This document describes HydrOffice QC Tools 4.9.9.

2.3.3.3. Common Flier Finder Techniques

This section provides guidance on which algorithms to use with Flier Finder.

2.3.3.3.1. Standard Use

2.3.3.3.2. Advanced Use

  • Enable these Checks on an as-needed basis, or to generate more flags:

    • Laplacian Operator [#1].

    • Isolated Nodes [#5]. This check will target grid nodes detached from the main body of the grid, thus, if the surface is a depth-threshold single-resolution grid, there may be many false flags created at the fraying edges of the grid’s depth threshold - ignore these.

    • Noisy Margins [#7]. This check will target edge fliers. The more edges there are, the more flags potentially could be returned. Most notably, if it is a set line spacing survey, it will therefore have an abundance of edges.

  • If too many flags are produced:

    • Enable the Masking and Filters. Flags will not be placed within a certain number of nodes horizontally, or within a certain depth vertically, to an S-57 feature VALSOU. This will reduce the occurrences of false flags on real features that are tall and narrow on the seafloor.

    • See the following note about organizing the output in separate layers, so checks returning excessive flags can be switched “off” as needed.

  • To organize the output and better target your flier search, running the flier checks in separate groupings may be helpful. This is common management:

    • A single output layer contains the results of checks #2, #3, and #4.

    • Separate output layers contain the results of checks #1, #5, and #7 (three separate layers total).

2.3.3.3.3. Summary Guidance Table

A summary of the guidance is shown in Table 2.1 (also see the How Does It Work? section to understand how each check works).

Table 2.1 Guidance on Current Algorithms

Criteria

#1 Lap

#2 Gau

#3 Adj

#4 Edg

#5 Iso

#7 Nsy

Flier search height can be auto-estimated (or forced)

X

X

X

Flier search height is determined by uncertainty

X

Flier search height is N/A

X

X

Prone to excessive flags

X

X

X

Standard use

X

X

X

Advanced use (use on an as-needed basis)

X

X

X

2.3.3.3.4. Tips and Tricks for Investigating Flags

  • Examine flags alongside various other layers for context. Consider using:

    • Grids, with vertical exaggeration increased.

    • High-density sounding selections, colored by depth.

    • Largest scale ENCs for reference, especially the ENC soundings.

    • The Final Feature File, filtered to only show new and updated features.

    • A critical soundings layer.

    • A side scan sonar contacts file.

  • When to look deeper into the underlying point clouds:

    • The flagged node(s) look anomalous compared to its neighbors.

    • The depth(s) are not in-line with the general trend of the seafloor topography.

    • Color the point clouds by line to help rule out anomalous data from real features.

  • While moving along in an investigation, it’s helpful to import candidate fliers into a separate layer so their location is not forgetten and you can return to them later.

  • To investigate edge fliers specifically (the output from flier check Noisy Margins [#7]), if excessive flags are returned:

    • If using CARIS, use the “Display Digits” option when perusing the grid edges and focus on those flags that have depths not in-line with the seafloor topography.

    • If available, enable a “Minimum Display Scale” so the grid is not bogged down by excessive digits when zooming out.

    • If a great abundance of edge flags are determined to be real fliers, consider using the Erode Raster tool to remove the outermost edges of grid nodes altogether.

  • Use 3D visualization. The human eye, scanning bathymetry in 3D, is an excellent judge of what is real and what is not - better than any algorithm we have.