LizardTech business partner Ascent GIS assists organizations and businesses that are interested in building and maintaining spatial information by providing GIS and image processing services. They use GeoExpress to handle large volumes of imagery, mostly in MrSID and GeoTIFF format, in their public and private partnerships across the U.S., providing the latest and most current imagery and LiDAR data to forestry, state and local government, engineering, and utilities industries.

We spoke with Russ Martin, GIS analyst for Ascent, about their experience with MG4, the new version of the MrSID format, and especially about how MG4′s alpha bands help with transparency, and we’d like to share his comments with you.

“Recently, LizardTech launched the new MrSID Generation 4 (MG4) format. In the past, displaying two MrSID images with irregular boundaries, like county mosaics, in ArcMap resulted in areas of nodata “noise” (black pixelation) between the images when the background layer was set to transparent. This occurred because some of the pixels around the edges of the MG3 images do not have the value of 0,0,0.

Noise (speckling) in MG3 caused by nodata. Note the non-zero pixel values. Image courtesy of NAIP.

“In the MG4 format, after the data is clipped inside of the nodata boundary, the non 0,0,0 pixels are still present, but setting the Alpha channel on the fourth band completely eliminates the “noise” between the MG4 images.

MG4′s alpha channel takes care of the no data pixels for a clean boundary. Image courtesy of NAIP.

“We receive so much data in MrSID format — roughly 1.5 terabytes for a single project recently — and we anticipate processing another 6 terabytes in the next two months. The continuing transition of the aerial photo industry to digital sensors has resulted in a great deal of 4-band imagery with very large file sizes, which can be compressed and stored in the MG4 format. This is essential for field and mobile GIS solutions on laptops and other mobile devices.”

There you have it — the straight scoop on alpha bands in MG4 from our friends at Ascent GIS! We wish it was always so easy to find people to give testimonials that are little technical papers at the same time! By the way, we call that black pixelation “speckling”, and GeoExpress has a way to “despeckle” MG3 images when they run afoul of nodata.  

To learn more about the evolution of the MrSID format see this article from the knowledge base on our website and this PDF detailing the history of GeoExpress software (viewing the PDF requires Adobe Acrobat Reader).