The GPS Filters button opens a window that allows the user to evaluate the GGA and VTG data for each transect and change the filter settings. Filter settings are applied to all transects and cannot be set for individual transects.
Summary Table
The summary table allows the user to select the transect to be graphed (by clicking on the desired row) and provides information about the quality of the GPS data. Because setting the thresholds for the GPS filters is based on best assumptions rather than statistical analysis of the data, the table lists the information about GPS quality indicators rather than the number of invalid ensembles based on each indicator. This table has no editable cells. The columns in the table are defined as follows:
Filename—Filename of the transect.
Ensembles—Total number of ensembles in the transect.
Invalid GGA—Total number of ensembles with invalid GGA data.
Invalid VTG—Total number of ensembles with invalid VTG data.
Unfiltered Diff Quality—A list of all unique differential quality values for unfiltered data.
0—no position fix
1—autonomous
2—differential correction (DGPS)
4—real-time kinematic (RTK)
5—float RTK
Unfiltered Delta Alt. —The maximum difference in altitude for an individual ensemble and the mean altitude of all ensembles using unfiltered data.
Unfiltered Max HDOP—The maximum horizontal dilution of precision (HDOP) value for all ensembles using unfiltered data.
Unfiltered Delta HDOP—The difference between the maximum and mean HDOP for all ensembles using unfiltered data.
Unfiltered # Sat Chg—The number of times the number of satellites used in the position fix changed.
Discharge Prev. —Total discharge prior to most recent filter change.
Discharge Now—Total discharge after applying the most recent filter change. If no filters have been changed, the discharge now will equal the discharge previous.
Discharge % Change—The percent difference between the discharge previous and the discharge now due to the last filter change.
The GPS Characteristics panel below the table on the left provides a time series graph of the GPS characteristics provided in the table above the panel (fig. 20). Radio buttons below the graph are used to determine what data are graphed (figs. 20–23). Data determined to be invalid by the filters are marked with a red circle.
The Speed panel provides a time series graph of the boat speed based on GGA, VTG, and BT data. The data displayed on the graph are controlled by the three check boxes at the bottom of the panel. Processed data (used to compute discharge) are shown in the graph as a solid line and raw data as dashed lines.
The ShipTrack panel allows the GGA, VTG, and BT ship tracks to be compared. Invalid data are identified with a symbol, which is defined in the legend. The water vectors are only graphed on the navigation reference currently selected for discharge computation. The data shown in the graph are controlled by check boxes at the bottom of the panel.
The Nav Ref panel that shows the current navigation or boat velocity reference. If this reference is not GGA or VTG, then changes to these filters will not change the discharge.