Small is beautiful!

Spatial data sets tend to be large, often needlessly so. If you're not using data, keep it compressed.

There are several standard compressions formats, including the following (with their extensions):

zip - .zip
gzip - .gz
unix compress - .Z

The popular Windows application WinZip can extract data from all three formats, and is a handy tool.

Unix programs will also extract each of these: for

.zip files, type
```
unzip filename
```
.gz files, type
```
gunzip filename
```
.Z files, type
```
uncompress filename
```

Sometimes files are "tarred" up first: for example, a directory will be "tarred" into a single file (using the UNIX tar facility), then compressed. For these files, there is a second step: given a file with extension .tar, let's say it's called file.tar, type

cat file.tar | tar -xvf -

to "untar" the file, creating the files stored in the tarball. Alternatively, you can generally proceed directly from the compressed tar files (without decompressing) as follows:


 zcat file.tar.Z | tar -xvf -
 zcat file.tar.gz | tar -xvf -
 zcat file.tgz | tar -xvf -

If zcat doesn't work, then try gzcat instead. If that doesn't work, ask your system administrator where the GNU unzip tools are. If they're not installed, you have a system administrator problem....

There are many options for these programs (unzip, gunzip, uncompress). To find out what the options are, use the unix manual page command, e.g.

man unzip

When you are done with a big file, zip it up: I prefer gzip compression (it tends to squash the most), so I'll type

gzip filename

when I'm done working with a file.

You'll have a chance to use unzip or WinZip very soon!

Website maintained by Andy Long. Comments appreciated.

aelon@sph.umich.edu