This document describes HydrOffice QC Tools 4.9.9.

2.3.4.2. How Does It Work?

The grid is scanned, and any empty grid nodes (“holes”) surrounded by populated nodes are identified. These are flagged as holidays based on 2024 NOAA HSSD.

A holiday is defined as a box of unpopulated nodes.

The holiday size is calculated in number of nodes based on the minimum allowable resolution and the grid resolution, per this formula:

floor((minimum allowable resolution X 3) / grid resolution)

Only in the case of a variable resolution input, all the tiles are re-sampled to create a single resolution grid (selecting the highest resolution among all the grid tiles).

The minimum allowable resolution is determined based on the Tables of Minimum Required Grid Resolution in the 2024 NOAA HSSD. The user defines the Feature Detection requirement in the Input Tab. The median value of all the node depths surrounding the perimeter of the holiday is determined to then evaluate against the NOAA Specification.

Note

The output of Holiday Finder is a sounding, with a value of “1” for certain holidays, and “2” for possible holidays.

In the example in (Fig. 2.21), General 1 requirements identify the hole of 12 grid nodes, because there it contains an instance of 3x3 unpopulated grid nodes. The holes with 7 and 2 nodes do not.

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Fig. 2.21 Example for General 1.

A candidate hole is flagged with a “1” (if certain) or a “2” (possible holiday).

Note that the default Upper holiday area limit (as a multiple of the rounded-up minimum holiday size) is set to 1000, meaning that an unpopulated part of the grid will be flagged only if smaller than 1000 times the rounded-up minimum holiday size (that is, the number is rounded up using a ceiling function). This setting exists so the search can be refined at the user’s discretion.

For example:

  • If desired to search for only the smallest of holidays, the Upper holiday area limit might be set at 100.

  • If desired to flag all unpopulated parts of the grid, regardless of their size, the Upper limit would be set at unlimited.

  • Settings in between are used at the discretion of the user, to identify holidays, while also preventing undue clutter in the output.