Processing and smoothing cruise tracks

Echoview is able to derive single source GPS and combined source GPS variables from NMEA data. Note that the data in position variables can contain a high degree of error. For example, these errors may originate from missing GPS fixes or inherent inaccuracies in the equipment, and can result in the creation of erratic cruise tracks. This in turn can negatively affect the quality of your data in Echoview. For example:

the geographic positions assigned to pings in echograms are derived from cruise tracks and will therefore inherit any inaccuracies present in the cruise track
surfaces, 3D regions, and other 3D objects that are created from variables linked to an erratic cruise track are generally of low quality

Echoview provides processing, smoothing and filtering to help you improve the quality of cruise tracks.

Processing

Echoview automatically processes GPS fixes to identify bad fixes that should not be used in the construction of cruise tracks. Echoview assigns good or bad status to each GPS fix based on the settings entered on the Data page of the Variable properties dialog box (see Processing GPS data).

Smoothing

If a cruise track appears erratic even after you have optimized the processing settings, you can apply a smoothing algorithm to improve it further. This is done by creating a virtual position variable (in the Dataflow window). The process consists of taking a raw position variable and applying a Kalman GPS smoothing filter operator to it. You can set the degree of smoothing on the Kalman Smoothing page of the Variable properties dialog box on the resulting variable. See also: Kalman filter.

Note: Processing and smoothing is only possible for position variables. Alongtrack display cruise tracks follow the cruise track of the platform with which that variable is associated (via its transducer).

Filtering GPS data

Modern sounders and sonars may record GPS fixes from a multiple sources. Numerous and/or variable quality GPS fixes from multiple sources may produce cruise tracks that are unnecessarily erratic. Erratic GPS fixes can result in unreasonable or even nonsensical apparent vessel speeds from fix to fix. Where GPS data is excessive or of mixed quality (high and low resolution), it is strongly advised that you filter the GPS fixes so that only the highest quality GPS fixes are used for a cruise track.

Echoview's automatic processing of GPS fixes applies a default set of filter conditions (see the Processing GPS data algorithm and Restore Default setting of the GPS filter operator).

To view the sources and number of GPS fixes use the Fixes page of the Variable Properties dialog box of the position GPS fixes variable.

Position data that contains multiple GPS sources - some of which may be in unexpected formats, can be filtered by Echoview. In the Dataflow window, apply a GPS Filter operator to a raw position variable. You can enter specific filter settings on the GPS filter page of the Variable properties dialog box on the resulting virtual variable.

Merging GPS data

Echoview offers you the ability to merge position data from multiple filesets with the Merge GPS operator to accompany processed and merged echogram data from multiple filesets.