|Images are the lingua franca of spatial Web sites. Realizing that the graphic elements of their pages are the main attraction, spatial Web site designers carefully watch image size, color palettes, and image server software performance. They pay attention to image size because the high information density of maps, air photos, and remotely-sensed imagery makes for large file sizes and long download times. They are concerned about color palettes because their users may be viewing the site with a monitor having limited color display options. They test performance because they fear an overwhelming amount of traffic to their sites will cause image delivery software to crash.
Fortunately, a variety of tools are available to help spatial Web site designers address these image problems. Because we all hate to wait for our spatial Web pages to draw, clever engineering teams at Kodak (www. kodak.com), AT&T (www.att.com), and numerous smaller research and development organizations have built image translators that compress images for speedy transport and decompress them again on arrival.
Some familiar acronyms in the alphabet soup of image translators are JPEG, MrSID, ECW, RLE, and LZW compression, each with its own benefits and shortfalls. Web-safe palettes are also available and documented for those intent upon serving readable maps to the widest possible audience. And, to the delight of organizations with very large image collections, companies such as LizardTech, Earth Resource Mapping, Xippix and GlobeXplorer are expanding the image-serving performance and reliability envelope with image pyramids, color-fidelity processes, and chip-level image server strategies.
DIG: Digital Imaging Group
ECW: Enhanced Compression Wavelet
GIF: Graphics interchange format
JPEG: Joint photographic experts group
MrSID: Multiresolution Seamless Image Database
RGB: Red, green, blue
RLE: Run length encoding
USGS: U.S. Geological Survey
TIFF: Tagged image file format