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ECW ( Wavelet compression for digital imagery and image web server from earth resource mapping pty. Ltd.)

ECW (Wavelet compression for Digital Imagery and Image Web server from Earth Resource Mapping pty. Ltd.)

Suresh Murthy
DGM (Tech support)
HOPE Technologies


As imagery becomes more affordable and easier to use, GIS and CAD users are increasingly demanding more powerful imagery capabilities. The amalgamation of spatial information, be it CAD, GIS, database, imagery or whatever has resulted in much closer working relationships between imagery and GIS companies. With the easy availability of High resolution Imagery in the near future and the integration required for mosaicing of a large number of imagery, airphotos, scanned topographical maps and other types of imagery, for various applications the size of image files can become very large. For. eg. it would takes 1.5TB (1,500GB) of color imagery to cover all of California at a resolution of one meter.

With ever more powerful computers, Data storage and Data retrieval systems capable of managing several thousand Terabytes of data it is possible to keep up with almost any amount of Imagery. The key issue is to easily use, manage and disseminate an always growing image database with limited equipment & funding. This is an important issue with most organisations.

It is now feasible to work with such large images and also serve high-resolution orthophoto mosaics for entire cities, real-estate companies and other non-technical markets using a combination of Compression Technologies like Wavelet compression and Image Web server (IWS) from ER Mapper.

What we are seeing is a definite trend where the user base for imagery is expanding rapidly. This is why companies like ER Mapper has expanded their product offerings into a whole solution range; ER Mapper (to preparing imagery), ECW (to use imagery inside any application), and the Image Web Server (to serve imagery into those applications using the Internet or Intranet).

What is a compressed image?
A compressed image is one that has been compressed to reduce the image file size, while still providing an almost perfect version of the original. With some compressed formats, such as the ER Mapper Compressed Wavelet (ECW) imagery format created using the ER Mapper Compression Wizard, you can use a compressed image almost. This is achieved by automatically selectively decompressing only the portion of the image, at the level of detail currently being viewed or printed.

There are various compression techniques. The most effective technique is wavelet based, which is a method of processing, quantizing and then encoding the image in a way that result in very high levels of compression, with an almost perfect reconstruction of the original image.

ER Mapper 6.1 offers wavelet based compression technology to compress images, resulting in very high quality results at very high compression rates. Typically a color image such as an airphoto can be compressed to less than 2% to 5% of its original size (50:1 to 20:1 compression ratio).

This means that, at 20:1 compression, 10GB (10,000MB) of color imagery will compress down to 500MB, which is small enough to fit on to a single CD_ROM. If your source image has less information content, such as scanned topographic maps or airphotos over areas with less land change, higher compression rates may actually be achieved – 100:1 or more is not uncommon for compressed scanned topographic maps.

Wavelet based compression: performance gains

In addition to the advantages of smaller files, (which requires lower seek time to access data from CD_ROM, DVD_ROM or Hard disk drive) compression of digital imagery offers higher performance, because compressed imagery trades slower disk speeds against faster CPU speeds. Compression is “compute bound” rather than “disk bound”, meaning that decompression is limited by CPU speed rather tan by disk IO or seek speed.

A 10GB image that has been compressed down to a 500MB ECW compressed image file can be read as quickly from a CD-ROM as the uncompressed imagery can be read from hard disk. This makes distribution and use of imagery directly from CD-ROM a practical reality.

Working with large images

The following table demonstrates the
reduction in file sizes when compressing imagery using the new
ER Mapper wavelet based compression technology, which can
compress very large images, at high compression rates, with
high quality results.

Image
Dimensions
Uncompressed size
Compression ratio
Compressed size

Color airphoto
3×8,000×8,000
192MB
50:1
4MB

Color airphoto mosaic
3×50,000×100,000
15,000MB
25:1
600MB

Usage of Compression

Compression is very useful for the following requirements.

  • Easy distribution of large amounts (10MB to multi TB) of imagery
  • Fast access directly from CD-ROM
  • Airphotos, orthophtos, satellite Imagery or scanned topographic maps.

One can use a lower compression factor, when some of the following criteria are important to the application.

  • Hyperspectral signature analysis is being carried out.
  • Important information is represented as sutable secondary changes to the date.
  • Your application is “mission critical”, where you want to retain the original quality of Image, for example navigation system using DEM data.

For example, a compression rate of 3:1 results in imagery that is very close to identical to the original image, while still offering faster access rates on slower CD_ROM media.