Updated: 03 July 2008

ACRES Image Gallery

Image Compression for Web Serving

ACRES Image Gallery presents this feature on image compression and image web serving technology.

Go straight to the viewing pages, or read the discussion page, below.

Image Compression

Electronic image files occupy larger data volume than text files. Geographic image data files are usually particularly large, as the raster structure has to accommodate a grid containing the individual cell values. These values can be brightness values, such as in satellite data, or height values, as in a digital elevation map. In addition, colour images show the values of three raster grids, displayed as (R)red, (G)green and (B)blue values, combining to generate the colour display.

For a number of reasons, including data storage or data transmission limitations, large image files need to be compressed to make smaller files.

Compressing image files is accomplished by trading off image quality and data volume. The greater the compression, the smaller the output file and the reduction in image quality compared to the original. Optimum image compression will reduce the file size while maintaining as much of the image integrity as possible.

• More information on image compression.

Image Serving via the World Wide Web

The speed of transmitting files over the World Wide Web is dependent on the type of Internet connection as well as the file size being sent.

Reducing the size of image files makes for faster transfer over the Web. This is especially helpful to users with slow Internet connections. There is also the technique of serving only "views" or "slices" of the original image file to the users' client browser, so that the full file does not need to be sent.

This particular image web serving technology allows the user to see and manipulate the image view served to their browser for pan, roam and zoom and other functions. By this technique, Geographic image data is provided for viewing in a more useful way to the user than simply viewing a static web image. In addition, the amount of data sent from the server to the client, is minimized.

Try Viewing Compressed Satellite Imagery via Two Example Image Web Servers

In the Gallery, two types of image Web server examples are presented for the purpose of allowing our data users to preview this increasingly important technology. Click on one of these two to start viewing. You are encouraged to try both of these to get the "full picture".

Note: Browser plug ins are required to use the viewers. You can down load these first, or allow the browser to notify you and retrieve and install the plug in automatically, when you try to use the viewer.