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FAQ: What happens when a MrSID raster is reprojected?

Question

What happens when a MrSID raster is reprojected?

Answer

MrSID (Multi-resolution Seamless Image Database) is a powerful wavelet-based image compression algorithm with multiple compression ratios. It has*.sid file format developed and patented by LizardTech for encoding georeferenced raster graphics, such as orthophotos.

When a MrSID raster is projected using a 'Project Raster' tool under ArcToolBox > Data Management Tools > Projections and Transformations, the Output raster can be saved using different file formats such as *.img, *.Tif, GRID; however, *.sid is not supported at this level.

Currently, all ArcGIS users can encode individual rasters into MrSIDs that are smaller than 50 MB, and anything bigger than this needs to have an additional MrSID encoder license. If the output raster is smaller than 50MB, it can be converted back into MrSID format. Output rasters larger than 50MB need to be saved using other supported raster formats other than MrSID.

During the projection process, MrSID compression is first taken away and the output raster is uncompressed. As a result, the output raster may end up with a much larger file size than the original MrSID input raster. This size discrepancy between input and output raster depends on the level of MrSID compression used on the original raster.

If the raster is stored in the geodatabase, different compressions such as LZ77, JPEG2000, and JPEG are available for application under the environment settings of 'Project Raster' tool. These help to reduce storage space for the output raster; however, the output raster remains relatively larger than the original SID file.

Such increases in raster file sizes particularly affect the portability of raster dataset, such as using raster tiles for GPS devices as backdrop layers.

On an extensive scale where one needs to project several MrSID files into a different coordinate system, be aware that the resultant rasters may consume much more disk space than the original input raster tiles. It is very likely that a situation will arise whenever MrSID rasters are processed using a tool that requires an output raster to be created.