This gridded data set represents the Seaward Landfast Ice Edge (SLIE) along the coast of Northern Alaska (West of Barrow) and Northwest Canada (East to the Mackenzie Delta) for the study area which ranges from approximately 133 to 160 degrees West and 68 to 72.5 degrees North. Landfast sea ice is a seasonal phenomena in the Alaskan Arctic 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 compiled by the University of Alaska Fairbanks to meet contract requirements for the US Mineral Management Service: Mapping and Characterization of Recurring Spring Leads and Landfast ice in the Beaufort and Chukchi Seas (AK-03-06)
Following is an operational definition of Landfast ice stability that will be refined as extensive data sets are analyzed for the MMS study area. (1) The general time period of stable landfast ice is bracketed by the first, sustained attachment in fall/winter and the break-up and decay of landfast ice in early summer. Preliminary work suggests that these dates have been shifting somewhat in the past 20 years. Here, the period of first, sustained attachment is defined as that time when a landfast ice cover of more than 1 km width is established that persists in-situ for more than 2 weeks over a given stretch of coastline. Break-up and decay are defined by the removal of substantial portions of the landfast ice to within less than 1 km from the beach, with concurrent substantial reductions in ice thickness through surface and bottom ablation. (2) During the general time period of stable landfast ice, it is the location of the seaward edge of the fast ice that determines stability in any given location. Hence, the simplest assessment of stability would include an indication of the amplitude of oscillations in seaward landfast ice edge during the course of the ice season as a measure of stability. In most cases, however, one can expect such oscillations to subside once the seaward sectors of the landfast ice are grounded by a combination of thick ice floes and pressure ridges. Thus, break-out events as major manifestations of fast-ice unstability could be characterized as breaking off of major sections of the landfast ice (ice fragments larger than 1 x 1 km in size) during this latter, "more stable" period of the landfast ice seasonal cycle. (3) In order to distinguish between full break-out events and the attachment and detachment of "floating extentions" which are often observed in the Beaufort Sea during periods of stagnating ice motion, one would furthermore discriminate between break-out of landfast ice inshore and offshore of the deepest grounded ridges (if present and identifiable) or of the isobath typically associated with the deepest grounded pressure ridges (generally between 20 and 25 m; Reimnitz et al., 1978). For a floating extension to qualify as part of the landfast ice, it would have to be present for two weeks or more.
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The funding for this effort was provided by the US Mineral Management Service for Mapping and Characterization of Recurring Spring Leads and Landfast ice in the Beaufort and Chukchi Seas (AK-03-06, MMS-71707)
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.
+/- 3 pixels based on co-location techniques of source imagery.
A combination of automated and manual techiques have been employed to derive the Seaward Landfast Ice Edge (SLIE) shown in this data set. The MMS study area was divided into 10 subregions in order to obtain subscenes from 500 x 500 km RADARSAT scenes that were free from mosaicking edges prior to calculating the gradient fields. Custom IDL routines were developed to derive the net difference between gradient fields of 3 consecutive colocated Radarsat subscenes. The average period between scenes was 8 days. An automated procedure was developed to consider 3 consecutive mosaics at a time representing an average period of 17 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 in some areas 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 sometimes be less distinct in some areas and bright regions sometimes occur along the coast particularly in springtime when surface flooding from rivers occurs. For this reason, the SLIE is manually delineated over gradient image mosaics compiled from the 10 subscenes. Monthly averages were processed and exported to GeoTIFF format using IDL routines. The final steps included converting GeoTiffs to ArcGIS format, defining the map projection and associating XML based metadata templates in preparation for posting to the project web site.
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Internal feature number.
903 Koyukuk Drive
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These data will be made available at the request of the US Mineral Management Service via online tools provided by the Geographic Information Network of Alaska (GINA) at the University of Alaska Fairbanks.
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