MrSID is an image compression format used to store large raster datasets in an extremely small amount of disk space. It is one of the two image formats supported by ArcPad, the other being georeferenced JPEG files. MrSID images have the advantage of higher image quality for the same amount of image compression. This set of instructions will lead you through the steps necessary to create a georeferenced MrSID file for use with ArcPad on the Compaq iPAQ handheld computers. Before you begin this tutorial you will need a georeferenced image. A georeferenced tiff image is probably the most common image type to use (e.g. filename.tif and associated filename.tfw world file). However, other filetypes, such as .lan, .img, and .bil are supported. You can check the MrSID program to see exactly which file types are supported.
Location of MrSID - MrSID is NOT an MIT site licensed program, so you can not access from Athena. Further, use of the program is restricted by a hardware key (also called a 'dongle') which must be physically inserted into the parallel port of the computer on which you wish to run MrSID. Currently the dongle is inserted into the black Dell PC on the left side of the geomorphology lab on the 10th floor of building 54. If you have an extremely compelling reason to run MrSID on your own PC, plus talk with Nathan Niemi or Kelin Whipple before removing the dongle, and to get the software for installation. Otherwise, please either transfer your image data via ftp or on CD to the computer in the lab.
Take a look at a MrSID image file. This is a 1-m resolution digital orthophoto quadrangle of the Goodsprings, NV 7.5-minute quadrangle, available from the U. S. Geological Survey. Originally a 40 Mb file, this is a 4.8 Mb image with MrSID compression.
If you can't view this image, you can download a MrSID plug-in for your browser from Lizard Tech.
1. Go to Start > Program Files> Lizard Tech > MrSID Geospatial Encoder. Note that under Program Files, there is also a program called MrSID Geospatial Viewer, which is only for viewing MrSID files, not creating them. When you start Geospatial Encoder, a window will appear which will look like the one below. To compress a single image, begin by pressing the far left yellow button to add an image. (The yellow button second from the left will allow you to mosaic multiple images together, as long as the total file size of all of the images doesn't exceed 500 Mb).
2. When you press the add image button (above), a standard windows dialog box appears, from which you should select the image that you want to compress. After selecting the image, the MrSID window will alter appearance as shown below. The image to undergo compression will appear in the left hand box, while information about the image will appear on the right. The image currently selected is an RGB tif image (TIFG). The image will be compressed at a ratio of 20:1, creating a 414 Kb MrSID image from the 8.1 Mb tiff file. Now select 'Options' from under the 'Tools' menu.
3. The 'Options' window will appear as below. For the most part, you can limit yourself to the 'Encode Settings' palette. For use with ArcPad, an Encoding Ratio between 20:1 and 10:1 is best. 10 will give a better image, but the trade off is file size. After you select an Encoding Ratio with this menu you can continue to adjust it on the previous window to test this trade off. Change the Zoom Level to '9' (the Target Thumbnail will adjust automatically). Click 'OK'.
4. Now that you have set the Encoding settings, you can adjust the 'Encoding Ratio' using the slider bar, shown on the window below, to check the trade off between file size and compression (image quality). When you are ready, press 'Encode' at the lower left, and the MrSID image will be created. If you began with filename.tif and filename.tfw, you will create filename.sid and filename.sfw. The filename .sfw file is a worldfile which contains the georeferencing information, and should always be copied together with the .sid file. If you wish to check that the compression worked, you can view the MrSID file using Arcview. Start Arcview and open a new project. From File go to Extensions, and place a check mark to the left of the 'MrSID Image Support' extension. Select OK. Now you can add a new theme to the view and open the MrSID image in Arcview.
