Frequently Asked Questions
- NSIDC has a bunch of sea ice data and information sites. How do I choose which one to use?
Icelights and All About Sea Ice answer general questions about sea ice. The Sea Ice Data Web pages gives an overview of NSIDC data products. Contact User Services for help in selecting the product that's right for you.
- Is there a better daily sea ice product at NSIDC?
NSIDC and the National Ice Center work together to make the daily, 4km Arctic-wide Multisensor Analyzed Sea Ice Extent product available. It may be better, depending on how you plan to use the data. See the MASIE FAQ for information on how it differs from the Sea Ice Index.
- The record minimum extent numbers I see for the 2012 Arctic Sea Ice News and Analysis Record Minimum Post, in Table 1, are different than the daily numbers for those days from Sea Ice Index data. Why?
There are slight differences because the ASINA site calls the minimum using a 5-day trailing average value, for reasons explained on the ASINA site. The archived Sea Ice Index daily value is not averaged in this way.
- Why do you later replace the NRTSI monthly product that you generate with the GSFC product?
The GSFC product has some additional quality control and provides the longest consistently processed record.
- Can I get the binary gridded monthly files from the NRTSI product that the SII processing computes and then uses to calculate ice extent and ice area?
No. You can calculate it yourself, however, by using the daily gridded data from the NRTSI product (NSIDC-0081).
- Why do you say "nominal" 25 km cell size?
Because the grid cells vary in size based on latitude when gridded data are in a polar stereo projection. In this context, the word nominal refers to a designated or theoretical size that may vary from the actual size. The 25 km comes from the size of the grid at the spot where this polar stereo projection is considered "true" (where the projection plane is tangent to the Earth's surface). For this projection, that is at 70 degrees latitude. See the NSIDC Polar Stereographic Projection and Grid Web page for more details.
- What is the difference between a pixel and a grid cell?
Pixel and grid cell are often used interchangeably. Pixel is the term for the smallest component of a digital image, while grid cell refers more specifically to gridded data. Gridded data are data that in turn can be represented in an image composed of pixels of unrelated size. We prefer the term grid cell here, to specify that we are displaying data. All About Sea Ice Terminology has more information on this.
- What is the difference between sea ice area and extent?
See the ASINA FAQ page: What is the difference between sea ice area and extent?
- Why do the line graphs plot anomalies and not actual extent values?
When anomalies are shown in percent they carry information that is more quickly and easily understandable than would be the extent numbers alone. A line graph of extent values would look exactly the same. Only the values and label on the y-axis would change.
- What is the relationship between the Sea Ice Index and Arctic Sea Ice News and Analysis (ASINA)? When to use one, when the other?
The Sea Ice Index is a data product. The Arctic Sea Ice News and Analysis (ASINA) Web site provides an interpretation of those data, and other data products, by NSIDC scientists. The ASINA site shows the daily sea ice extent graph that is a Sea Ice Index product; however, when the news site reports a day's extent value, the scientists are using a 5-day average value, not the un-averaged value that the Sea Ice Index archives. For example, in 2012, the lowest Sea Ice Index extent value was 3.37 million km sq, while the ASINA site reported 3.41 million km sq as the 2012 minimum value. An ASINA post from April 2012, Daily Graph Changes, explains why the ASINA site uses an average.
- In Antarctica, the Sea Ice Index seems to have a "land" boundary that is different from that given on standard world shapefiles. Is this related to ice shelves?
Yes. The Sea Ice Index Antarctic "land mask" is based on the mask used in 2000 by the Sea Ice Concentrations from Nimbus-7 SMMR and DMSP SSM/I Passive Microwave Data. The mask used for processing to get ice area and ice extent is this mask, and it has not changed. The Antarctic shapefile derived from this mask may have undergone small changes over the life of the Sea Ice Index data product.