This document explains how to generate custom monthly mean sea ice concentration maps with ArcGIS and will help users who want to calculate different ice thresholds. The steps listed below are the same as those NSIDC used to create the 15 percent and 50 percent threshold sea ice maps. Flat binary (.n and .s) images of monthly mean sea ice concentration are also available via FTP in a separate directory. These are used to create ice concentration maps in GIS format.
The following steps show an example of creating a Northern Hemisphere sea ice map for May 1979-2002 with a 30 percent ice threshold. These steps assume you have already established a workspace in ArcInfo where the sea ice images and headers will reside.
Rename the monthly mean sea ice concentration image in flat binary format (avg.may.19792002.n) to a new file with a band-interleaved-by-line (BIL) extension, for example, may_n.bil.
Download the ASCII header file that corresponds to your data. The header files must be renamed so that they have the same name as the image but with an .hdr extension instead of .bil. When you run the Raster To Other Format command in the next step, ArcMap must read the header file in order to successfully convert it.
Start ArcMap and open the ArcToolbox. Select Conversion Tools > To Raster > Raster to Other Format. The Raster to Other Format window will appear. In the Input Raster box, browse and select a monthly means sea ice concentration data file with a .bil extension (Refer to Step 1.). In the Output Workspace box, select the location to store the new data set. In the Output Format box, select the format to create the new data set; this can be one of three formats: ESRI Grid, ERDAS Imagine, or TIFF. Click OK.
In ArcToolbox, navigate to Data Management Tools > Projections and Transformations > Define Projection. Navigate to the new data set, then select the button next to the coordinate system box. In the window that appears, choose New... > Projected... and specify a new name for the projection. Next, select Stereographic_(North or South)_Pole as the projection. Modify the Standard_Parallel_1 to equal (+/-) 70. Modify the Central_Meridian to -45 for the Northern Polar Stereo projection.
In the Geographic Coordinate System box select New.... In the new window enter a new name, then select the Datum to be <custom>. Leave the Spheroid option box <custom> and change the Semimajor axis to be 6378273 and in the Semiminor axis box enter 6356889.4. Next, change Angular Units to Degrees and choose Greenwich for the Prime Meridian box. Click OK.
The data set is now appropriately projected.
This step is not necessary for creating Southern Hemisphere ice concentrations maps. NSIDC provides a land mask for the Northern Hemisphere to distinguish land from areas unsampled by the Scanning of Multichannel Microwave Radiometer (SMMR) instrument in 1978-87, around the North Pole. Download the file landmask_n_grid.e00 to your workspace directory. In ArcToolbox go to Coverage Tools > Conversion > To Coverage > Import from Interchange file. Leave the Feature Type drop down box as AUTO. Select the landmask_n_grid.e00 file as the input and name the output data set landmask_n.
You will then have to bring the land mask into your ArcMap session and give it the appropriate projection info. In ArcToolbox go to Data Management Tools > Projections and Transformations > Define Projection. The Input Data set is your landmask_n from the previous step. The coordinate system will show up as your predefined Polar Stereographic projection from Step 4, but the Central Meridian will need to be modified to line up correctly with your data. Click on the tab to the right of the Coordinate System window and then click Modify in the Spatial Reference Properties dialog window. In the Projected Coordinate System Properties Dialog, change the Central_Meridian to 0.
Using a 30% ice threshold as an example, the translation will be as follows.
|Input Value||Output Value|
|"may_n.bil" < 30%||Ocean (1)|
|"may_n.bil" >= 30%||Ice (2)|
|"landmask_n" ≠ 0||Land (3)|
|"may_n.bil" = 64736||Unsampled by SMMR (4)|
|Input Value||Output Value|
|"may_s.bil" < 30%||Ocean (1)|
|"may_s.bil" >= 30%||Ice (2)|
|"may_s.bil" = 64736||Land/ice sheet (3)|
To translate the values, in ArcToolbox go to Spatial Analyst Tools > Map Algebra > Multi Output Map Algebra. Enter one of the following expressions in the Map Algebra window, depending on hemisphere (make sure to enter the full path of each of the rasters).
may30N = CON (may_n < 300,1,may_n >= 300 && may_n <= 1000,2,landmask_n ne 0,3,may_n == 64736,4,9999)
may30S = CON (may_s < 300,1,may_s >= 300 && may_s <= 1000,2,may_s == 64736,3,9999)
In ArcToolbox go to Conversion Tools > From Raster > Raster to Polygon.
Select the may30N or may30S grid as the Input Raster and specify a new name for the output polygon file.