Extensis - has a free
viewer (GeoViewer Pro) that allows you to work with .sid files.
are now available. Some counties along the middle and western portions of the
state are split into two .sid files, this is because half the county was
produced by USGS and the other portion by TVA. The X-resolutions did not
match so a seamless countywide file was not possible for 10 counties: Bolivar,
Claiborne, Clarke, Copiah, Issaquena, Jasper, Simpson, Smith, Warren, and
Washington. Please download both sets of files for these counties. DRG's are digital copies of 7.5
minute - 1:24,000 topographic maps published by the U.S.Geological Survey.
The USGS produces their DRG product by scanning paper copies of the map at
500dpi and then resampling them to 250 dpi.
USGS topographic maps covering Mississippi were published over a number of years, with the most recent original versions and updates ranging from the early 1960's to mid 1990's. The DRG's are output as geotif image files georeferenced to UTM NAD 83 coordinates.
The MARIS staff has reprojected the DRG tif files into the MSTM projection and then compressed them using MrSID compression software. The files are stored by blocks designated by degrees of latitude and longitude (for example - 3390 identifies maps with a longitude of 33 degree and longitude of 90 degrees, etc.). Individual file names follow the USGS naming scheme (3390a1.sid, 3089g3.sid, etc.) MrSid desiginates the image file as a .sid file with an associated .sdw file as a world geo-reference file. The .sid file is binary while the .sdw file is ASCII. These files are NOT available in any other format or projection from the MARIS Technical Center.
If there are any general questions about DOQQs and downloading or acquiring the data through other means, please contact Steve Walker at email@example.com or 601-432-6149.