Geospatial Compression and Viewing Software options Continued ... Geospatial Commercial Image Compression Programs Although JPEG can be a useful tool in a GIS there are commercial image compression programs designed specifically for use in a geospatial environment. The two principle image compression software packages for use in this arena are ECW from Earth Resource Mapping and MrSID from LizardTech, Inc.. These two companies have arisen from fascinating backgrounds. LizardTech acquired compression algorithms from the Computer Research and Applications Group at Los Alamos National Laboratory. ER Mapper was founded by Stuart Nixon who was raised on a 300,000-hectare cattle ranch with no television or telephone in the Australian outback. The compressed files created using ECW or MrSID can be viewed in most if not all of the major GIS and image processing programs. Many come with the standard feature of being able to directly read these compressed files. Some require that an extension or plug in be installed. These extensions can be downloaded at no cost from the ER Mapper or LizardTech websites. For all intents and purposes both of these compression formats have become industry standards. The list of companies with direct working arrangements or with software support with these two key players is quite impressive. They include: ESRI , ERDAS, SPOT Image, Space Imaging, PCI Geomatics, Autodesk , Bentley , Intergraph, and many others. Both of the programs are quite user friendly and allow the user to set the level of compression desired. Wavelet compression technology is used in both the ECW and MrSID programs. Both companies suggest that compression ratios of 50:1 for color and 20:1 for grayscale be used. Setting compression ratios higher than this can result in perceivable degradation of the image quality. Although both programs use wavelet technology for image compression the way in which they use it is quite different from each other. The MrSID program does the compression in a block by block fashion while ECW does it one line at a time. Because of this the MrSID process can require the formation of very large temporary files to accommodate the blocking process. The ECW method needs very little memory space since it is only working with one line at a time. Not only does the MrSID methodology require more hard drive space it also needs more RAM to function properly. To compress an image 3 GB in size or larger LizardTech suggests 512 MB of RAM where as ER Mapper suggests 128 MB. The flagship versions of both programs cost approximately the same amount, $5,000 for MrSID and $4,950 for ER Mapper. With MrSID you get compression software. With ER Mapper you get compression software plus all of the image processing, mosaicking, and GIS functions that come standard in that program. If you want to compress and mosaic separate images into one seamless mosaic both programs can do this. ER Mapper has very powerful tone matching capabilities in addition to the compression portion of the program. Because of this the mosaic that it creates truly looks like one large, seamless image. A seamless, tone matched, compressed mosaic was created from 2,980 USGS DOQQ's of the Southern California area. The MrSID software has no tone matching capabilities. This can result in a seamless, compressed mosaic with a patchwork or checkerboard appearance. Both companies also offer no cost compression and viewing software that can be downloaded from their websites. These programs can be very useful as a tool in a low cost GIS, for a graphic artist or CAD operator, or for a typical family that needs a simple method to compress and email some snapshots. Images compressed using LizardTech's free compression software Images compressed using ER Mapper's free compression software Standalone Versions of Compression and Viewing Programs The stand-alone versions of the compression and viewing programs from both LizardTech and ER Mapping were evaluated for this research. These were the ERViewer 2.0b, ECW Free Compressor 2.2B1, MrSID Photo Edition 1.1, and the MrSID Stand Alone Viewer. Grayscale and color imagery in TIFF format was compressed into JPEG, MrSID, and ECW formats. The ease of use and capabilities of the software were noted and the compressed image quality compared. All of the programs were easy to download and install on a PC. The basic operations are straightforward however the MrSID compression and viewing programs are rather limited in their capabilities when compared to the ER Mapping programs. The MrSID compressor will not accept images larger than 1600 x 2100 pixels while the ECW compressor will accept an image up to 500 MB in size. The ECW compressor gives a progress report while the compression is being done as well as a final on screen report listing the target and actual compression ratios, the processing time, and the file size of the compressed image. Unfortunately the MrSID compressor did none of this. The stand-alone viewers also have widely varying capabilities. The MrSID viewer can only view MrSID compressed images while the ERViewer can display TIFF, ECW, and JPEG images. While both viewers can export data, the MrSID viewer can export in TIFF format only. The ERViewer has the ability to export as Windows BMP, GeoTIFF/TIFF, JPEG, and several other formats. The MrSID viewer was also much slower when opening an image, zooming, and refreshing than was the ERViewer at the same tasks. Summary To compare the LizardTech and Earth Resource Mapping flagship and no cost programs strictly on a bang-for-the-buck basis the clear winner would be ER Mapper. ER Mapper, ERViewer, and the ECW compressor all have so many features and capabilities not available in the MrSID products. MrSID does have one option that is not available for ER Mapper, it is available for the UNIX platform. An older version of ER Mapper is available for the UNIX platform but does not include ECW compression. Both companies are at the moment disputing each other in the courts regarding patent infringements. This action has been instigated principally by LizardTech. Hopefully this will be resolved soon so that they can put more time and energy into improving their products and less on court procedures. When compared to what was available to the general public 5 to 10 years ago the MrSID and ECW programs are amazing tools. As this technology continues to improve, image and raster data compression will undoubtedly find use in more and more fields. About Author David B. Davis has over 9 years of experience in photogrammetry, image processing, and GIS . He also works part time as a GIS consultant, specializing in Latin American projects. Carpincho@att.net September issue GISVision "Notes from URISA 2000" GISCafe TechnicalPapers "Implementing GIS in Developing Countries" ER Mapper compression software Mr.SID compression software ER Mapper Viewing Software Mr.SID Viewing software