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Orthoimagery Download Files Now Use the MrSID Format

May 31

Written by:
5/31/2016 2:36 PM  RssIcon

We all know image collections can become large...very large. Whether it’s a personal photo collection or years’ worth of statewide imagery these data sets can get huge. Fortunately, there are different image file formats that allow for the balancing of accuracy, performance, and storage size. There isn't one format that is the best for every situation. However, moving forward, MrSiD fits the bill for NC OneMap imagery. This is now the format of files downloaded from the NC OneMap GeoSpatial Portal.

Imagery Formats and File Size
The image format of a picture taken with a GoPro or cell phone is not much different from one taken from an aerial camera. Commonly, these are stored as TIFF (tagged image file format) files. These files provide an accurate display of the colors/values for each pixel in the captured image. Having accurate values for each pixel in the image is important if the imagery will be used for raster analysis. However, this accuracy comes at a cost. These files are large in size and having a hefty collection of imagery can easily require multiple terabytes of storage. However, if the imagery is only going to be used for visual purposes the files sizes can significantly be reduced by converting the TIFF's to an alternative format that implements some form of compression.

Switching to MrSID
Since 2010, we have offered imagery for download in a TIFF format with JPEG compression applied to it. While these were still in the TIFF format, applying the JPEG compression did convert some of the pixel values (which is why we always explain that these should be used for visual purposes only). We considered this a happy medium that balanced file size vs. performance. As we have continued to add imagery from the subsequent years however, our storage limitations have become more of a priority. Switching to MrSID format will result in a 25% reduction is storage size. This means we found 3 more terabytes! What will we do with all that space? Well, we will be able to:

  • Continue to Provide Historical Imagery - Creating more available space will allow us to avoid having to remove historical imagery in order to have sufficient capacity for upcoming image collections.
  • Provide County-Wide Downloads - More space means not only will we have enough space to store the upcoming imagery for 2017-2019, but we will also be able to offer the most recent imagery for anywhere in the state as a county-wide mosaic for download.
  • Streamline Internal Processes - The NC Orthoimagery Program creates a set of MrSID files as part of their internal processes. By using MrSID files for downloads, we can simply take the distribution files from the Orthimagery Program and plug them into the GeoSpatial Portal rather than running our own processes to apply JPEG compression to the TIFF files.

We realize that switching to MrSID is a bit of a change but, considering what having the additional storage space will provide, it’s one that we think is well worth it. As always, we welcome your feedback so get in touch with us and let us know what you think.

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