A flat mosaic combines all of the input tiles into a single homogeneous file. The input tile information is not preserved when creating a flat mosaic.
A composite mosaic is an MG3 or MG4 file that contains copies of the input tiles and displays them together. The individual tile’s history can be referenced in the metadata of the output composite mosaic file.
Creating a flat mosaic involves the encoder. If the data cartridge is decremented (as when input file formats other than MrSID, JPEG 2000 and NITF are used), the amount deducted is equal to the final output file size of the mosaic or the sum of the input tile sizes, whichever is smaller. Creating a composite mosaic does not involve an encode operation, which means that not only is it a fast operation but it also does not deduct from the data cartridge.
A flat mosaic in any output format can be made of input tiles with different resolutions. Composite mosaics can only be made of input tiles of differing resolutions if the output is MG4. MG3 composites do not support multiresolution mosaicking (an existing MG3 compoiste may be updated with an image tile in a different resolution, but it must subsequently be flattened).
A flat mosaic can be made of input tiles in any format. MG3 composite mosaics can contain MG2 and MG3 input tiles, and MG4 composites can only contain MG4 input tiles.
The decoding of composites can be slower than the decoding of flat mosaics, but composites are more versatile in some applications, and you can update only desired portions of a composite instead of the whole image. An MG4 composite also has an overview that enables more efficient viewing.