Digital Asset Management
with digiKam

Welcome to part II

of this article by Gerhard Kulzer.
This article treats many aspects of how to preserve your digital assets, in particular your photographs and scans. Much of what is being explained here is equally and readily applicable to manage other digital assets such as music files, movies, documents, art work, etc.

Protect your images from data corruption and loss

Themes: disk errors, disk failures, power surges, ECC, transmission errors, storage media deterioration, recovery, redundancy, disaster prevention, lifetime, temperature, data size, common myths

What are then the main factors of digital data loss?

Of course we're not talking about losing CDs on the road or in a fire - that kind of loss is just the same as traditional paper copies or negatives. We are talking about problems with the so called "New Media".

Problems with digital data can roughly be categorized into the following areas of concern:

1) the physical deterioration of the media (all media deteriorate at different time scales)

2) undetected transmission errors during data transfer

3) the lack of support for long-date, undoubtedly proprietary, digital formats

4) ancient hardware.

Kroll Ontrack, the worlds largest data recovery firm, have some interesting statistics on what actually causes data loss.

Cause of data loss

Perception Reality
Hardware or system problem 78% 56%
Human error 11% 26%
Software corruption or problem 7% 9%
Computer viruses 2% 4%
Disaster 1-2% 1-2%

So let us analyze those cases step by step!

Physical deterioration

CD, DVD, optical drives

Physical deterioration of the media happens more rapidly with paper and CD-Rs than the average of film. Yet while film lasts longer (sometimes decades longer) than other forms of media, the right kind of backup of digital media never loses anything. Film decays - digital 1's and 0's do not, and film starts to decay the moment it's created and developed. It will never have the same color, contrast, etc. that it did have a moment before. Digital doesn't do that. However, digital is susceptible to corruption! And yes, physical media such as floppies and magnetic hard drives are also susceptible to the decay of the medium, just like CDs are. They just last longer.

To combat the problem of CDs/DVDs, they need to be properly cared for and not trusted for more than a few years. Thankfully you can purchase archive-quality CDs and DVDs which last longer, though they are much more difficult to obtain and are much more expensive. There are offers out there for gold-plated DVDs, $2 a piece claiming 100 years storage life (if you care to believe it).

CD/DVD disks may become unreadable, but you can reduce the risk using good disks and a good recorder, and storing them in a correct way. The best DVD recorders are not much more expensive than the cheapest, but they write in a much more reliable way. It's a matter of choosing the right one.

Essentially, CDs and DVDs are very prone to errors, even in a freshly written state. That's why they are heavily protected with a checksum mechanism (75% of data are effective data, the rest is formating and checksum overhead). But even with that massive amount of protection they will suffer deterioration from chemical aging, ultra-violet exposure, scratches, dust, etc.

For damaged CDs and DVDs, there is an inexpensive program called "IsoBuster" from which will do seeming miracles on CDs and DVDs. It runs on Windows and Linux but not (yet) on Macs. Similarly, there are applications designed to get data from damaged floppies, hard drives, flash media such as camera memory and USB drives, and so forth.

Optical media: Blu-ray disks seem to win the format war against 'HD DVD'. A dual-layer Blu-ray disc can store 50 GB, almost six times the capacity of a dual layer DVD at 8.5 GB. Everything that has been said about CDs/DVDs applies to Blu-ray disks as well.

Best practice:

Burn them slowly with a good recorder on archive quality media in an open, non-proprietary format, read the data back to verify, label them with some descriptive text + date & author, lock them away where it is clean, dark, animal safe and dry. And do not forget to copy them over to the next generation of media before you throw away your last piece of hardware or software able to read them.

Hard disks (hard drives, HDD)

Disk manufacturers keep their statistics to themselves. A manufacturer guaranty buys you a new disk, but no data. Google for one has done a large scale study on HDD failure mechanisms:

In a nutshell: Disks run longest when operating between 35°C and 45°C, at lower temperatures the error rates increases dramatically. Controller parts (electronics) are the foremost sources of failure, SMART does not diagnose any of this. Some SMART errors are indicative of imminent failure, in particular scan errors and relocation counts. Lifetime expectancy is 4-5 years.

But all depends much on the real use case and some luck. For example I have a Fujitsu notebook that is running 24/7 since 1998, almost ten years without the slightest hick up. Just luck? In general and contrary to intuition or ecological considerations, running a hard drive permanently results in a longer lifetime than switching it on and off all the time. It has even been reported that aggressive power management spinning down the drive can harm it quickly. Making it working hard shortens the lifetime somewhat. The worst factors for HDD probably are vibrations, shocks, and cold temperatures.

If your disk is making weird noises, normal file recovery software isn’t going to work. Do a quick backup if that is going to happen to you. (Use dd utility if possible, not a normal file backup since dd reads in a smooth, spiraling stream from beginning to end and doesn't stress the mechanics). The are specialist companies that can recover data from otherwise destroyed drive, but they are costly, plan for 2000$ minimum charge.

Power surges

As much as 1% of all computers are affected by lightning and power surges every year.

(This is about total data loss due to power surges. Off course you can have the occasional data loss due to power loss before saving files. But those losses can normally be restored without major difficulty.)

You don’t have to wait for the next thunderstorm to be concerned about how a sudden fluctuation in electric power may affect your computer system. Recent statistics have shown that as much as 63 percent of all electronics casualties are due to power problems, and most computers are subject to two or more power anomalies a day. Since power surges or blackouts can occur anywhere and at any time, it only makes sense to protect your computer by investing in some sort of surge protection device.

How surges happen

A power surge occurs when the power line voltage increases over nominal values for more than 10 milliseconds. Sixty percent of all power surges are caused from within the home or office, generally when a device with a motor (such as a hair dryer, refrigerator, or water pump) shuts off and the power it was using is diverted elsewhere as excess voltage. The remaining 40 percent of power surges are generated by factors such as lightning, utility grid switching, line slapping, poor wiring, and so on.

While most average electricity-using devices are not affected by power surges, devices relying on computer chips and high-speed microprocessors are susceptible to serious damage. For your computer, power anomalies can result in keyboard lockup, complete data loss, hardware degradation, damaged motherboards, and more. Failure to protect yourself from the inevitable can result in a loss of both time and money.

Surge protectors

The most common defense against power surges is a surge protector or suppressor, a device that works by absorbing some of the excess energy and diverting the rest of it to the ground. These are usually found in the form of a power strip (one of those long devices that have six or so outlets and a single, grounded plug). Bear in mind, however, that not every power strip serves as a surge protector.

When selecting your surge protector, you want to be sure it is listed as meeting the UL 1449 standard, which guarantees a certain minimum of protection. You should also look for one that offers protection against lightning (not every one does) and provides insurance for equipment that is properly attached.

Because a power surge can follow any path to your computer, be sure that each peripheral connected to your system is protected. this includes your phone line or cable modem, as power can surge through these routes as well. A number of manufacturers are now producing surge suppressors that feature a phone jack for your modem along with the electrical outlets, while others have coaxial cable jacks for those who use a cable modem or TV tuner card.

If you have a notebook computer, you will want to carry a surge suppressor as well. A variety of suppressors designed specifically for notebooks are available, small in size and possessing both electric and phone outlets that make them ideal for use on the road.

Uninterruptible power supply (UPS)

While a surge suppressor will protect your system from minor fluctuations in the power lines, it won’t help you if the power should black out completely. Even an outage of just a few seconds can result in the loss of valuable data, so you might find it worthwhile to invest in an uninterruptible power supply.

Besides serving as surge suppressors, these devices automatically switch to battery power when a power outage occurs, giving you the opportunity to save data and shut down your system. Some models will even allow you to keep working until power is restored. When purchasing a UPS, be sure that it has the same qualities that you would seek in a surge suppressor, but also check out the battery life and included software.

Considering the potential risk to your computing system, ensuring its safety from power disturbances is a worthwhile investment. A quality surge suppressor will cost you upward of €20, a 500W UPS can be had for less than €40. It’s a small cost to pay for the peace of mind you’ll gain knowing your computer is well protected. In the very least pull all lines to your computer when you go on holidays.

Solid state drives: USB sticks, memory cards, flash disks

SSDs are mechanically more robust than HDDs and suffer much less on that front when they are plugged into the computer. But since they are mostly mobile devices, their exposure to drops, accidents and electrostatic discharges is much higher. So, for different reasons, SDDs are as likely to fail if not more than hard drives. Add the danger of theft and longevity and limited capacity, and SDDs will become prohibitive as permanent data storage devices.

One major cause for data loss (often recoverable) is the unsafe removal of SDDs from a computer. Before data is saved from a computer memory to any attached device, it remains for some time in buffers. In hard drives this means seconds at most, whereas with SDDs it can be tens of minutes. Therefore, before you disconnect a flash device, always activate data flushing through software (often called "safely remove device").

There is a new technology trend coming up, to replace hard drives with SSD flash drives. By 2010 they may be competitive in price to HDDs. Data retention is an issue with SDDs, it cannot be overwritten an infinite amount of times. SDDs wear in use. Wear then depends much on the location data is written, and how often it is written. Linux has developed a special driver avoiding writing to the same spot too often. But this is all premature information. Keep your eyes and ears open.

Magnetic media

Magnetic tapes are used in backup systems, much more in professional environments than in home use. Tapes have issues with data retention and changing technology, but they are safer in one aspect than CDs and DVDs: they are less exposed to scratches and dirt and writing deficiencies. On the other hand they are susceptible to magnetic fields. Throw a magnet next to a tape and it's gone! Tapes should be re-copied every 5-8 years, otherwise too many bits will fail and escape the checksum protection. The downside of magnetic tapes is often the recorder price and the restore time (20x longer than from HDD). Tape backup system have seen their best days.

Logical errors

Web storage services

Amazon Web Services includes S3 - Simple Storage Service. With appropriate configuration, you can mount S3 as a drive on Linux, Mac, and Windows systems, allowing you to use it as a backup destination for your favorite software. Google Shared Storage is another popular offer where one can store infinite amount of data.

It is expensive compared to hard drives at home - 100 GB cost $150 a year, 1TB cost $1500 (as of 01/2012). And you have to transfer the images over (a comparatively slow) internet.

I think as a safeguard against localized data loss of the most essential images it's not a bad idea at all, but it is not a general backup solution, much too slow for that.

Picasaweb (Google), Flickr (Yahoo) and Foto-Community provide online storage services specialist on photographie. Their free space is limited to 1 GB and you don't want to have full resolution images online. But the pro-accounts offer more, in the case of Flickr, dramatically more. For a mere 25$ a year you get unlimited (sic! reality check needed here) space.

Update Sept. 2008: If you look here or here you'll find offers for less then $5 a month with unlimited disc space. All you need is to order such a provider, even if you do not have a website. Just use it with ftp to upload all your files. This is by far the cheapest web storage solution, and it is quite basic too in it's features, no mounted drive, no WebDav and such.

Other online backup services include Carbonite, Backblaze, Spider Oak, Mozy, SugarSync, Elephant Drive, Livedrive, SOS, MiMedia,ZumoDrive, Usenet Backup,Symantec Online Backup, Files Anywhere, Crashplan, Barracuda Backup Service, Acronis, ASUS Web Storage and Dolly Drive. And here is a comparison of them.

In terms of data retention the web space solution is probably pretty safe. Transmission errors are corrected (thanks to the TCP protocol) and the big companies usually have their own backups included plus distributed storage so that they are disaster proof within themselves.

Transmission Errors

Data does not only get lost from storage devices, it also gets lost when traveling inside the computer or across networks (although the network traffic itself via TCP is error protected). Errors occur on buses and in memory spaces. Consumer hardware has no protection against those bit errors, whereas it is worthwhile to look into such. You can buy ECC (error code correction) protected memory (which is expensive, granted). With ECC RAM at least the memory will be scrubbed for single bit errors and corrected. Double bit errors would escape that scheme but they occur too infrequently.

transmission chain

Illustration 1: Data transmission chain within a commodity computer

This diagram depicts the transmission chain elements in a computer, all transitions are susceptible to transmission errors. The zfs and btrfs file system at least ensure the OS to disk path of data integrity.

Illustration 2: System failure projections based on multi-core CPUs doubling every 18, 24 or 30 months

The Byte Error Rate BER for memory and transmission channels is in the order of some10-7 bit. That just means that 1 in 3000 images has an error only due to transmission problems. Now how dramatic that is for an image is left to chance, it could mean that the image is destroyed or that a pixel somewhere changed its value, due to the compression used on almost all images one can't predict the gravity of a single bit error impact. Often one sees some partial images instead of the full image.

The worst of all that is that nobody tells you when a transmission error occurs, not your hardware. All those glitches go down unheard until one day you open the photograph, and to your surprise it's broken. It is quite worrisome that there should be no protection within a computer, nobody seems to have thought of it. The internet (TCP protocol) is much saver as a data path than inside a computer.

Flaky power supplies are another source of transmission losses because they create interference with the data streams. With normal files systems those errors go unnoticed.

Even if you are not overly concerned today with transmission problems, have a look into the future at illustration 2. Already in 2010 we'll see thousands of errors per year!

'Oracle' or 'Rising Sun' at the file system horizon?

ZFS from Sun Microsystems seems to be one of two candidates to deal with disk errors on a low level, and it is highly scalable. It is Open Source, heavily patented, comes with an GPL incompatible license, and is available on Solaris and Leopard. Let us hope that it will soon be available for Linux and Windows. Link

This is for the courageous ones: Fuse

Oracle has also started an initiative with its btrfs file system, (pronounced "Butter FS"), which still is in an alpha stage. It employs the same protection technique as zfs does, and it's available on Linux, although it is not yet part of the stock kernel. Recent progress has let to version 0.12, and the time line promises it to be on board the kernel in 2008. I've been running btrfs for a few weeks now on a partition with heavy compiling activity - I must say I'm impressed with its speed, it beats ext3 by a factor of 2. Otherwise there is a remarkable absence of errors, alerts, problems as you would expect from any production file system.

Certainly, these promising technologies play their role in the war games between Sun, Oracle, Linux, Redhat and MS. Sun tries to pull customers away from Linux to OpenSolaris, Oracle works against Redhat and Sun (who has acquired MySql), Linux is yet to trust Oracle. And all of them try to stay technologically ahead of the Mastodon.

Human errors

Theft and accidents

Do not underestimate it! Those two factor account for 86% of notebook and 46% for desktop system data losses. For notebooks, theft counts for 50% alone.


Data loss due to viruses is less grave than common wisdom make you believe. It accounts for less damage than theft or re-installations, for example. And it is limited to Microsoft OS users. Apple users experience very few viruses and under Linux they haven't been around for quite some time now.

Panic is a factor in data loss

Human error, as in everything, is a major problem in data loss. Take a deep breath and stop! Panic is a common reaction, and people do really stupid things. Experienced users will pull the wrong drive from a RAID array or reformat a drive, destroying all their information. Acting without thinking is dangerous to your data. Stop stressing about the loss and don’t do anything to the disk. Better yet, stop using the computer until you have a plan. Sit down and explain you plan to a laymen or better, laywoman. You will be amazed how many stupid ideas you'll discover yourself in such an exercise.

If your disk is making weird noises, normal file recovery software isn’t going to work. Do a quick backup if that is going to happen to you. If the drive is still spinning and you can’t find your data, look for a data recovery utility and backup to another computer or drive. (Non-Linux users: Google for “free data recovery software” for some options, including one from Ontrack). The important thing is to download them onto another drive, either on another computer, or onto a USB thumb drive or hard disk. It is good practice to save the recovered data to another disk. dd is your friend on *nix systems.

Common myths dispelled

I'd like to dispel some common myths:

  • Open Source file systems are less prone to data loss than proprietary systems:
    Wrong, NTFS is rather a tiny notch better than ext3, ReiserFs, JFS, XFS, to name just the most popular file systems that often come as default FS with distributions. A brilliant article about it is here: Vijayan Thesis.pdf

  • Journaling files systems prevent data corruption/loss:
    Wrong, they only speed up the scan process in case of a sudden interrupt during operation and prevent ambiguous states. But if a file was not entirely saved before the mishap, it'll be lost.

  • RAID systems prevent data corruption/loss:
    Mostly wrong, RAID0 and 1 prevent you from nothing, RAID5 can prevent data loss due to disk-failures (but not from disk or file system errors). Many low-end RAID controllers (most mother board controllers are) don’t report problems, figuring you’ll never notice. If you do notice, months later, what is the chance that you’ll know it was the controller’s fault? One insidious problem is corruption of RAID 5 parity data. It is pretty simple to check a file by reading it and matching the metadata. Checking parity data is much more difficult, so you typically won’t see parity errors until a rebuild. Then, of course, it is too late.

  • Viruses are the biggest thread to digital data:
    Wrong. Theft, and human errors are the primary cause of data loss.

Make your budget: Data size, required storage volume estimation

Digital camera sensors are 1-2 aperture stops away from fundamental physical limitations. What I mean is this: as technology evolves, there is a natural limit to its progress. Sensitivity and noise characteristics for any kind of light sensor are not far from that limit.

Today's cameras tend towards 10 mega pixels sensors, although this resolution is already too high for compact cameras and deteriorates the end result. Given the sensor size and quality of optics, 6 mega pixels are optimum for compact cameras. Even DSLR cameras run into their limits at 10-12 mega pixels, for higher resolutions one has to go for full frame sensors (24x36mm) or even bigger formats.

So, taking into account the manufacturer mega pixel propaganda it seems save to say that the bulk of future cameras will see less than 20 mega pixels. This gives us an estimation for the necessary storage space per photograph in the long run: <15 MB per image. Even if file versioning will be introduced (grouping of variations of a photograph under one file reference), the trend is to implement scripting of changes so that a small overhead will be recorded only and not a whole different image per version. With faster hardware this concept will see it's maturity quite soon.

In order to estimate the amount of storage space you have to plan for, simply multiply the number of photographs you take per year (easy with digiKam's timeline sidebar) and multiply it by 15 MB. Most users will keep less than 2000 pictures per year which requires less than 30 GB/year. Assuming that you will change your hard disk (or whatever media in the future) every 4-5 years, the natural increase in storage capacity will suffice to keep you afloat.

The more ambitious ones out there will need more space, much more maybe. Think of buying a file server, Giga-Ethernet comes integrated into motherboards today and it's a flick to fetch the files over the local network. Speaking about modern mobos: they now have external SATA connectors. This makes it really a trifle to buy an external SATA drive and hook it up to your machine. 1 TB drives will hit the market this year (2008). These are terrific compact storage containers for backup swapping: keep one drive at home and one somewhere else.

Back it up, backup, backup, recover!

A 750GB HD costs €100 today. Do not blame anybody else for data loss! 6% of all PCs will suffer an episode of data loss in any given year. Backup your data often according to a plan, and back it up and test the backup before you do anything dramatic like re-installing your OS, changing disks, resizing partitions and so on.

Disaster prevention

Say, you religiously do your backups every day on a external SATA drive. Then comes the day where lightning strikes. Happy you if the external drive was not connected at that moment!

Disasters strike locally and destroy a lot. Forget about airplane crashes: fire, water, electricity, kids and theft are dangerous enough to our data. They usually cover a whole room or house.

Therefore disaster control means de-localized storage. Move your backups upstairs, next house, to your bureau (and vise versa), whatever.

There is another good aspect to the physical separation: as said above, panic is often the cause of destroying data, even the backup data. Having a backup not at hand right away may safe your ass one day.

Some backup technicalities explained for laymen.

  • Full Backup: A complete backup of all the files being backed up.
    It is a snapshot without history, it represents a full copy at one point in time.

  • Differential Backup: A backup of only the files that have changed since the last full backup.
    Constitutes a full snapshot of two points in time: the full backup and the last differential one.

  • Incremental Backup: A backup of only the files that have changed since last whatever backup
    Constitutes multiple snapshots. You can recreate the original state at any point in time such a backup was made. This comes closest to a versioning system except that it's is only sampled and not continuous.

Best practice: The IT-layman's backup cookbook

  1. do a full backup in a external storage device.

  2. verify its data integrity and put it away (disaster control)

  3. have another storage device for frequent backups

  4. swap the devices every other month after having verified data integrity

useful rsync recipe to backups

Rsync is a wonderful little utility that's amazingly easy to set up on your machines. Rather than have a scripted FTP session, or some other form of file transfer script - rsync copies only the differences of files that have actually changed, compressed and through ssh if you want to for security. That's a mouthful.

A reasonable backup approach for images could be this one:

  1. backup important images right away (after dumping them to a computer) to DVD/optical media

  2. do a complete backup (c)

  3. do daily incremental backup (i) of the work space

  4. do a weekly differential backup (d) and delete integral backups of week-2 (two weeks ago)

  5. do a monthly differential backup (d) and delete the daily and weekly of the same day

  6. if not physically separated already, separate the backups now (swapping-in another backup drive)

A graphical representation on a daily rythm base could look like this:

At any occasion of a letter on the line you will be able the restore the full archive.

This protocol tries to leave you enough time to spot losses and to recover fully at the same time keeping the backup volume at <130% of the working space. You end up with a daily version of the last 7-14 days, a weekly snapshot for at least one month, and a snapshot of every month. Any more thinning should be done by hand after a full verification.

script to follow....TBD

For German speaking *nix users: Heinlein Support

Preserve your images through the changes of technology and owners

Themes: metadata, IPTC stored in image files, XMP files associated, keep the originals, storage, scalability, media, retrieval of images and metadata, copying image data over to the next generation of media, applications, operating systems, virtualization, viewing device... use of the www.

In order for your valuable images to survive the next 40 years or so (because that's about the time that you will become really interested to revisit those nice old photographs of you as a child, adolescent etc.) there are two strategies to be observed:

  1. Keep up with technology, don't lag behind more than a couple of years.

  2. Save your photos in an open, non-proprietary standard.

How to keep up with technology?

As the future is unforeseeable by nature, everything said today is to be taken with caution, and to be reviewed as we advance. Unfortunately there is no shortcut possible to some basic vigilance. Every 5-8 years at least one should ask oneself the question of backwards compatibility of current systems. The less variants we used in the past the less questions are to be answered in the future.

Of course every time you change your computer system (machine, operating system, applications, DRM) you have to ask yourself the same questions. Today, if you want to switch to Windows Vista, you have to ask yourself three times if you still can import your pictures, and, more important so, if you are ever able to move them onto some other system or machine. Chances are good that you can't. I see many people struggling around me, because Vista enforces a strict DRM regime. How can you proof to Vista that you are actually the owner of your pictures copyright?

Basically the questions should be answered along the line explained in this document: use and change to open standards supported by a manifold of applications.

Virtualization becomes available now for everybody. So if you have an old system that is important for reading your images, keep it, install it as a virtual machine for later.

Otherwise the advice is quite simple: every time you change your computer architecture, your storage and backup technology, your file format, check it out, go through your library and convert to a newer standard if necessary. And keep to open standards.


Scalability is the tech-geek expression of the (easy) capability of a system to be re-sized, which always means up-sized.


Lets assume you planned for scalability and dedicated the container you want to increase to a separate disk or partition. On *nix systems like Linux you then can copy an re-size the container to the new disk:

Check with dmesg if your new disk is recognized b the system, but don't mount it.

$ dd if=/dev/sdb[#] of=/dev/sdc # source is /dev/sdb, new disk is /dev/sdc
$ parted re-size /dev/sdc1 0 <disk size in MB> # works on ext2,3, fat16, 32 and reiserfs
$ re-size2fs /dev/sadc1 #re-size_reiserfs in case


Use open, non-proprietary standards as file formats

The short history of the digital era in the past 20 years has proven over and over again that proprietary formats are not the way to go when you want your data to be intelligible 10 years into the future. Microsoft is certainly the well known culprit of that sort because of its domineering market share. But other companies are actually (if inadvertently) worse since they may not stay long enough in the market at all or have a small user/contributor base only. In the case of Microsoft one has at least the advantage of many people sharing the same problems. Finding a solution has therefore much more success. Still, in some cases Microsoft is using Open Source documentation to understand their own systems, so badly maintained have been their own documentation. Usually with any given MSoffice suite one cannot properly read a document created with the same application two major versions earlier.

Image formats have had a longer live time than office documents and are a bit less affected by obsolescence.

Open Source standards have the huge advantage of having an open specification. Even if one day in the future there will be no software to read it anymore, one can recreate such software, a task becoming simpler every year.

JPEG has been around for a while now, and whilst it's a lossy format losing a bit every time you make a modification and save it, it is ubiquitous, supports JFIF, EXIF, IPCT and XMP metadata, has good compression ratios and can be read by all imaging software. Because of its metadata limitation, lossy nature, absence of transparency and 8 bit color channel depth, I do not recommend it. JPEG2000 is better, can be employed lossless, but lacks in user base.

GIF is a proprietary. patented format and slowly disappearing from the market. Don't use it.

PNG has been invented as a Open Source standard to replace GIF, but it does much more. It is lossless, supports XMP, EXIF and IPTC metadata, 16 bit color encoding and full transparency. PNG can store gamma and chromaticity data for improved color matching on heterogeneous platforms. Its drawback are a relatively big footprints (but smaller than TIFF) and slow compression. I recommend it.

TIFF has been widely accepted as an image format. TIFF can exist in uncompressed form or in a container using a lossless compression algorithm (Deflate). It maintains high image quality but at the expense of much larger file sizes. Some cameras let you save your images in this format. The problem is that the format has been altered by so many people that there are now 50 or more flavors and not all are recognizable by all applications.

RAW format. Some, typically more expensive, cameras support RAW format shooting. The RAW format is not really an image standard at all, it is a container format which is different for every brand and camera model. RAW format images contain minimally processed data from the image sensor of a digital camera or image scanner. Raw image files are sometimes called digital negatives, as they fulfill the same role as film negatives in traditional chemical photography: that is, the negative is not directly usable as an image, but has all of the information needed to create an image. Storing photographs in a camera's RAW format provides for higher dynamic range and allows you to alter settings, such as white balance, after the photograph has been taken. Most professional photographers use RAW format, because it offers them maximum flexibility. The downside is that RAW image files can be very large indeed.

My recommendation is clearly to abstain from archiving in RAW format (as opposed to shooting in RAW format, which I recommend). It has all bad ingredients: many varieties and proprietary nature. It is clear that in a few years time you can't use your old RAW files anymore. I've already seen people changing camera, losing their color profiles and having great difficulty to treat their old RAW files correctly. Better change to DNG format!

DNG Digital Negative file format is a royalty free and open RAW image format designed by Adobe Systems. DNG was a response to the demand for a unifying camera raw file format. It is based on the TIFF/EP format, and mandates use of metadata. A handful of camera manufacturers have adopted DNG already, let's hope that the main contenders, Canon and Nikon will use it one day.

-> All you ever wanted to know about DNG.

I strongly recommend converting RAW files to DNG for archiving. Despite the fact that DNG was created by Adobe, it is an open standard and widely embraced by the Open Source community (which is usually a good indicator of perennial properties). Some manufacturers have already adopted DNG as RAW format. And, last not least, Adobe is the most important source of graphical software today, and they of course support their own invention. It is a good archival format, the raw sensor data will be preserved as such in either linear TIFF format, embedded RAW or undemosaiced raw sensor data combined with a color profile inside DNG, so that the risk associated with proprietary RAW formats is alleviated. All of this makes migration to another operating system a no-brainer as far as metadata preservation is concerned.

In the near future we'll see 'non-destructive editing', where files are not changed anymore but rather all editing steps will be recorded (into the DNG as it where). When you open such a file again, the editing script will be replayed on the original data. This takes computation power, but it is promising as it leaves the original intact. However, this non-destructive editing will not be standardized as it depends on the tools one uses. DNG has an extensible section where everyone can throw in there own cook book. There is little hope that this part is going to be compatible between applications and platforms.

Here is some -> controversial reading on the DNG format, read the comments to the article as well, very instructive indeed!

XML (Extensible Mark-up Language) or RDF (Resource Description Framework). XML is like HTML, but where HTML is mostly concerned with the presentation of data, XML is concerned with the “representation” of data. On top of that, XML is non-proprietary, operating-system-independent, fairly simple to interpret, text-based and cheap. RDF is the WC3's solution to integrate a variety of different applications such as library catalogs, world-wide directories, news feeds, software, as well as collections of music, images, and events using XML as an interchange syntax. Together the specifications provide a method that uses a lightweight ontology based on the Dublin Core which also supports the “Semantic Web” (easy exchange of knowledge on the Web).


That's probably one of the reasons why, around 2001, that Adobe introduced its XML based XMP technology to replace the “Image resource block” technology of the nineties. XMP stands for“Extensible Metadata Platform", a mixture of XML and RDF. It is a labeling technology that lets users embed data about a file in the file itself, the file info is saved using the extension“.xmp” (signifying the use of XML/RDF).

XMP. As much as ODF will be readable forever (since its containing text is written in clear text), XMP will preserve your metadata in a clearly understandable format XML. No danger here of not being able to read it later. It can be embedded into the image files or as a separate accompanying file (sidecar concept). XMP can be used in PDF, JPEG, JPEG2000, GIF, PNG, HTML, TIFF, Adobe Illustrator, PSD, PostScript, and Encapsulated PostScript. In a typical edited JPEG file, XMP information is typically included alongside Exif and IPTC data.

Embedding metadata in files allows easy sharing and transfer of files across products, vendors, platforms, customers, without metadata getting lost. The most common metadata tags recorded in XMP data are those from the Dublin Core Metadata Initiative, which include things like title, description, creator, and so on. The standard is designed to be extensible, allowing users to add their own custom types of metadata into the XMP data. XMP generally does not allow binary data types to be embedded. This means that any binary data one wants to carry in XMP, such as thumbnail images, must be encoded in some XML-friendly format, such as Base64.

Many photographers prefer keeping an original of their shots (mostly RAW) for the archive. XMP suits that approach as it keeps metadata separate from the image file. I do not share this point of view. There could be problems linking metadata file and image file, and as said above, RAW formats will become obsolete. I recommend using DNG as a container and putting everything inside.

The Dublin Core Metadata Initiative is an open organization engaged in the development of interoperable online metadata standards that support a broad range of purposes and business models. DCMI's activities include work on architecture and modeling, discussions and collaborative work in DCMI Communities and DCMI Task Groups, annual conferences and workshops, standards liaison, and educational efforts to promote widespread acceptance of metadata standards and practices.

Best practice

  • Use surge protectors (UL 1449 standard), possibly combined with a UPS

  • use ECC memory to verify correct data transmission (even just saving files)

  • watch your hard drives (temperature, noise...), make backups

  • Keep backups at another location, locked up, use web storage space

  • use archival media and burners

  • Don't panic in case of data loss, explain your recovery plan to a layperson.

  • choose your file system, partitions, folders to cater for easy scalability

  • Use open, non-proprietary standards to manage and save photographs

  • Do a technology/migration review at least every 5 years

Typical DAM workflow with digiKam

  1. import images from camera, card reader or scanner. As long as the images are stored on the camera media, you can use that as temporary backup.

  2. RAW are converted to DNG and stored away into an RAW archive

  3. rate and cull, write-back metadata to the DNG archive

  4. make a backup e.g. on DVD, optical drive or tape

  5. tag, comment, geo-locate

  6. edit and improve photographs

  7. For layered editing use external applications. Back in digiKam, re-apply the metadata, which was probably lost or curtailed by the other applications.

  8. run the routine backup with following data-integrity checks

  9. protect processed images for copyrights with Digital Watermarking
    export to web galleries, slide shows, MPEG encode, contact sheets, printing etc.


-> return to part 1