Ontologies, Egg Codes, and SIGRID3 data
Currently, SSIII has seven Ontologies (see Figure 1) : sea ice, seaice concentration, seaice development, seaice form, ice of land origin, egg code, and Sea Ice Grid (SIGRID), also called SIGRID-3, since this ontology describes a data format that has been through three updates. SSIII recognizes the existing operational use of the egg code and SIGRID-3 at National Ice Centers around the world. We hope that eventually these ontologies can aid the creation and reuse of operational sea ice charts. We have also worked to ensure our ontologies use terminology from the World Meteorological Organization (WMO) Sea Ice Nomenclature.
Figure 1: The Version 2 SSIII Ontologies and the documents from which their contents were derived
On sea ice charts, see Figure 2, sea ice parameters are represented by symbols with accompanying numbers giving the values of the sea ice parameters. The symbols varied depending on what nation was compiling the sea ice chart until the 1980s, when an international standard called the egg code 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. Refer to Figure 3. Scientists and sea travelers use the egg code to describe ice conditions around the world. The egg code describes sea ice concentration (amount of the sea surface that is covered in ice), stage of development (thickness), and form of ice (floe size) for a given area.
Figure 2: An example of a sea ice chart from Russia that uses egg codes. Taken from the flashtalk by Mark Parsons titled "The Semantic Sea Ice Interoperability Initiative" with co-authors Ruth Duerr, Siri Jodha Singh Khalsa, Peter Pulsifer, Peter Fox, Deborah McGuinness, James McCusker given in 2013.
The letters in the egg code describe the parameters such as sea ice concentration, thickness, and size. The numbers in the egg code, inserted by people who observe the sea ice directly from ships or aircraft, or indirectly through remote sensing images, represent the stages of the sea ice development such as thickness, type, size, and concentration. Technicians print ice-code ovals on top of ice maps, and captains use the egg code to avoid thick ice and find the best way to get where they're going. Egg codes are also used for lake ice in large bodies of fresh water. The U.S. National Ice Center has a more detailed explanation of the egg code.
Figure 3. The WMO System for Sea Ice Symbology, a.k.a. the Egg Code. The numbers in the egg give total concentration (C), usually as a range; partial concentration of the first (C1), second (C2), and third (C3) thickest ice; stage of development of the first (D1), second (D2), and third (D3) thickest ice; and other information such as form (i.e., Floe size) of the first (F1), second (F2), and third (F3) thickest ice, if available.
SIGRID is an alphanumeric coding of ice chart information originally obtained by overlaying a grid on the original paper chart and encoding the ice information in each grid cell. SIGRID-3 evolved from earlier SIGRID formats and incorporates much of their content. The SIGRID-3 format is a WMO standard with two main components: the chart information itself in shape file format, and metadata describing the chart.
Figure 4: An example of an ice chart showing how individual polygons are associated with egg code content.