AVHRR data was geolocated, calibrated and reprojected in GeoTIFF format using TeraScan software. A coast mask is applied in ENVI. The fraction of leads within each pixel is then determined based on the brightness temperatures or reflectances of open water and the surrounding thick ice, as described by Lindsay and Rothrock (Arctic sea ice leads from advanced very high resolution radiometer images. Journal of Geophysical Research, 100(C3), 4533-4544, 1995). Here, we have corrected for regional differences in surface temperature and reflectance as well as thin cloud cover by determining the fraction of leads within a moving 50 by 50 pixel (55 x 55 km) square window, with the fraction of thick ice based on the upper and lower quartile reflectance and brightness temperature, respectively. For a few cases of more expansive stretches of open water, manual adjustments of open water extent have been made. The resulting image indicating the fraction of open water is then binarized about a threshold corresponding to an open water fraction of 25% in each pixel, with pixels containing less open water designated as ice and those at or above 25% designated as leads. This information is then brought into NIH Image for derivation of lead location, shape and size statistics, open water fractions and other information for the different study subregions.
An IDL script was used to export binarized files into GeoTiff format. The batch utility in ArcToolbox was used to convert the GeoTiffs to ArcGIS Grid format. Another batch file was used in ArcCatalog to define the map projection and datum. Each Grid file was then exported to an ESRI interchange format *.e00 using batch utilities within ArcToolbox to ensure clean data transfer. Low resolution browse images (*.jpg format) were generated for display on the project web site. Final processing included a batch import of the appropriate FGDC compliant metadata template. Final data sets were bundled with Winzip for posting to the web to ensure clean data transfer of the components of the grid, plus associated metadata record and browse image.