APPENDIX C: PROCEDURE USED TO CREATE THE COMPOSITE COOK
INLET ICE CHARTS
Three time-series map sets are shown in Section 4. The first set of maps shows the
mean ice conditions that have occurred, based on arithmetic averaging of the digitized
ice concentration and stage of development data from the available NWS ice charts
from December 1984 through March 1999. The second and third sets show the probabliity
of occurrence for two specific ice conditions: the occurrence of any marine ice at all,
and the occurrence of any ice type having at least 5/10ths concentration.
We developed these maps in the following manner. We transferred the microfiche
records to digital images in the tagged information file format (*.tif files) and then used
ArcInfoTM GIS software to process each image file. The images were georeferenced to
image pixels onscreen. We then used ArcInfo's Registration tool to calculate the rota-
tion and translation parameters required to generate a new image file that was refer-
enced to the geodetic coordinate space. When the root mean square of the transforma-
tion least-squares residuals achieved an order of magnitude of 3, the registration was
accepted. Using the Rectification tool, we applied the transformation parameters in
creating a new georectified image. We then placed a digitized Cook Inlet coastline
from a USGS 1:250,000 quad sheet over the image to check for proper image registra-
tion. It also guided the technician in making minor spatial corrections from the regis-
The technician then digitally traced the chart's ice features onscreen, establishing
each as a separate and unique polygon, which was linked to a database populated with
all of the classification attributes assigned by the NWS forecaster. Two additional data
fields were filled with numerical equivalents by the GIS technician--ice concentration
and stage--which enabled statistical calculations for these attributes. The database fields
and their contents are as follows:
The letter designation for each polygon that was assigned by the NWS
(e.g., A, B, C, etc.).
The ice concentration range reported by the NWS, in tenths (e.g. 79).
The various ice stages of development reported by the NWS. Because
several types may be assigned to a single polygon, the next seven fields
(RptStage_1, ... RptStage_7) are available to be filled as needed.
The NWS description of the most advanced ice type for the polygon. It
may be shown, for example, as Brash, Strips, Pancake, or Nilas. Because
the NWS charts were inconsistent with respect to this classification, we
did not use this value in our analyses.
Atlas_Stage The numerical equivalent assigned by the GIS technician for the most
advanced stage of development for the polygon. It is a numeric field
with values from 1 to 5, with the more mature ice assigned a higher
value. We intended the value to be a conservative estimate of navigation
difficulty. The atlas stage codes correspond with the WMO and NWS
codes according to the table below.