A Guide to NSIDC's Polar Stereographic Projection
NSIDC's Polar Stereographic Projection was originally designed to be optimal for sea ice applications, though it is now used for many other products.
Northern Hemisphere (left) and Southern Hemisphere (right) NSIDC Polar Stereographic Projection coverage maps. — Credit: National Snow & Ice Data Center (NSIDC)
It specifies a projection plane or grid tangent to the Earth's surface at 70° N/S (Figure 1), which means that the grid cells at 70° latitude are exactly equal to the nominal grid resolution. This translates to a 6% distortion of the grid at the poles but means there is little or no distortion of the grid in near the marginal ice zones. Many data sets archived at NSIDC, including many brightness temperature and sea ice products, use the NSIDC Polar Stereographic Projection precisely because of this minimal distortion around the marginal ice zone.
References
To learn more about projections and grids, including the polar stereographic grids discussed here, please refer to the following:
Knowles, Kenneth W. 1993. Points, Pixels, Grids, and Cells: A Mapping and Gridding Primer. Unpublished report to the National Snow and Ice Data Center, Boulder, CO USA.
Pearson, F. 1990. Map projections: Theory and applications. CRC Press. Boca Raton, Florida. 372 pages.
Snyder, J. P. 1987. Map projections - a working manual. U.S. Geological Survey Professional Paper 1395. U.S. Government Printing Office. Washington, D.C. 383 pages.
Snyder, J. P. 1982. Map Projections Used by the U.S. Geological Survey. U.S. Geological Survey Bulletin 1532.
Projection details
Projection details vary by region. Older passive microwave data sets archived at NSIDC use the NSIDC Sea Ice Polar Stereographic projections described in Table 1 (Northern Hemisphere: EPSG 3411) and Table 2 (Southern Hemisphere: EPSG 3412). These are based on the Hughes 1980 ellipsoid. There are two other Polar Stereographic projections, described in Table 3 (Northern Hemisphere: EPSG 3413) and Table 4 (Southern Hemisphere: EPSG 3976), that newer NSIDC data sets use. These projections are based on the WGS 1984 ellipsoid (WGS 84/NSIDC Sea Ice Polar Stereographic North and WGS 84/NSIDC Sea Ice Polar Stereographic South). The differences between the two projection pairs (EPSG 3411/3412 and EPSG 3413/3976) are minimal - the diagonal distance across a WGS 84/NSIDC Sea Ice Polar Stereographic 25 km grid cell differs about 1 m from the original NSIDC Sea Ice Polar Stereographic grid cell. However, users should note that EPSG codes 3411 and 3412 are deprecated, and NSIDC encourages all new products to use EPSG codes 3413 and 3976.
Table 1. Northern Hemisphere Projection Based on Hughes 1980 Ellipsoid
| Geographic Coordinate System | Unspecified datum based upon the Hughes 1980 ellipsoid |
|---|---|
| Projected Coordinate System | NSIDC Sea Ice Polar Stereographic North |
| Longitude of True Origin | -45 |
| Latitude of True Origin | 70 |
| Scale factor at longitude of true origin | 1 |
| Datum | Not_specified_based_on_Hughes_1980_ellipsoid |
| Ellipsoid/spheroid | Hughes 1980 |
| Units | meter |
| False Easting | 0 |
| False Northing | 0 |
| PROJ4 String | +proj=stere +lat_0=90 +lat_ts=70 +lon_0=-45 +k=1 +x_0=0 +y_0=0 +a=6378273 +b=6356889.449 +units=m +no_defs |
| EPSG Code | https://epsg.io/3411 |
Table 2. Southern Hemisphere Projection Based on Hughes 1980 Ellipsoid
| Geographic Coordinate System | Unspecified datum based upon the Hughes 1980 ellipsoid |
|---|---|
| Projected Coordinate System | NSIDC Sea Ice Polar Stereographic South |
| Longitude of True Origin | 0 |
| Latitude of True Origin | -70 |
| Scale factor at longitude of true origin | 1 |
| Datum | Not_specified_based_on_Hughes_1980_ellipsoid |
| Ellipsoid/spheroid | Hughes 1980 |
| Units | meter |
| False Easting | 0 |
| False Northing | 0 |
| PROJ4 String | +proj=stere +lat_0=-90 +lat_ts=-70 +lon_0=0 +k=1 +x_0=0 +y_0=0 +a=6378273 +b=6356889.449 +units=m +no_defs |
| EPSG Code | https://epsg.io/3412 |
Table 3. Northern Hemisphere Projection Based on WGS 1984
| Geographic Coordinate System | WGS 84 |
|---|---|
| Projected Coordinate System | WGS 84 / NSIDC Sea Ice Polar Stereographic North |
| Longitude of True Origin | -45 |
| Latitude of True Origin | 70 |
| Scale factor at longitude of true origin | 1 |
| Datum | WGS 1984 |
| Ellipsoid/spheroid | WGS 84 |
| Units | meter |
| False Easting | 0 |
| False Northing | 0 |
| PROJ4 String |
+proj=stere +lat_0=90 +lat_ts=70 +lon_0=-45 +k=1 +x_0=0 +y_0=0 +datum=WGS84 +units=m +no_defs |
| EPSG Code | http://epsg.io/3413 |
Table 4. Southern Hemisphere Projection Based on WGS 1984
| Geographic Coordinate System | WGS 84 |
|---|---|
| Projected Coordinate System | WGS 84 / NSIDC Sea Ice Polar Stereographic South |
| Longitude of True Origin | 0 |
| Latitude of True Origin | -70 |
| Scale factor at longitude of true origin | 1 |
| Datum | WGS 1984 |
| Ellipsoid/spheroid | WGS 84 |
| Units | meter |
| False Easting | 0 |
| False Northing | 0 |
| PROJ4 String |
+proj=stere +lat_0=-90 +lat_ts=-70 +lon_0=0 +k=1 +x_0=0 +y_0=0 +datum=WGS84 +units=m +no_defs |
| EPSG Code | http://epsg.io/3976 |
Grid dimensions
The grid size varies depending on the region and channel, as shown in Table 5. Latitude/longitude pairs are geodetic, with positions on the Earth based on an ellipsoid rather than a sphere.
Table 5. Grid Dimensions
|
Region |
Nominal Gridded Resolution (km) |
Number of Columns |
Number of Rows |
|---|---|---|---|
| North | 6.25 | 1216 | 1792 |
| North | 12.5 | 608 | 896 |
| North | 25 | 304 | 448 |
| South | 6.25 | 1264 | 1328 |
| South | 12.5 | 632 | 664 |
| South | 25 | 316 | 332 |
Grid coordinates
The approximate outer boundaries of the Northern and Southern grids are defined in Table 6 and Table 7, respectively. Corner points are listed and should be read clockwise from the upper left. Interim rows define boundary midpoints.
Table 6: Northern Hemisphere Grid Coordinates
|
X (km) |
Y (km) |
Latitude (deg) |
Longitude (deg) |
Description |
|---|---|---|---|---|
| -3850 | 5850 | 30.98 | 168.35 | corner |
| 0 | 5850 | 39.43 | 135.00 | midpoint |
| 3750 | 5850 | 31.37 | 102.34 | corner |
| 3750 | 0 | 56.35 | 45.00 | midpoint |
| 3750 | -5350 | 34.35 | 350.03 | corner |
| 0 | -5350 | 43.28 | 315.00 | midpoint |
| -3850 | -5350 | 33.92 | 279.26 | corner |
| -3850 | 0 | 55.50 | 225.00 | midpoint |
Table 7. Southern Hemisphere Grid Coordinates
|
X (km) |
Y (km) |
Latitude (deg) |
Longitude (deg) |
Description |
|---|---|---|---|---|
| -3950 | 4350 | -39.23 | 317.76 | corner |
| 0 | 4350 | -51.32 | 0.00 | midpoint |
| 3950 | 4350 | -39.23 | 42.24 | corner |
| 3950 | 0 | -54.66 | 90.00 | midpoint |
| 3950 | -3950 | -41.45 | 135.00 | corner |
| 0 | -3950 | -54.66 | 180.00 | midpoint |
| -3950 | -3950 | -41.45 | 225.00 | corner |
| -3950 | 0 | -54.66 | 270.00 | midpoint |
Grid distortion
To minimize distortion in the (typical) marginal ice zones, the Polar Stereographic projections are true at 70 degrees, which translates to a 6% distortion at the poles. Distortion in the rest of the grid increases as the latitude decreases because more of the Earth's surface falls into a given grid cell. This distortion can be quite significant. In the Northern Hemisphere, there is a 31% distortion around the grid edge, while in the Southern Hemisphere the maximum distortion is 22%.