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DataIntegrity

Page history last edited by PBworks 12 years, 11 months ago

data integrity

 

A place to collect random notes / thoughts on data integrity and methods of preservation.

 

digital photograph datetime stamps

Perhaps worth its own page, but for now, kept as a subsection of DataIntegrity.

 

Digital photograph datetime stamps (both in the modification date of the photo files, e.g. the JPEGs, and the date taken in the EXIF) are really easy to mess up, that is, be inconsistent with the actual physical wall clock time of when the photograph was taken - what that information in/about the photo file is supposed to represent.

 

Simplest working scenario live only in Arizona

If you live in Arizona, and never travel outside of Arizona, and set all your clocks (including your camera clock) to Arizona time, and set your computer to be in the Arizona time zone, then everything will work just fine.

 

However, here are some easy ways to change the above scenario to mess up your datetime stamps.

 

Live only in California where Daylight Savings Time is observed

If all you change to the above scenario is to live only in California instead of Arizona you get the Daylight Savings Time problem - datetime stamps off by an hour. Same problem happens if you live anywhere that observes Daylight Savings Time.

 

Say you set all your clocks in January, during "standard time", including the clock in your camera. Note that your camera has no notion of what time zone its clock is set to

 

Now you take a bunch of photos. You upload the photos to your computer which is set to Pacific time (and automatically knows to be in Pacific Standard Time (PST) because of clever programming and databases and what not).

 

Everything works fine because your computer assumes the datetime stamps it gets from the photos in the camera are in the *exact* same time zone as the computer itself, and because you set the camera clock that way, they are.

 

It's February. You take a bunch more photos. You forget to upload them to your computer.

 

It's now April. Your computer (remember the cleverness?) has automatically switched to Pacific Daylight Time and set its clock one hour forward. Your camera (which doesn't know anything about time zones) is still effectively set to a PST time.

 

You upload your February photos to your computer. All the photo's modification datetimes are now off by an hour. Data integrity lost.

 

Solution

In March/April, just after your computer has switched from PST to PDT, when you still have photos taken during PST in your camera:

 

  • change computer timezone to Alaska Daylight Time (AKDT -0800).
  • upload camera photos taken during PST to computer.
  • set computer timezone to SF/PDT.
  • remove memory card from camera (in case setting the clock can affect the memory card)
  • set camera clock forward 1 hour, that is sync it with PDT
  • insert a different memory card into the camera and erase it (again, in case setting the clock can affect the memory card)

 

In October/November (as twittered), just after your computer has switched from PDT to PST, when you still have photos taken during PDT in your camera:

 

  • change computer timezone to Arizona.
  • upload camera photos taken during PDT to computer.
  • set computer timezone to SF/PST.
  • remove memory card from camera (in case setting the clock can affect the memory card)
  • set camera clock back 1hr, that is sync it with PST
  • insert a different memory card into the camera and erase it (again, in case setting the clock can affect the memory card)

 

Travel to different time zones

If you travel the different time zones, the above Daylight Savings Time switchover and subsequent one hour of error in datetime stamps can turn into several hours, or in the late evening/morning hours, effectively change the date of the datetime stamp as well.

 

The *simplest* solution to the traveling to different timezones problem that I have found is to:

  • DO NOT adjust the time zone on your computer to your travel destination- yes the clock will still show your home time (which can actually be useful if/when you are communicating with folks back home).
  • DO NOT change the clock on your camera.
  • DO wear a watch which you set to the local time so you can at a glance see what time it is where you are.

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