Frequently Asked Questions

1. What are the polar stereographic and EASE-Grid projections? Which one should I use?
2. What is the difference between the NASA Team algorithm and the Bootstrap algorithm?
3. What are the differences between the NSIDC and GSFC sea ice products?
4. Which ancillary product should I use?
5. I just want to see a picture of sea ice extent or concentration or a plot of ice trends by month. Where should I look?

Answers

1. What are the polar stereographic and EASE-Grid projections? Which one should I use?
   
   The two grids are based on different map projections. The polar stereographic projection was initially used with ice concentration products developed at the NASA Goddard Space Flight Center (GSFC) and has been retained for historical consistency. It specifies a projection plane tangent to the earth at 70° latitude, which was selected so that little or no distortion would occur in the marginal ice zone. For a more in-depth description, see the Polar Stereographic Projection and Grid Web page.
   
   The Equal-Area Scalable Earth Grid (EASE-Grid) is based on a philosophy of digital mapping and gridding definitions developed at NSIDC. It is intended to be a versatile tool for users of global-scale gridded data, specifically remotely sensed data, although it is gaining popularity as a common gridding scheme for data from other sources as well. The EASE-Grid consists of a set of three equal-area projections, combined with an infinite number of possible grid definitions. It allows diverse data sets to be compared on the same grid. For a more in-depth description, see the EASE-Grid: A Versatile Set of Equal-Area Projections and Grids Web page.
    
2. What is the difference between the NASA Team algorithm and the Bootstrap algorithm?
   
   For general analyses or creation of simple images, either algorithm will suffice. The Bootstrap sea ice concentration data set is believed to be more useful for modeling and process studies in the polar regions because it is generally free of residual errors that could not be removed by conventional techniques. A temporally more consistent time series of sea ice concentrations is provided, offering improved accuracy over the ice concentration maps created from the original Bootstrap algorithm.
   
   For in-depth analyses, input into model fields or other scientific research, users should first study the differences between the algorithms. For more information, see the NASA Team and Bootstrap algorithm descriptions.
    

   	NASA Team	Bootstrap
   Channels	19V, 19H, 37V	Frequency mode: 19V, 37V (seasonal ice) Polarization mode: 37V, 37H (perennial ice)
   Methodology	Ratios of brightness temperatures	Interpolation between clusters of pure ice types
   Tiepoints	Summer and winter only	Multi-seasonal
   Strengths	Not sensitive to changes in surface temperature	Less sensitive to thin ice and layering within snow and ice
   Weaknesses	Sensitive to thin ice and layering within snow and ice	Sensitive to changes in temperature (thin ice); tiepoints switch by date.
   Regions of highest errors	Regions where melt or refreeze occurs; algorithm underestimates thin ice	Regions with very cold surface temperatures, such as the Antarctic coast in winter and central Arctic in winter

   
    
3. What are the differences between the NSIDC and GSFC sea ice products?
   
   NSIDC developed the DMSP SSM/I Daily and Monthly Polar Gridded Bootstrap Sea Ice Concentrations. Sea ice products from the NASA Goddard Space Flight Center (GSFC) include Sea Ice Concentrations from Nimbus-7 SMMR and DMSP SSM/I Passive Microwave Data and Bootstrap Sea Ice Concentrations from Nimbus-7 SMMR and DMSP SSM/I.
   
   NSIDC and GSFC products include sea ice concentration estimates from two algorithms: NASA Team and Bootstrap. For general qualitative views of sea ice, either product and algorithm is suitable; however, for scientific studies, one product or one algorithm may be more suitable than the other in certain situations (see FAQ #2 above). Biases exist between algorithms and between products, so they should not be used interchangeably.
    

   	NSIDC	GSFC
   Length of Record	June 1987 through three to six months before present	October 1978 through December 2004 (NASA Team) and December 2004 (Bootstrap)
   Frequency of Update	Every 3-6 months; the near-real-time product is available daily.	Every 1-2 years
   Quality Control	No tiepoint adjustments Standard automated weather filtering No removal of coastal contamination; expanded land mask can be used to eliminate such pixels.	Algorithm tiepoints are adjusted to ensure intersatellite consistency; SMMR is used as a baseline. Additional weather filtering to remove spurious ice over water Pixels with coastal contamination (mixed ocean and land pixels) are removed.

   
    
4. Which ancillary product should I use?
   
   The NSIDC Sea Ice Index products and the Sea Ice Trends and Climatologies from SMMR and SSM/I both encompass the SSM/I time series and include trends and anomalies of total ice-covered area and extent. The Sea Ice Index products are developed from the NASA Team algorithm and are updated monthly. The Sea Ice Trends and Climatologies from SMMR and SSM/I are developed from the NASA Team and Bootstrap algorithms and are updated every three to six months.
   
   Ancillary products developed from GSFC sea ice data provide a longer term time series, with data extending back to October 1978. In addition to total ice-covered area and extent, these ancillary products also include ice persistence, maximum ice extent, and ocean masks. The GSFC ancillary products are updated less frequently (every one to two years) compared to the NSIDC ancillary products.
    
5. I just want to see a picture of sea ice extent or concentration or a plot of ice trends by month. Where should I look?
   
   The Sea Ice Index Web page provides images derived from passive microwave satellite data that depict the most recent daily sea ice conditions.