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The mosaic dataset item references a large raster dataset that is stored uncompressed.
Using large uncompressed raster datasets in a mosaic dataset doesn't have a deprecating impact on the mosaic dataset; however, your organization of the data could be more space-efficient.
There is no solution for this warning, unless you compress your source data.
Recommendations for optimizing image formats: some imagery can be slower to read than others due to their storage format or compression and it is recommended that you convert these into more optimal formats. For example, an ASCII DEM image format is slow to read; therefore, it is recommended that you convert it to a format such as TIFF. Also, if the image is very large and not tiled, it is recommended that you convert this to a tiled TIFF format to optimize disk access. Also, when converting imagery, consider using either lossless (for example, LZW) or lossy (for example, JPEG) compression. You could choose to use wavelet-based compressions, such as JPEG 2000, but these are generally more CPU intensive to decompress while providing only marginally better compression. When converting imagery isn't an option, you can build overviews on the mosaic dataset that start at a very low pixel size