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1999 Louisianna DOQQs MrSid Compressed, 50:1 compression ratio Universal Transverse Mercator, NAD 83 This dataset is a 3.75 minute color infrared (CIR) rectified digital photo for the SE quadrant of the North Islands quadrangle, Louisiana. The photo was created from color infrared aerial photography film. Color infrared film is manufactured to be sensitive to red, green and near-infrared wavelengths. This type of film makes near-infrared reflected energy visible to the human viewer by capturing that energy and representing it as a visible color. The dyes that are used in the film render green objects as blue images, red objects as green images, and objects that primarily reflect infrared as red images. Because healthy vegetation reflects strongly in the near infrared, areas of vegetation appear red or pink. The appearance color for water in CIR imagery is highly variable and dependent on the depth and amounts of floating vegetation and suspended matter. It can range in color from olive drab through turquoise, deep blue and to black. Barren areas can be highly reflective at all wavelengths, thus frequently appear very bright in a CIR image. This image has an overedge of approximately 70 meters on all sides, so the area encompassed is actually somewhat larger than 3.75 minutes by 3.75 minutes. Each pixel in these orthophotos represents an area of one square meter on the ground. The image is in Multi-Resolution Seamless Image Database (MrSID) form. Like the other common photographic image compression methodology, JPEG, MrSID compression maintains high visual image quality even at high compression levels. The advantages MrSID offers over JPEG is a faster display rate and a 'smooth' appearing image at any magnification level, the latter being a manifestation of the 'Multi-Resolution' qualities of this imagery. The compression levels obtained make it possible to distribute this MrSID imagery over the web, even to users with modem-only access to the Internet, and to put a hundred or more MrSID orthos on a single CD-ROM. Imagery in MrSID form is supported in most major GIS software. It can also be manipulated in Adobe Photoshop or viewed, subset, and converted to standard TIFF format imagery with a free stand-alone viewer available from LizardTech ( The 'k' prefix indicates that it is over the area of the Kisatchie National Forest and that there is no identical product available from the USGS. The 'm' prefix indicates that it is positioned on the Mississippi-Louisiana border and that there is no identical product available from the USGS. The 't' prefix indicates that it is positioned on the Texas-Louisiana border and that there is no identical product available from the USGS. The 'z' prefix indicates that the photo was created to be reference imagery for the near-coastal or extended water areas of Louisiana that are not depicted by orthophotos. The user should be aware that this photo has only been rectified. It is not an orthophoto and does not possess the horizontal positional accuracy of that product. |
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