GDSIDB Data Formats
The data formats for the GDSIDB are Sea Ice Grid (SIGRID) sometimes called SIGRID-1, SIGRID-2, and SIGRID-3. These and related formats are described below.
On sea ice charts, ice parameters are represented by symbols and accompanying numbers giving the values of the ice parameters. The symbols varied depending on what nation was compiling the chart until the 1980s, when an international standard was developed by the WMO. The egg code, which gets its name from the shape of the symbol used to embody the WMO standard sea ice information, is now used for most sea ice charts. The U.S. National Ice Center Web site has an explanation of the egg code and examples of operational ice charts.
SIGRID is an alphanumeric coding of ice chart information obtained by overlaying a grid on the original paper chart and encoding the ice information in each grid cell. Thompson (1981) provides a description of the proposal for the SIGRID-1 format. The official 1989 version of SIGRID-1 is described in the Annex to Recommendation 11 on pages 81 to 111 of the Abridged Final Report of the Tenth Session of the WMO Commission for Marine Meteorology which contains a number of differences from the 1981 proposal. In 1994, a simplified version of the original format, SIGRID-2 (WMO, 1994), was adopted and is described in the report Format to Provide Sea Ice Data for the World Climate Program (SIGRID-2) (Also available as a PDF, 272 KB).
SIGRID encodes the information in each egg as illustrated by the following example:
The nominal resolution of a SIGRID grid is 15 minutes latitude. The longitudinal resolution varies with latitude, as illustrated by the drawing at left and the plot at right:
While converting data from SIGRID to EASE-Grid makes it easier for most to use, SIGRID and EASE-Grid formats are limited in that they do not contain all the information in the original chart; therefore, they cannot be used to reconstruct the original chart. The illustration below shows why. In the SIGRID raster format, a single point within a cell represents the entire cell, and the shape of the boundary between regions is lost. To preserve this information and to avoid losing spatial resolution, a vector rather than raster format is needed.
In 1997, AARI proposed a vector format called CONTOUR 2; however, the sea ice community did not adopt this format. In 1999, work on a new vector format began based on ESRI shapefiles. The vector format preserves the chart information in a series of vertices that define the spatial extent of each ice region or polygon, and in a table with a record of attributes for each polygon encoded in SIGRID descriptors for that polygon's egg code. Background information on this new format, SIGRID-3, is found in the International Ice Charting Working Group's Ad Hoc Format discussion and on the SIGRID-3 development Web site. The SIGRID-3 format was adopted by JCOMM as an official WMO format in May, 2004. It may be referenced as "SIGRID-3: A Vector Archive Format for Sea Ice Charts. JCOMM Technical Report Series No. 23, 2004, WMO/TD-No. 1214."
Most ice centers now use GIS tools for ice chart production. SIGRID-3 is relatively easy for those centers to output as part of their production routine. This should lead to increased ice center participation in the GDSIDB.
A chart in SIGRID format is an ASCII file that must be interpreted by a computer program before the information it contains can be viewed. Because there are ten layers of information per chart, the format makes the data difficult to compare and analyze. The ten layers reflect the egg code: total ice concentration, concentration of the first thickest ice, stage of development of the first thickest ice (these are WMO-designated stages such as nilas, second year ice, and so forth), form of first thickest ice (generally fast ice is the only form used, out of several allowed by SIGRID), concentration of the second thickest ice, stage of development of the second thickest ice, form of second thickest ice, concentration of the third thickest ice, stage of development of the third thickest ice, and form of the third thickest ice. To simplify viewing and data analysis, NSIDC and AARI reformatted the collection of GDSIDB data from AARI and created the data set, Sea Ice Charts of the Russian Arctic in Gridded Format, 1933-2006. Data were re-gridded from SIGRID to the Equal-Area Scalable Earth Grid (EASE-Grid). EASE-Grid is a grid and projection combination used at NSIDC and elsewhere. It is described in the All About EASE-Grid Web page. One advantage of EASE-Grid is that the grid cell size remains the same with latitude.
The above figure illustrates how information in the ASCII SIGRID file is regridded and displayed. In this case the concentration of first year ice is shown.
IICWG. 2004. SIGRID-3: A Vector Archive Format for Sea Ice Charts. JCOMM Technical Report Series No. 23, WMO/TD-No. 1214.
Thompson, T. 1981. Proposed Format for Gridded Sea Ice Information (SIGRID).Unpublished report prepared for the World Climate Programme.
World Meteorological Organization. 1994. Format to Provide Sea Ice Data for the World Climate Program (SIGRID-2). World Meteorological Organization.
World Meteorological Organization. 1989. Abridged Final Report of the Tenth Session of the WMO Commission for Marine Meteorology. Paris, France. February 1989. World Meteorological Organization.