conditions integrated from data usually collected over a 72-hour period. The NIC sea-ice charts
incorporate discrete polygons to characterize homogenous sea-ice conditions with information on
sea-ice concentration, stage of development, and form as mandated by the World Meteorological
Organization (WMO 1970).
Methods of Analysis
This section discusses the calculations of sea-ice thickness from both datasets, their
comparison, and the analysis of sea-ice conditions. The in situ ship measurements were
imported into ArcGIS 8 as a spatially referenced geodatabase data structure which catalogues
properties for each of three partial categories (thickest, middle, thinnest partial) of ice
concentration, thickness, type, topography, amounts of ridged ice and ridge sale heights, and
snow depths, as well as overall open water and open water fraction. Estimates of sea-ice
thickness were calculated, in centimeters, for the level ice (Tlevel), ridged ice (Tridge), and snow
thicknesses (Tsnow) which together are used to estimate the total ice thickness (Schellenberg
Ttotal = Tlevel + Tridge + Tsnow
Each in situ sea-ice concentration and thickness estimate has an associated uncertainty.
Empirically determined uncertainties for sea-ice concentration and thickness estimates are
computed from ship-based observations following standard rules of error propogation
(Schellenberg 2002: 34). These calculations were implemented utilizing the ArcMap's field
Thickness estimates, in centimeters, calculated from weekly NIC ice charts use sea-ice
concentration (Ci) and stage of development (Si) observations for the thickest (1), middle (2), and
thinnest (3) ice within each sea-ice polygon as is also done for the in situ ship observations.
Stage of development is a quantifiable indicator of ice type determined from remotely sensed
data. Specific ice types are associated with a thickness range that serves as a proxy for sea-ice
thickness and used here to estimate total (TNIC) ice thickness (Schellenberg 2002).
S1 + 2 S 2 + 3 S 3
As with the ship thickness estimates, NIC ice chart concentration and thickness estimates has an
associated level of uncertainty (Schellenberg, 2002: 31-34).
The next step is to evaluate the utility of sea-ice thickness estimates derived from satellite
products produced by the NIC. The in situ and NIC datasets are spatially defined in their
respective native coordinate systems and projection in ArcCatalog and reprojected to a common
South Pole Lambert Azimuthal Equal Area spatial reference in ArcToolbox. Next, the two
datasets are temporally and spatially merged by temporally querying the hourly ship-based
observations coincident with a weekly ice chart using ArcMap's attribute query tool (attribute
query search on date field date ≥ #5/7/1998# and date ≤ #5/12/1998#), and then, spatially