Friday, December 02, 2005

Sessions 101 and New versus Open in GTViewer and GTVx

GTViewer and GTVx both offer the same two dialogs for New and Open and there is sometimes confusion about what the difference between these two commands. The differences are minor, but there are some differences. This blog posting will try to clarify these two commands and explain how a Session relates to them.

GTViewer and GTVx are session based. When you are viewing your GIS data, you are always in a session. The session information is separate from the GIS data in that it contains only the user-specific information such as the current view extents, display filter settings, and redlines. The GIS data is generally represented by a .GTM file (unextracted GTViewer data), and the session it uses is a .GTS file. A single .GTM file can have any number of sessions created against it, and these sessions can all be open simultaneously by one or more users.

This separation between the GIS data and the session info is straightforward until you consider an Extract File (.GTX) which contains both the GIS data and the session information in a single file. The .GTX file was initially designed to be single-user since its primary purpose was to provide data for Pocket GTViewer (a single-user environment). However, user affinity for managing their data with a single, self-contained .GTX file on the desktop guided product development to support simultaneous users each with their own session from a single .GTX file. By extending the .GTS session file to support .GTX files as well as .GTM files, multi-user support is provided for both .GTM and .GTX files. For more info, see: Unextracted versus Extracted Data, Creating Extract Files

This convoluted introduction to sessions is the key to understanding the difference between New and Open.

  • You will always be in a session if you are viewing GIS data from a .GTM file or a .GTX file.
    If you are using a .GTX file in single-user mode, the session information is embedded in the .GTX file (along with the GIS data, but only the session information is modifiable by the user).
  • If you use File/Open to open a .GTX file, the embedded session is used.
  • If you use File/Open to open a .GTM file, a new session is created using the .GTM file as the source. When you exit the session, you will be asked if you want to save the session. The session will be saved as a .GTS file.
  • If you use File/New and specify a .GTX file, a new session is created. When you exit the session, you will be asked if you want to save the session as a .GTS file.
  • If you use File/New and specify a .GTM file, a new session is created. When you exit the session, you will be asked if you want to save it or not.
  • With a .GTM file, File/Open and File/New are identical.
  • Double-clicking a file in the File Exploring is identical to File/Open.
  • Select a file from the Most Recently Used (MRU) list under the File menu is identical to using File/Open.
  • The File/Open command does provide a fancier dialog for creating a session. A history of the last ten source files is listed along with their descriptions (from inside the .GTM or .GTX files). You can select a file in the history list to create a session from or you can browse for a source file. (The Browse button will also take the path typed into the prompt as a starting point to speed along browsing)

The difference between File/New and File/Open are really pretty trivial; however, know the differences between the two command can help you get even more use out of your data, especially if you are wanting to share a .GTX file amongst multiple users. I have noticed from observing users that the File/New command is sometimes never used. There is nothing wrong with this approach, but the File/Open’s history list is sometimes very useful if you deal with several different datasets.

Creating sessions from .GTW and .GTG files is also supported, but this will be another blog posting at a later date.

See Also: Sessions 102

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