Job optimization with ArcGis Geodatabases: 2-Compression

The different
types of geodatabases

A little reminder before continuing, there is not just one
geodatabase but several. There are personal geodatabases
and enterprise geodatabases.
Personal
geodatabases can be created with ArcCatalog in your computer, or in a network
drive. There are two available formats: the personal geodatabase
that uses an Access format and the file geodatabase that uses an ESRI brand
format.
To
create an enterprise geodatabase you will need to use ArcSDE and the format
depends on the DBMS being used (Oracle, for example).
In
this article, we will not discuss the enterprise geodatabases.
While
there are two formats of personal geodatabases, we will discuss the file geodatabases
because the access databases just maintain compatibility with the old
geodatabases. Indeed, the geodatabase of the
ArcGis first version used the format Access, but the use constraints (limitation
to 2Go for the size, etc) led ESRI to develop its own format. Currently
there is no valid reason to keep on creating access geodatabases.

It
is rather important not to confuse the compression of a file geodatabase with
the compression of an enterprise geodatabase (which is outside the scope of
this article). Compressing a file geodatabase
does not delete the data it contains. However,
it prevents the content from being updated.

Compression of a file
geodatabase

Do not confuse compression and compaction. We
will discuss compaction in a next article (Job optimization with ArcGis 3-
Geodatabases Compaction). Compressing a geodatabase is
like compressing an image: the only goal is to save disk space by reducing its
size.
When
compressed, the geodatabase is marked as read-only. This
means that you will not be able to perform any editing operation on the
geodatabase. If you try to use ArcMap to
modify it, you will get a message saying there is no editable layer and that
the geodatabase is compressed.
To
compress a file geodatabase, perform the following steps:
1. Open ArcCatalog.
2. Locate and right-click the file geodatabase, check
Administration, and then click Compress file geodatabase.

You
will notice the existence of the inverse command (Uncompressing a file
geodatabase) that allows to restore the original geodatabase and to edit it.

The
only option in the Compression window is a “Lossless Compression”
check box.
The
compression operation may be performed with or without loss of data.
Loss compression was the only option available before ArcGIS
10.0 file geodatabases. For geodatabases that run on
versions 10 and later, you have two options: lossless and loss compression.

Do
not ask me what can be used for data loss compression, for a database manager
it’s heresy!
Loss compression is the process by which the content
is compressed while losing some of its content. It’s
an irreversible operation. This is what can be done with
image compressions where, from a certain degree of compression there has been
lost some definition of the image. On
the other hand for attributesdata it is absolutely necessary to avoid this type
of compression.
Lossless compression compresses the content while
preserving the data. It’s a reversible operation.

Compression
can be used to save a lot of disk space, especially for geodatabases with a
large number of entities. If the geodatabase is mature
enough and has not been modified for a long time, it is advisable to use
lossless compression to reduce its size.
Before
using compression, always remember to create a backup of your file geodatabase.
Compression can sometimes corrupt the geodatabase and make
it inaccessible.

For
more information on how to compress ArcGis geodatabases, visit the ArcGis
Online Help (ESRI)

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