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Snow Data Assimilation System (SNODAS) Data Products at NSIDC

Frequently Asked Questions

1. File extensions have changed from ".grz" to ".tar.grz". Has anything changed with the data?
2. How do I extract data from the ".grz" files?
3. How can I extract the .hdr and .dat files from the .grz file on a Windows machine?
4. I downloaded a SNODAS file to my Windows computer, and the SWE .dat file seems too large.
5. What do the letters and numbers in the SNODAS file names indicate?
6. Is subsetting an option for this data set?
7. What is the difference in the output between ordering SNODAS data through the GISMO versus ordering SNODAS data through the FTP site?
8. What if I need SNODAS data in real or neal-real time?
9. Where can I find additional information about this data set?

Acronyms used on this page:

FAQ: Frequently Asked Questions
GISMO: Graphical Interface for Subsetting, Mapping and Ordering
NOHRSC: National Operational Hydrologic Remote Sensing Center
SNODAS: SNOw Data Assimilation System, produced by NOHRSC

1. In the past, I have downloaded files from your FTP site that ended in “.grz”. Now, the files end in “.tar.gz”. Has anything changed with the data?

No. The files on our FTP site were renamed from <filename>.grz to <filename>.tar.gz in January of 2006 (example: us_ssmv11034tS__T0001TTNATS2006012005HP001.grz has been renamed to us_ssmv11034tS__T0001TTNATS2006012005HP001.tar.gz). Renaming the files only changes the file extensions and does not change the content of the data.

2. Since I have SNODAS data files that are in the .grz format, how can I extract the data files (the .Hdr and .dat files) from the .grz file?

Rename the file to <filename>.tar.gz and then use the gunzip and tar commands to uncompress and untar the data.

For example, on a Unix machine, run the following commands:

> mv <filename>.grz <filename>.tar.gz

> gunzip <filename>.tar.gz

> tar xvf <filename>.tar

Here is an example, using a SNODAS file from January 20, 2006:

> mv us_ssmv11034tS__T0001TTNATS2006012005HP001.grz us_ssmv11034tS__T0001TTNATS2006012005HP001.tar.gz

> gunzip us_ssmv11034tS__T0001TTNATS2006012005HP001.tar.gz

> tar xvf us_ssmv11034tS__T0001TTNATS2006012005HP001.tar

>us_ssmv11034tS__T0001TTNATS2006012005HP001.Hdr

>us_ssmv11034tS__T0001TTNATS2006012005HP001.dat

The results should display two files: a .hdr file and an associated .dat file.

3. Since I still have SNODAS data files in .grz format, how can I extract the .hdr and .dat files from the .grz file on a Windows machine?

Follow these steps:

• Rename the <filename>.grz file to <filename>.tar.gz on your Windows computer.
• Use WinZip to extract the <filename>.grz file. The results should display a <filename>.gr file.
• Since the <filename>.gr file is actually a tar file, rename the file extension from .gr to .tar
• Use WinZip one more time to extract the .tar file. The .hdr and .dat files should now be available.

4. I downloaded a SNODAS file to my Windows computer.  After extracting a Snow Water Equivalent file (that contains “1034” in the file name) to my Windows machine, I noticed that the file size for the .dat file seems too large for a 2-byte integer with 6935 x 3351 dimensions.  I am using WinZip software.  Do you have any suggestions?

• Extract the “.tar.gz” file using WinZip. The WinZip software may ask you something similar to: "Should WinZip decompress it to a temporary folder and open it?"
• Answer "Yes."
• The two files should then appear in the WinZip window.
• In WinZip, click on Options -> Configuration -> Miscellaneous
• Uncheck the "TAR file smart CR/LF conversion" option
• Click "OK".
• Now try transferring the files from WinZip to your Desktop.

Note that these instructions may vary, depending on which version of WinZip you are using.  The “.dat” file should be 44.3 MB in size.

5. What do the letters and numbers in the SNODAS file names indicate?

For information on the output naming convention, please review Appendix C of NSIDC Special Report - 11.

6. Is subsetting an option for this data set? For example, I would only like to order snow depth data over Michigan for October 10, 2004.

Yes. Go to the Graphical Interface for Subsetting, Mapping and Ordering (GISMO). Click on the “Registration” link and fill out the contact form. After you submit this form, a user name and password will appear on the web screen. Return to the main GISMO web page and click on “SNODAS Interface”. Enter in the user name and password provided on the previous screen. You can then subset the data set and select an output format.

7. What is the difference in the output between ordering SNODAS data through the GISMO versus ordering SNODAS data through the FTP site?

Data ordered through the GISMO are usually a subset of the entire SNODAS data set. Here are some of the main differences between a regular SNODAS file (.tar.gz) and a subsetted file:

• File names differ:
  • Normal file name: <filename>.tar.gz
  • Subsetted file name: subset_<filename>.dat.<user_selected_output>
    (example: "subset_us_ssmv11036tS__T0001TTNATS2005122605HP001.dat.bil.Z")
• If you order SNODAS data via GISMO, you can select your output extension from a list and subset the data set, whereas with a normal file, the file extension is “.tar.gz” and covers the continental United States.
• When you order data via the GISMO, you will see .hdr, .pcf and .prj files in the first tar file of the GISMO request; this tar file ends in “-0.tar”.
  (example: “2006119161725-0.tar)
  • The .hdr file – contains information about the subsetted data file
  • The .pcf file – lists all of the input parameters that you selected in the interface
  • Note: The "Map Projection" parameter in the .pcf file is set to “CYLINDRICALEQUIDISTANT”. This is a synonym for the “GEOGRAPHIC” projection.
  • The .prj file – this is a projection file and information from this file can be used to import the data into ArcGIS, for example (if you selected one of the GIS output options in the GISMO)

8. What if I need SNODAS data in real or neal-real time?

NSIDC has scripts that run several times a day. If new SNODAS files are found, these scripts will automatically post them to our FTP server. If you need data sooner than the normal timeframe that NSIDC uploads files to our FTP site, you can contact NOHRSC, as they distribute the data for operational users.

9. Where can I find additional information about this data set?

NSIDC Special Report - 11

NSIDC SNODAS data set documentation

For additional questions, please contact NSIDC User Services.