One of the reasons for its popularity is that it can compress photo data without making photos look too bad. This is the most widely used format for digital cameras. JPEG is a file format from the "Joint Photographic Experts Group". Fortunately, ArcGIS and BlueSpray will read either. Sometimes this is done and sometimes folks will add a World File and a PRJ file to a TIFF file. TIFF was not a GIS file format at birth but it is very extensible and the "GeoTIFF" format describes how to add georeferencing and spatial references into a TIFF file. You can get your masks back (see the "issues with rasters" web page). This means that sometimes your masks will work and sometimes they will not. The problem is that there are different ways of supporting "NoData" (or a "mask"), the standard way and the way that Esri does it. It is well supported and one of the only issues is the support of "NoData". The Tagged Image File Format or "TIFF" is the preferred format for distributing data. Note that this is a proprietary format from ERDAS and is not a good format for data exchange. This file format is faster, supports larger rasters, and is more robust than the GRID file format that we used to recommend. IMG or "Imagine" files are the preferred raster file format to use with ArcGIS and other Esri products. You should see an image something like the one on the right where the pixels with slashes are transparent. A good activity is to make your own ASCII grid file like the one below in a text editor such as NotePad and then save it with a ".asc" extension and load it into a GIS application. The ASCII GRID format is pretty clever and allows you to see the raster data in a text editor. The extension is created by removing the second letter in the original extension and adding a "w" for "World" on the end. You can georeference a JPEG file by adding a text file like the one above and adding an extension of "jgw".
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