One confused image monkey

Melanie Harlow’s blog @ ESRI has left me like one of these …

Hmmm? Conversion from 89MB of MrSID’s to a JPEG2000 balooned to 922MB. What exactly does 75% compression mean in that context? Sounds pretty incorrect to me unless 75% means lossless which isnt exactly a fair test. Recompress to ~1:15 lossy jp2 and I would expect the total dataset size to be largely the same as the original MrSID.

Hmmm? I do believe you are missing a format in your comparison. ECW will save you space, use a complex algorithm, and improve the performance of reading the data. This statement is completely wrong. Sticking with a more verbose format like TIFF will require you to read more data, more files and offer less effective caching mechanisms

Before you go off and convert all your raster data to a highly compressed format to save space, please note that the more complex the compression the slower it is to read the data. Therefore, we generally recommend when speed is your #1 concern, that you use a TIFF raster dataset with JPEG compression

Hmmm? If the output mosaic is only 30,000 x 20,000 pixels then why not do a proper mosaic into a single MrSID, JP2 or ECW file? Keeping small tiled datasets removes the benefits these wavelet formats provide, gives worse performance and requires you to store redundant overview data thats simply not required if kept as a single file.

A good example on a larger dataset was the FOSS4G 2010 Benchmarking raster over Barcelona. 3 band, 8 bit 220,000 x 150,000 pixel dataset. Mosaiced to 1 ECW = 4.8 GB. The equivalent JPEG Compressed TIFF tiles totalled 112 GB. Thats a significant difference without even looking at the performance gains.

I will wait for Part Duex that I’m sure will clarify this =)