This document describes the HydrOffice QC Tools 2 application (2.7). For the project page, go here.

2.2.5. Grid QA

2.2.5.1. How To Use?

Computes grid statistics to ensure compliance to uncertainty and density requirements.

  • Select the Grid QA tab (Fig. 2.19) on the bottom of the QC Tools interface.
  • In Parameters, check the box to calculate the TVU QC layer regardless of whether or not the grid contains the layer already.
  • Since Grid QA v5, it is also possible to select the plots to create.
  • In Execution, click Grid QA v5.
fliers tab

Fig. 2.19 The Grid QA tab.

  • After computing, the output window opens automatically, and the results are shown (Fig. 2.20).
fliers tab

Fig. 2.20 The output message at the end of Grid QA v5 execution.

  • From the output window, view each plot to assess the grid compliance to uncertainty and density specifications.

Note

The Plot depth vs. density and Plot depth vs. TVU QC can potentially require a large amount of memory (i.e., when the input grid contains hundred of millions of nodes). As such, both plots are unflagged by default. You can flag them if you need their output. If having both plots selected triggers a memory error, you may try to flag these plots once at a time.



2.2.5.2. How Does It Work?

The Depth, Uncertainty, Density (if available), and a computed Total Vertical Uncertainty (TVU) QC layer (optional) are used to compute particular statistics shown as a series of plots.

The TVU QC is either given to the program in the grid input, or calculated on-the-fly. It is determined by a ratio of uncertainty to allowable error per NOAA and IHO specification:

TVU\, QC = Uncertainty / \sqrt{A^2 + (B * Depth)^2}

where A = 0.5, B = 0.013 for Order 1 (depths less than 100 m), and A = 1.0, B = 0.023 for Order 2 (depths greater than 100 m).

The following plots are the output of Grid QA:

  • The Depth layer is plotted as a distribution (plot entitled “Depth Distribution”).

  • The Density layer is plotted as a distribution (plot entitled “Object Detection Coverage”).

    • Percentages of nodes less than 5 soundings per node fall in the red shaded region of the plot and together must be less than 5% of all nodes in order to “pass”.

  • Density is plotted against the corresponding Depth of the node (plot entitled “Node Depth vs. Sounding Density”).

  • TVU QC is plotted as a distribution (plot entitled “Uncertainty Standards”).

    • Percentages of nodes with TVU QC greater than 1.0 (indicating that the allowable error has been exceeded) fall in the red shaded region of the plot, and together must be less than 5% of all nodes in order to “pass”.

  • TVU QC is plotted against the corresponding Depth of the node (plot entitled “Node Depth vs. TVU QC”).

  • Only for Variable Resolution grids, a histogram with the percentage of nodes at the prescribed resolution is created. This histogram can be used to evaluate whether “95% of all surface nodes [..] have a resolution equal to or smaller than the coarsest allowable resolution for the node depth” (NOAA HTD 2017-2 “Caris Variable Resolution Grids”).