EASE-Grid
All of the gridded fields on the Atlas are in EASE-Grid format. EASE-Grid is a set of equal-area projections and grids developed at the National Snow and Ice Data Center (NSIDC) to be a tool for users of global-scale gridded data. The Atlas gridded fields are in an azimuthal equal-area projection centered on the North Pole. The grid cell size is 250 km. There are a total of 529 cells in a 23- by 23- cell array, with cells numbered 0 to 22. Cell 11,11 is centered on the North Pole. The gridded field values are in ASCII files that can be accessed from the data directory, or by clicking on a browse image of a field. A "missing" data point is coded with nines (in the same format as a real value would be, for instance, "9999.99" for snow depth, or "999.999" for cloud cover).
The corners of the grid, with grid cell center coordinates, are shown in the table below.
Table 1. EASE-Grid corners.
Corner | Row | Column | Latitude (degrees North) | Longitude (degrees East) |
Upper left | 0 | 0 | 54.36 | 225.00 |
Upper right | 0 | 22 | 54.36 | 135.00 |
Lower left | 22 | 0 | 54.36 | 315.00 |
Lower right | 22 | 22 | 54.36 | 45.00 |
The center latitude and longitude for each grid cell is in file N65-250km.latlon.num. The grid is oriented so that the 180° meridian is at the top center, and the 0° meridian is at the bottom center. Figure 1 shows the location of the grid cell centers.
Fig. 1. EASE-Grid cell centers used for this Atlas. Red cell centers mark land cells
The advantage of the EASE-Grid's azimuthal equal-area projection is that it combines minimal shape distortion with no areal distortion on a hemispheric scale. No areal distortion means that a grid cell at the North Pole covers the same area as one at 45 degrees North. In contrast, a polar stereographic projection is conformal (has no shape distortion) but suffers large areal distortion on a hemispheric scale. The cylindrical equal-area projection (sometimes referred to as the "latitude-longitude grid") suffers from both shape and area distortion.
The EASE-Grid is defined by
r = 2*R/C * sin(lambda) * sin(PI/4 - phi/2) + r0
s = 2*R/C * cos(lambda) * sin(PI/4 - phi/2) + s0
h = cos(PI/4 - phi/2)
k = sec(PI/4 - phi/2)
where
r = column coordinate
s = row coordinate
h = particular scale along meridians
k = particular scale along parallels
lambda = longitude in radians
phi = latitude in radians
R = radius of the earth = 6371.228 km
C = nominal cell size
r0 = map origin column
s0 = map origin row
The values of C, r0 and s0 are determined by the grid that overlays the projection. For the Atlas EASE-Grid, r0 = 11; s0 = 11; and C = 250 km. The projection is based on a spherical model of the earth with radius R = 6371.228 km. This radius gives a sphere with the same surface area as an ellipsoid using the International Datum. Brodzik [1998]. (http://nsidc.org/NASA/GUIDE/EASE/ease_maps_info.html) and Armstrong and Brodzik [1995] have more information on the NSIDC EASE-Grid.