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Fledermaus 7

Technical Note: Grid convergence computations for sonar files


Version 7.5.2 of the Fledermaus Suite, released February 25th, 2016 includes a fix in FMGT to compute the grid convergence for sonar files. This fix involves computing the convergence between the True North heading, stored in most sonar files, and Grid North heading, in FMGT's projected space.

What are the changes?

FMGT now correctly computes the convergence factor for each ping in a survey line and applies it to the reported heading from the survey line. This will change the True North heading, coming from the raw sonar file, into a projected heading in FMGT.

Who does this impact?

Problems due to convergence in previous versions would have appeared as a heading offset and a mis-match between a bathymetry reference grid (if it was corrected for convergence) and the backscatter mosaic produced in FMGT. If no bathymetry was used for comparison, a user may not notice the offset. The extent of a heading mis-match will depend on the latitude and longitude where the survey data was collected and how far away from the longitude origin the ping is.

Since most users use the auto-UTM projection selection for FMGT, the convergence error that would have been introduced is typically small, less than a few degrees with maximum values upwards of 3 degrees at high latitudes at the boundary between UTM zones. Users with unique projection needs, such as polar stereographic projections, would have been strongly affected. These users should consider evaluating the impact of the error prior to determining whether to reprocess their data.

ARA and Statistics layers will be less affected than Mosaic layers due to the coarser spatial resolution of the ARA and Statistics layers.

Users with existing data may wish to establish how much they are affected prior to re-processing large data sets.  This can be done by creating a single-line mosaic with a sample line in their existing FMGT installations, then opening in the new version, clearing the "Adjust" processing flags, re-processing and creating a second mosaic.

For users with existing projects, to benefit from this correction, you will need to clear the "Adjust" processing flags and re-build your Mosaic, ARA and/or Statistics layers.