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Help for Downloading and Uncompressing Files

Many publications on geopubs include very large files. To deliver these files more easily via the web, many files are archived in packages (.tar or .zip) and compressed using a variety of compression schemes.

Step 1. Download the files

Sometimes clicking on an FTP hyperlink to download a file will display, or attempt to display the file in your web browser rather than downloading it.

To force a file to download:

on a Macintosh computer, hold the mouse button down for a moment to display a pop-up menu and select "Save this link as..." (Netscape Navigator) or "Download Link to Disk" (Internet Explorer) and navigate to where you want the file saved.

on a Windows computer, use the right mouse button to display a pop-up menu and select "Save Link As..." (Netscape Navigator) or "Save Target As..." (Internet Explorer) and navigate to where you want the file saved.

Step 2. Uncompress the Files

A variety of compression schemes are used to compress large files that make up publications on this server.

One of the more common file types for files on geopubs is .tar.gz, which is a group of files that have been archived (using the tar command) and compressed using gzip.

Another common file type for files on geopubs is .tar.Z, which is a group of files that have been archived (using the unix tar command) and compressed (using the unix compress command).

For PC users

An application called Winzip for Windows computers includes the gzip compression code and can decompress .gz and tar.gz files. Winzip will also extract tar archives. It is available from the Winzip website.

Renaming the extension might be necessary if expanding the file in some older Winzip software. Microsoft Internet Explorer changes the extension of downloaded files in the following cases: 

• .TAZ to .TGZ 
  
• .TGZ to .TAR 
  
• .Z to .TGZ 
  
• .TAR.Z to .TAR.TAR 
  
• .TAR.GZ to .TAR.TAR

An alternative to Winzip is gzip software available from the Free Software Foundation or on the gzip website. The gzip website provides links to gzip software for PC platforms. Gzip will uncompress both .Z and .gz files.

Once you uncompress the files using gzip, you'll also need to untar the files, if in fact it is a group of files. Almost all digital data packages contain more than one file and therefore are archived using tar. Typically, you will see one large plot file that has been compressed only using gzip or compress. However, some publication include a plot file package that consists of several files for plotting—in this case, the package will also be archived using tar.

To extract a tar archive, you can use Power Archiver.

Please note that gzip is freeware but Winzip is shareware, so register Winzip if you use it.

For Macintosh users

An application called StuffIt Expander for Macintosh computers can decompress .gz files and .tar.gz files. StuffIt Expander is available from Aladdin Systems. StuffIt expander will both decompress files and extract tar archives.

StuffIt Expander can also decompress .zip files, which are used in some publications on geopubs.

You can also use the application Mac Gzip to uncompress both .Z and .gz files. If using Gzip, you'll also need the application called Mac tar if you use Mac Gzip to uncompress the files. Mac Gzip can also extract a .zip file, but only if the .zip file does not contain multiple files.

Another application for handling .tar and .gz files on the Macintosh is called Cold Compress, which is available as freeware.

For Unix users

For .Z files, you can uncompress using the command:

uncompress [options] [files]

For .gz files, you can uncompress using the command:

gunzip [gzip options] [files]

To extract a .tar archive use the command:

tar [options] [files]
typically, the options for extraction will be xvf

For .zip file, you can uncompress using the command:

unzip [options[modifiers]] zip file . . . [extraction options]

Please refer to the unix manual on your system for more information

Step 3.

You are now ready to use the files

Please note that the mention of product names on this page does not imply endorsement by the U.S. Government