layout: default title: findShoals parent: Grid Tools nav_order: 32 —

findShoals

Description

findShoals is a feature detection utility that scans a JHC-format 32-bit floating-point grid file (DTM/DEM) to identify all local maxima. In bathymetric terms, these local high points represent “shoals” or peaks on the seafloor.

The tool uses a 3x3 sliding window to examine each pixel. If a pixel’s value is strictly greater than all eight of its immediate neighbors, it is classified as a shoal. The geographic coordinates (latitude, longitude) and depth of each identified shoal are then written to an ASCII output file.

Usage 

findShoals <depthfile.r4> <output_asciifile>

Arguments

Argument Description Example
<depthfile.r4> Required. The input JHC-format 32-bit floating-point grid file to be analyzed. my_bathymetry.r4
<output_asciifile> Required. The name for the output ASCII file that will contain the list of detected shoals. shoal_list.txt

How It Works

  1. File Opening: Opens the input .r4 depth file and the output ASCII file.
  2. Header Reading: Reads the JHC_header from the input file to get grid dimensions and geographic information.
  3. Data Loading: Loads the entire input grid data into memory.
  4. Local Maxima Detection: Iterates through each pixel in the grid (excluding the outermost boundary):
    • For each pixel, it compares its value with the values of its eight immediate neighbors in a 3x3 window.
    • If the central pixel’s value is strictly greater than all eight of its neighbors, it is identified as a local maximum (a shoal).
    • The geographic coordinates (latitude, longitude) and depth of the shoal are calculated using the grid’s geographic information.
  5. Output: Writes the latitude, longitude, and depth of each detected shoal to the output ASCII file.

Output Files

  • <output_asciifile>: A three-column ASCII file (latitude longitude depth) listing all detected shoals.

Dependencies

  • array.h: For JHC_header structure and DTM data handling.
  • support.h: For general utility functions and error handling.
  • j_proj.h: For coordinate projection functions.

Notes

This tool is simple yet effective for identifying bathymetric high points that might be navigational hazards or features of geological interest. The definition of a “shoal” here is purely morphological (local maxima). Further analysis might be needed to determine significance. The depth values are inverted to be positive as is common in oceanographic conventions.