This gridded data set represents the Seaward Landfast Ice Edge (SLIE). Landfast sea ice is a seasonal phenomena and throughout its annual existence, between formation in late fall and break-up in late spring, it is shaped by a range of thermodynamic and dynamic forces (Barry et al. 1979; Shapiro and Metzner, 1989). The most apparent changes are those in landfast ice area and extent when floes of ice attach to and break off from its seaward edge. The position of the SLIE over the course of the year generally advances offshore to a stable position in mid-winter before retreating with the onset of spring. However, higher frequency changes in position also occur on timescales of days to weeks, which are generally small but occasionally affect the full width of the landfast ice. The location and stability of the SLIE at any point in time affects the activities of people and wildlife in the coastal arctic as it marks the boundary between stationary, continuous sea ice and drifting deforming pack ice. It is a vital consideration for people hunting or working on the ice in determining where food sources might be or where spilt oil might go. Since landfast ice occupies the shallowest water in the arctic, its presence or absence is also important for coastal process such as erosion and sediment entrainment.
These data were processed to meet contract requirements for the US Bureau of Ocean Energy Management, Regulation and Enforcement (BOEMRE): Mapping and Characterization of Recurring Spring Leads and Landfast ice in the Beaufort and Chukchi Seas Coastal Marine Institute (CMI/BOEMRE09HQPA0004T).
Data files have been processed to cover the western and eastern portions of the study area. The westen portion spans the coast from Russia's Chukotka Peninsula across the Bering Straight to Wainwright, Alaska. The eastern portion extends from Wainwright, Alaska to Mackenzie Bay, Canada. The files names associated with the landfast ice grid data derived from Radarsat mosaics are prefixed with an "r". The year and time period covering each delineation is noted with a range consisting of the day of the year for the first and last Radarsat mosaic used. For descriptive purposes, the following abbreviations are used as placeholders for real values to illustrate the naming convention: "d" = day of year and "Y" = Year. This convention follows as "rYY_ddd_ddd" where the gridded landfast ice extent derived from 2001 Radarsat mosaics covering a 16 day period would be r01_279_295 (ArcGIS grids have a 13 character file naming convention.) Definition of landfast sea ice: The seaward landfast ice edge has been delineated from colocated Radarsat SAR imagery based on the following criteria, which are used to define landfast ice 1) The ice is contiguous with the coast 2) The ice exhibits no detectable motion over approximately 20 days The pixel size of the imagery is 100m, with a stated geolocation accuracy of 200m. The time period for which the ice must remain stationary was chosen to represent a number of synoptic periods and therefore exclude ice that temporarily comes to rest adjacent to the coast without any mechanism to hold it in place while weather patterns change. The exact time period cannot be stated universally, since ice motion, or lack thereof, is determined by examination of 3 consecutive mosaics, the time interval between which is not constant. Furthermore, each mosaic represents a time-span of between 3 and 4 days and so there maybe different time seperations between different points in any two mosaics. A more detailed explanation of the definition and delineation of the seaward landfast ice edge can be found at mms.gina.alaska.edu
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The funding for this effort was provided by the US Bureau of Ocean Energy Management, Regulation and Enforcement (AK-03-06, MMS-71707) and the Coastal Marine Institute (CMI/BOEMRE09HQPA0004T). Matching funds are provided through support of Shell and Conoco-Phillips.
Map projection and datum were confirmed and checked to conform with base data sets and imagery. It should be noted that water on the surface of the ice will affect the backscatter, without necessarily affecting the stability of the ice or causing it to move. This situation is common in the Spring. Thus, if the ice immediately beyond a flooded area exhibits consistency, then the SLIE line is drawn beyond the flooded area. This is consistent with our definition of SLIE (Seaward Landfast Ice Edge.) As the ice at the mouth of a river becomes unstable during the Spring, a "hole" may develop in the landfast ice which we will not detect with this approach. This will be discussed further in reports associated with this study.
Horizonal Positional accuracy is typically between 0-300 m when comparing images with the Alaska coastline (derived from 1:63360 and AlaskaMapped.org data sources) in ArcMap. Offset larger than 300 m are reported to ASF for investigation. A few mosaics lacked coverage for Kotzebue. In these cases, the landfast ice delineation from the previous sequence was used.
A combination of automated and manual techiques have been employed to derive the Seaward Landfast Ice Edge (SLIE) shown in this data set. Custom IDL scripts were utilized to derive the net difference between gradient fields of 3 consecutive, colocated Radarsat subscenes. The average period between scenes is 10 days. A semi-automated procedure was developed to consider 3 consecutive mosaics at a time representing an average period of 20 days. From these a magnitude image of the horizontal and vertical components of gradient difference is created. The landfast ice is characterized by dark regions of low gradient difference adjacent to the land and typically bounded by bright, linear regions of high gradient difference. The SLIE is identified by bright regions of high gradient difference. After thresholding the image at a gray value corresponding to a net backscatter gradient difference of 8 dB / km, the SLIE is more easily detected and can be manually delineated. A technique for automatically delineating the SLIE from the gradient difference images proves elusive at this time, since the SLIE can be less distinct in some areas or misidentification of a SLIE, e.g., in areas of ice surface flooding can occur. For this reason, the SLIE is manually delineated over gradient image mosaics using the corresponding source imagery for interpretation.
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SLIE is manually delineated over gradient image mosaics using ENVI image processing software. Each SLIE delineation is processed and exported to GeoTIFF format . ArcGIS batch utilities are used to convert each GeoTiff to a 100 meter cell ArcGIS Grid file. Low resolution browse images (*.bmp format) are generated using ArcCatalog. These browse images are then converted to *.jpg format for display on the project web site. Final processing includes importing FGDC metadata, export to ESRI Interchange format (*.e00) and bundling with Winzip for posting to the project web site to ensure clean data transfer of the ESRI interchange file (*.e00), plus associated metadata record and browse image.
Internal feature number.
Grid cell value (where 255 = landfast ice)
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