We’ve had some emails from people who are having trouble viewing MrSID images in Esri’s ArcGIS Explorer. We dug around and found out that our friends at Esri had already been notified of the same issue and they’d already discovered that the problem was undefined projections. MrSID images are sometimes created from TIFFs that have no projection information, or the projection is written to an AUX (.aux) file but not included in the actual MrSID image. Esri has posted an article on how to fix the problem for immediate viewing using ArcCatalog. Click here or enter this URL in a browser: http://blogs.esri.com/info/blogs/arcgisexplorerblog/archive/2009/ 03/05/projections-and-mrsid-images-for-explorer.aspx Of course, defining the projection in ArcCatalog only writes the information to an AUX file, not into the MrSID file itself, which would be a better practice. Users should ask their data provider to include the projection system (LizardTech’s GeoExpress® software calls this the coordinate reference system or CRS) in the metadata of the MrSID files they deliver. As for any images you have already, if you know what their projection is and have access to a licensed copy of GeoExpress, simply add any number of MrSID images of the same projection to the GeoExpress Edit Metadata tab and specify that projection for all of them at once. You can also use the GeoExpress command line tool mrsidgeometa to add a well known text (WKT) string. The syntax is as follows: Source.