order
Big-endian (Motorola, MM)
Image Width
2448
Image Height
3264
Encoding Process
Baseline DCT, Huffman coding
Bits Per Sample
8
Color Components
3
Y Cb Cr Sub Sampling
YCbCr4:2:0 (2 2)
---- JFIF ----
 
JFIF Version
1.01
---- EXIF ----
 
Image Description
Test
Make
Apple
Camera Model Name
iPhone 5
X Resolution
72
Y Resolution
72
Resolution Unit
inches
Software
10.3.3
Modify Date
2018:08:27 12:58:05
Y Cb Cr Positioning
Centered
Exposure Time
1/20
F Number
2.4
Exposure Program
Program AE
ISO
80
Exif Version
221
Date/Time Original
2018:08:27 12:58:05
Create Date
2018:08:27 12:58:05
Components Configuration
Y, Cb, Cr, -
Shutter Speed Value
1/20
Aperture Value
2.4
Brightness Value
2.737327805
Exposure Compensation
0
Metering Mode
Multi-segment
Flash
Auto, Did not fire
Focal Length
4.1 mm
Subject Area
1631 1223 1795 1077
Sub Sec Time Original
483
Sub Sec Time Digitized
483
Flashpix Version
100
Exif Image Width
2448
Exif Image Height
3264
Sensing Method
One-chip color area
Scene Type
Directly photographed
Exposure Mode
Auto
White Balance
Auto
Focal Length In 35mm Format
33 mm
Scene Capture Type
Standard
Offset Schema
0
GPS Latitude Ref
South
GPS Longitude Ref
East
GPS Altitude Ref
Above Sea Level
GPS Time Stamp
02:58:04.34
GPS Speed Ref
km/h
GPS Speed
0
GPS Img Direction Ref
True North
GPS Img Direction
259.425641
GPS Dest Bearing Ref
True North
GPS Dest Bearing
259.425641
GPS Date Stamp
2018:08:27
XP Title
Test
XP Subject
 
Lens Info
4.12mm f/2.4
Lens Make
Apple
Lens Model
iPhone 5 back camera 4.12mm f/2.4
Padding
(Binary data 2060 bytes, use -b option to extract)
Compression
JPEG (old-style)
Thumbnail Offset
5638
Thumbnail Length
3435
Thumbnail Image
(Binary data 3435 bytes, use -b option to extract)
---- MakerNotes ----
 
Run Time Scale
1000000000
Run Time Value
6.57328E+13
Run Time Epoch
0
Run Time Flags
Valid
Acceleration Vector
-0.02057652645 -0.3014756349 -0.9511387164
---- XMP ----
 
About
uuid:faf5bdd5-ba3d-11da-ad31-d33d75182f1b
Title
Test
Description
 
Headline
Test
---- ICC_Profile ----
 
Profile CMM Type
Lino
Profile Version
2.1.0
Profile Class
Display Device Profile
Color Space Data
RGB
Profile Connection Space
XYZ
Profile Date Time
1998:02:09 06:49:00
Profile File Signature
acsp
Primary Platform
Microsoft Corporation
CMM Flags
Not Embedded, Independent
Device Manufacturer
IEC
Device Model
sRGB
Device Attributes
Reflective, Glossy, Positive, Color
Rendering Intent
Perceptual
Connection Space Illuminant
0.9642 1 0.82491
Profile Creator
HP
Profile ID
0
Profile Copyright
Copyright (c) 1998 Hewlett-Packard Company
Profile Description
sRGB IEC61966-2.1
Media White Point
0.95045 1 1.08905
Media Black Point
0 0 0
Red Matrix Column
0.43607 0.22249 0.01392
Green Matrix Column
0.38515 0.71687 0.09708
Blue Matrix Column
0.14307 0.06061 0.7141
Device Mfg Desc
IEC http://www.iec.ch
Device Model Desc
IEC 61966-2.1 Default RGB colour space - sRGB
Viewing Cond Desc
Reference Viewing Condition in IEC61966-2.1
Viewing Cond Illuminant
19.6445 20.3718 16.8089
Viewing Cond Surround
3.92889 4.07439 3.36179
Viewing Cond Illuminant Type
D50
Luminance
76.03647 80 87.12462
Measurement Observer
CIE 1931
Measurement Backing
0 0 0
Measurement Geometry
Unknown
Measurement Flare
1.00%
Measurement Illuminant
D65
Technology
Cathode Ray Tube Display
Red Tone Reproduction Curve
(Binary data 2060 bytes, use -b option to extract)
Green Tone Reproduction Curve
(Binary data 2060 bytes, use -b option to extract)
Blue Tone Reproduction Curve
(Binary data 2060 bytes, use -b option to extract)
---- Composite ----
 
Aperture
2.4
GPS Altitude
573 m Above Sea Level
GPS Date/Time
2018:08:27 02:58:04.34Z
GPS Latitude
35 deg 15' 21.86" S
GPS Longitude
149 deg 7' 35.44" E
GPS Position
35 deg 15' 21.86" S, 149 deg 7' 35.44" E
Image Size
2448x3264
Megapixels
8
Run Time Since Power Up
18:15:32
Scale Factor To 35 mm Equivalent
8
Shutter Speed
1/20
Create Date
2018:08:27 12:58:05.483
Date/Time Original
2018:08:27 12:58:05.483
Circle Of Confusion
0.004 mm
Field Of View
57.2 deg
Focal Length
4.1 mm (35 mm equivalent
Hyperfocal Distance
1.89 m
Light Value
7.2

This article is accurate and true to the best of the author’s knowledge. Content is for informational or entertainment purposes only and does not substitute for personal counsel or professional advice in business, financial, legal, or technical matters.

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