Flow of GNSS Session Data

We’ve seen some posts in the ArcGIS forums that contain questions about the flow of GNSS ‘session’ data from the field back to the office.  GNSS sessions are a combination of raw GNSS satellite measurements (required for post-processing) and “construction” data (required to rebuild feature geometries after post-processing).  Although the Trimble Positions Mobile extension and Trimble Positions ArcPad extension both collect GNSS session data, the storage and flow of this data is quite different between the two products.

In ArcPad, session data is collected and stored in an SSF file that sits adjacent to the AXF file in the project folder.  Each time you open that project and start data collection, session data is appended to this SSF file.  (Note: Please refer to documentation for best practices regarding SSF management.)  When you bring that data back to the office, you will generally copy the entire project folder onto your desktop computer and perform the check-in step from Trimble Positions Desktop add-in.  At this point, the Esri check-in logic will apply the data edits to the geodatabase and the Trimble Positions logic will extract feature and session data from the AXF + SSF and store it in the Trimble Positions Desktop database (the Jet or ODBC data source you setup using the Configuration tool).  From this point on in the workflow, Trimble Positions Desktop add-in will only use the data in the Desktop database and will no longer refer to the project files on disk.

In ArcGIS for Windows Mobile, sessions are managed and stored differently.  Each time you open the project and start data collection on a feature, a T0X file is created on the device to store the session data.  That session is kept open until you close the project or close the application (which automatically closes the project first).  Sessions can also be started and stopped manually from the GNSS Sessions task in the Task List.  When this session is ended, either automatically or manually, a feature record is written to the TrimbleSessions layer in the mobile cache (remember the GNSS Enabling step on the desktop?) and the binary T0X file is streamed in as an attachment to this layer.  In fact, if you navigate into the file structure of the mobile cache, you should see a folder for the TrimbleSessions attachments that will contain these T0X files, at least until you synchronize them.  At this point you would either copy the mobile cache over to the desktop and use the ‘Synchronize Mobile Cache’ geoprocessing tool to apply the edits to the geodatabase, or you would use the ‘Synchronize’ task on the mobile device to post your edits through a mobile service on ArcGIS for Server through to the geodatabase.  In addition to posting the edits to your feature layers, the synchronization has also posted the new data in TrimbleSessions to the geodatabase.  If you were to view the TrimbleSessions layer in ArcMap at this point, you’d see a polygon indicating the extent of the session and the binary T0X attachment.  When you use the ‘Check for New Sessions’ button in the Trimble Positions Desktop add-in, the session (and feature) data is extracted from TrimbleSessions and moved into the Trimble Positions Desktop database.  Assuming that process is successful, the ‘session’ features in the TrimbleSessions feature class are deleted.  In this way, TrimbleSessions is only used as transient storage for session data for the purpose of synchronization.

graphic of GNSS session data flow

Flow of GNSS session data in Trimble Positions

Careful readers will also note something else: if the ArcPad workflow does not use TrimbleSessions, is it even necessary to ‘GNSS Enable’ the geodatabase and MXD you intend to use in an ArcMap-only workflow?  The answer is no, it is not necessary.  One caveat here however…by GNSS enabling your MXD you are ensuring that a transformation is selected in the case where your data collection layers are exclusively non-WGS84.  The TrimbleSessions layer will always be in WGS84 and thus adding it to your MXD (through GNSS enabling) will prompt you to select a transformation.  So if you are using the ArcPad workflow and elect not to GNSS enable your MXD, you will need to use the ‘Datum Transformations’ functionality within the Administration window in order to define a transformation from WGS84 to your working coordinate system.

This entry was posted in Post Processing, Under the Hood, Workflow. Bookmark the permalink.