This article describes procedures used to adjust a typical GPS network with GeoLab. A sample GPS network is used to show how your IOB files should normally be organized.

Important: You should read the Getting Started with GeoLab article before reading this one.

Importing Your Measurements
After the field work has been completed, the first step in the adjustment process is to get your data into GeoLab's "IOB" format. This step is usually done by using GeoLab's File/Import menu command. GeoLab supports many different file formats, and you simply have to select the correct format in the "Import Foreign Format File" dialog as shown in the following screen shot:

The "Import Plugins" section lists all the installed import filters. In the above picture, the Ashtech O-File filter is selected. Once you have selected the correct filter, click the "Import Text File(s)" button, select the files your instrument produced, and click the "Open" button. GeoLab will convert your coordinates and measurements into a new IOB file. Note that for Trimble's SSF files, you must first load those files into TGO (Trimble Geomatics Office) and then export the data into Trimble's "Data Exchange Format" which GeoLab can import directly.

Organizing Your IOB File
We will use the example GPS network's IOB file to help with describing how you should organize your IOB file. Note that we will not use a geoid model in this particular adjustment, but you can see how that is done in the Getting Started with GeoLab article.

The following general IOB file structure can be used for any type of adjustment you wish to perform in GeoLab (the sections used are in the same order as they appear in our actual IOB file, and the line numbers for each section are given in the following table):

Note that we have renamed the IOB file so that your browser knows how to display the text file. If you want to try this adjustment in GeoLab, just save the IOB file as the text file "gpsnet.iob".

Note that you don't have to, and you normally shouldn't, edit the lines of the IOB file manually. You should always use GeoLab's Record Editor to add and edit records. Please see the Getting Started with GeoLab article for information about using the Record Editor. The Minimal Constraint Adjustment Once you have your IOB file set up as described above, you are ready to start adjusting the network. The first adjustment you do should always be a minimal constraint adjustment. This allows you to analyze the quality of your measurements without any distortions that may be caused by fixing all of your control stations. For our network, we fix one control station for this adjustment. The IOB file we provide is set up for the over-constrained adjustment, with two control stations fixed.

For the minimal constraint adjustment, the following lines were used in the "Control Stations" section:

For information on fixing coordinates in GeoLab, please see the Getting Started with GeoLab article. Note that we fixed only the first control point in the records above.

The minimal constraint adjustment is where you will normally do most of your adjustment work. The normal procedure for performing this adjustment is as follows:

1. Perform the adjustment (see the Getting Started with GeoLab article for information about getting GeoLab to do the adjustment).
2. Analyze the standardized residuals (see below) and edit the measurements if necessary. If measurements are edited, do steps 1 and 2 again.
3. Once the standardized residuals are all acceptable (see below), use the estimated variance factor from the adjustment listing in a VSCA record (measurement variance scale) at the beginning of the Measurements section (see the IOB file for an example) and then run the minimal constraint adjustment one more time (this last adjustment is done to record the results of the minimal constraint adjustment).

The analysis of the standardized residuals is the most important quality control process in an adjustment. To discuss this further, we will use a portion of the listing file from our GPS adjustment as an example (for more information about listing files, please see the Getting Started with GeoLab article):

The standardized residuals are listed at the right of the first line for a measurement. For example, the GPS vector measurement listed has a DZCT standardized residual of -3.4639. Note that the "critical value" for the standardized residuals is given at the top of the listing page ("critical value = 2.799"). This is the theoretical upper limit for the magnitude of acceptable standardized residuals, and GeoLab will underline (with "^^^^...") any standardized residual greater in magnitude than this critical value (i.e. the measurement is "flagged" for possible removal). Normally there will be only a few such "bad" measurements.

Using the Adjustment Summary Dialog
When an adjustment is finished, GeoLab displays the "Network Processing Completed" dialog. When you click the OK (or Draw Network) button, GeoLab will then display the Adjustment Results Summary dialog as follows:

Note that the Standardized Residuals section (in bottom left of this dialog) shows that two standardized residuals are larger than 2.7. If you decide that the first observation should be removed from the adjustment, you can double-click the value in the list and GeoLab will move the text caret in the listing window to the location of the measurement's standardized residuals. You can then see which observation group the measurement is in, and easily find that group in your IOB file. To remove the measurement, click on the corresponding GRP record in the IOB file and press the F4 function key to comment it out. Then keep pressing the down arrow and F4 keys until all records in the observation group are commented out. Once this is done you can re-run the adjustment. You continue in this way until you have found and removed all "bad" measurements. Note that you should only remove one measurement group at a time (the one with the largest standardized residual) because the measurement with the largest residual may be actually causing other measurements to receive large residuals.

After you have removed (commented-out) all "bad" measurements by doing your minimal-constraint adjustments, you should enter the resulting variance factor in the VSCA record (see the IOB file for an example).

The Over-Constrained Adjustment
The final step in adjusting our GPS network is the over-constrained adjustment. We start with the IOB file from our minimal constraint adjustment, but now we will fix both control stations. The "Control coordinates" section was changed to the following for this adjustment:

The main difference you will normally see in the results of the over-constrained adjustment is the value of the estimated variance factor. Because of the additional control coordinate constraints, the variance factor will increase somewhat depending on the relative quality of the control and our measurements. Please have a look at our Weighted Stations article for a description of a method for analyzing the quality of the control. After ensuring that the standardized residuals are still acceptable in this adjustment, you should revise the VSCA record to reflect the new estimated variance factor (the value in this record should now be changed to the product of the minimal constraint variance factor and the variance factor from this over constrained adjustment).

Post-Adjustment Tasks
Once you have completed the over-constrained adjustment, you can use GeoLab's tools to produce other information from the adjustment. For example, you can use the "Network/List/Station Data..." menu command to produce a list of the adjusted coordinates from the adjustment.

This was done for this GPS network adjustment (to see instructions for using the "User-Defined Station Data Lister" dialog, just click the dialog's Help button), and we produced the following comma-delimited file of station coordinates:

This file is useful because it can be imported into many other programs.

Please let us know if you would like to see this article expanded in any way. Thanks.