Importing SNODAS Data into ArcGIS

Overview

If you are using SNODAS products in ArcMap, the data will display improperly and the values will be off because these data are signed integers and ArcMap can only read unsigned integers. You will need to perform the Map Algebra function in ArcMap in order to fix the byte swapping problem.

These steps will work for all eight snowfall variables:

• SWE
• Snow Depth
• Snow Melt Runoff at the Base of the Snow Pack
• Sublimation from the Snow Pack
• Sublimation of Blowing Snow
• Solid Precipitation
• Liquid Precipitation
• Snow Pack Average Temperature

Preparing the File

In this section, you will prepare the file for execution in ArcMap. Depending on if you received the data from the regular FTP access link or GISMO, the steps you follow to prepare the file will be different because the data from the regular FTP access link is nonsubsetted data, and the data received from GISMO is subsetted data. To prepare the file for data received by the regular FTP access link, you will change the file extension name and create a new header file using a tool such as WordPad or Notepad. It is important to save the new header file in the same directory where you saved the .bil file. To prepare the file for data received by GISMO, you will only change the file extension name. The header file created by GISMO does not need to be changed since it is created from subsetted data.

1. Depending on if you received the data from the regular FTP access link or GISMO, choose one of the following steps:

1. If you received the data from the regular FTP access link, change the filename extension from .dat to .bil.
   
   For example, from us_ssmv11036tS__T0001TTNATS2007020105HP001.dat
   to us_ssmv11036tS__T0001TTNATS2007020105HP001.bil
   
   
2. If you received the data from GISMO, uncompress the file. This will change the filename extension from .dat.bil.Z to .bil.
   
   For example, from subset_us_ssmv11036tS__T0001TTNATS2005122605HP001.dat.bil.Z
   to subset_us_ssmv11036tS__T0001TTNATS2005122605HP001.bil

2. If you received the data from the regular FTP access link, perform the following step. However, if you received the data from GISMO, you can use the header file created by GISMO.

1. Create the following header file using a tool such as Notepad.




byteorder	M
layout	bil
nbands	1
nbits	16
ncols	6935
nrows	3351
ulxmap	-124.729583333331703
ulymap	52.871249516804028
xdim	0.00833333333
ydim	0.00833333333




2. Give the new header file the same name as the .bil filename but with a .hdr extension.
   
   For example: us_ssmv11036tS__T0001TTNATS2007020105HP001.hdr
   
   
3. Save the new header file in the same directory where you saved the .bil file.
   

Performing the Map Algebra Function

In this section, you will perform the Map Algebra function in ArcMap in order to byte swap the data.

1. Start ArcMap.
   
   
2. Open Arc Toolbox.
   
   
3. Select Conversion Tools > To Raster > Raster to Other Format (multiple).
   
   Note: The Raster to Other Format window displays.

4. In the Input Raster's box, browse and select the filename with a .bil extension.
   
   
5. In the Output Workspace box, select the location to store the new data set.
   
   Note: You can rename the file or use the default name.
   
   
6. In the Raster Format (optional) box, choose one of the three raster formats to create the new data set. The three raster formats are:

• ESRI Grid
• ERDAS Imagine
• TIFF

7. Click OK.


8. In ArcGIS, open up the Arc ToolBox
   
   
9. Select Spatial Analyst Tools > Map Algebra > Multi Output Map Algebra.
   
   Note: In Step 10, the whole file path name must be included. For example, if the file is located in the D directory, the file path name is D:\ filename. Since SNODAS is a 16 bit-signed integer, Arc imports the data in an unsigned integer. Thus, the data needs to be formatted that way.


10. In the new input box, type the following command in the Map Algebra Expression box:
    
    D:\filenameconv = con(D:\ filename >= 32768, D:\ filename - 65536, D:\filename)
    
    
11. Click OK.
    
    Note: Make sure that the syntax for the expression is followed exactly how it is displayed here, for example, spaces between numbers and/or file name otherwise the conversion will not work or will be incorrect.