TerraMatch allows for the correction of data that has position errors that vary over time, for example mobile data that was gathered in downtown areas with GPS drop-out. This is accomplished by computing tie line fluctuations that compute a correction curve along the trajectory to account for positional uncertainty. Each discrete position along a trajectory can have its own accuracy estimate assigned, and these values can be used to weigh the magnitude of the corrections to be made to that position. The worse the accuracy estimate the more correction can be applied.
The ‘Max rate’ value in the Find Tie Line Fluctuations tool defines how smooth this correction curve will be. It is a multiple of the trajectories accuracy that defines how much change can be applied per 100 meters.
So the Trajectory position accuracy estimates can have a great influence on this type of calibration process.
How to determine your position accuracy estimates:
There are several ways to determine what accuracy estimates are assigned to your trajectories.
1. Estimates for the accuracy for trajectory positions can be set in the TerraMatch Settings under ‘Default trajectory accuracy.’ This shows the default accuracy values for Easting, Northing, Elevation, Heading, Roll, and Pitch. The default values are those recommended for mobile data collected along rural roads. If there are no accuracy estimates available from post-processing software, then the values in these settings will be used in the calibration process. Also, these values will apply to all of the trajectories in your project.
2. To see if accuracy values are assigned to your individual trajectory positions, simply select ‘view positions’ in the Manage Trajectories dialog. This will produce a list showing each of the trajectories’ positions and their attributes, including Xy, Z, Heading, and Roll/Pitch accuracy values. If there are no accuracy values assigned to these positions (as in the example below) then the matching algorithms will use the Default Trajectory Accuracy settings discussed previously. However, it is possible to directly edit each position’s accuracy estimate using the “Set accuracy’ button.
3. Once you import trajectories that do not contain accuracy estimate values, then there is an option to import a separate file that contains these values. Select the ‘Import accuracy files’ option in the Manage Trajectories dialog, navigate to your accuracy files and import. These will populate the accuracy estimate columns in the View trajectory positions table. Some trajectory files contain the accuracy estimates and does not require any additional steps.
4. If there is no accuracy file to use, then there is a way to manually set the accuracy values on a per trajectory basis. This can be accomplished by using the ‘Set trajectory accuracy’ option in the Manage Trajectories dialog.
Note: Older methods of calibration (i.e. surface matching) used flight line quality to prioritize position corrections. The trajectory accuracy is used in fluctuations to fulfill this same role. For example, in an aerial project, the ‘Set trajectory accuracy’ option can be used to assign overall accuracy to each individual trajectory. A lower accuracy value can be applied to the trajectories that you wish to have greater adjustments.