Daily Gridded and Mean Gridded Files
The daily gridded files provide ice motion vectors by merging the raw ice motion vectors together using a set of rules. See the Processing Steps section of this document for more information on this merging process.
The mean gridded files provide averages of the daily gridded data at different resolutions: weeks and months and the complete time series. Both the daily and mean gridded fields are projected to Northern and Southern Hemisphere EASEGrids. Data are stored in 2byte integer binary format (little endian) and are pixelinterleaved threeitem vectors (u, v, 3). Each vector represents three variables:
 U Component (cm/sec)—Scaled by a factor of 10; divide by 10 to revert to original units.
 V Component (cm/sec)—Scaled by a factor of 10; divide by 10 to revert to original units.
 Third Variable—Varies for daily and mean grids (see below). For both daily and mean grids, a pixel value of 0 in the third variable indicates no vectors at that location.
Third Variable for Daily Grids
For the daily grids, the third variable contains the square root of the estimated error variance, scaled by a factor of 10, at a given location. The error variance is the estimated error of that vector obtained from the optimal interpolation process. The input vectors from the individual sources (NCEP/NCAR Winds, SSM/I, SSMIS, SMMR, AMSRE, and AVHRR) are weighted separately based upon crosscorrelations with buoy vectors. The optimal interpolation uses these weights, along with their distances from the location being estimated, to obtain the final error variance.
If the closest input vector was greater than 1250 km, then a value of 1000 is added to this variable. Because interpolation was applied to a surface map from passive microwave data, coastlines may contain false ice. In this case, the third variable was assigned a negative value to allow users to remove these vectors near coastlines (within 25 km). For example, a value of 1035 indicates all of the following conditions:
 The vector was near a coastline
 The nearest sampled vector was further than 1250 km
 The vector had a σ value of 3.5, or the estimated error variance (σ^{2}) is 12.25
Third Variable for Mean Grids
For the mean grids, the third variable is the number of daily gridded values that contributed to the mean value. For example, at a grid point in the weekly product, the number of vectors would be between 1 and 7, indicating the number of days of the week with a valid vector at that grid point; for a monthly product, the number would be between 1 and 31. Generally, the greater fraction of days in the mean field that contain valid values, the higher the data quality.
Thus, the information contained in the third variable provides a means of characterizing data quality, in addition to the "near coastline" check described above. For example, a data user might choose to filter out vectors with error variances above a certain level or values for which the nearest observed vector was beyond a particular distance.
Raw Ice Motion Vector Files
The raw ice motion vector files provide the motion vectors from each specific sensor in spacedelimited ASCII text format. Each daily file contains a variable number of vectors that are described at the top of every file in a oneline header containing three numbers as described in Table 1.
Number  Description 

First  Specifies the number of vectors (lines) in the file 
Second  Original grid dimensions (x) 
Third  Original grid dimensions (y) 
After the header line, the data are listed in five columns for all files, except for the IABP buoy data which contains six columns. The columns are described in Table 2.
Column  Name  Description  

1  x 
EASEGrid row number for the start of the vector (vector starts in the center of the grid cell). The upper left corner is represented by x = 0.5  
2  y 
EASEGrid column number for the start of the vector (vector starts in the center of the grid cell). The upper left corner is represented by y = 0.5  
3  u 
The horizontal vector component in cm/sec  
4  v 
The vertical vector component in cm/sec  
5^{1}  z 
Source of the data; z value varies depending on instrument:


^{1} For the buoy data, there are six columns with the fifth column containing time of day in Universal Coordinated Time (UTC) and the sixth column containing the z value with the IABP buoy number. 
See the Sample Data Record for an example of a raw vector file derived from SSM/I.