Switch

Description

Switch.c is a utility designed to convert a Time-Angle (TA) lookup table (raster) into a Depth-Across (DA) lookup table. This conversion is crucial in ray tracing applications, where initial ray tracing results are often in TA space (time of flight vs. outgoing beam angle), but practical applications require results in DA space (across-track distance vs. depth).

The tool takes pre-computed lookup tables for beam angle (.beam_angle), time of flight (.time_lag), and an angle-time depth lookup table (e.g., from Ray.c), and uses them to interpolate depth values into a new grid representing across-track distance and depth.

Usage 

Switch -locate <ATFilename> -pro <svp_handle>

Arguments

| Option | Description | |—|—| | -locate <ATFilename> | Required. Path to the Angle-Time (AT) depth lookup table file (e.g., from Ray.c). This file provides the depth for given angle and time. | | -pro <svp_handle> | Required. Base name for the velocity profile. The tool expects: * <svp_handle>.beam_angle: Beam angle lookup table. * <svp_handle>.time_lag: Time of flight lookup table. * It will create <svp_handle>.Switched as output. | | -v | Enable verbose output. | |

How It Works

  1. Initialization: Parses command-line arguments to get input/output filenames.
  2. File Opening and Header Reading:
    • Opens the Angle-Time depth lookup table file (ATFilename) and reads its JHC_header (AThead).
    • Opens the beam angle (.beam_angle) and time of flight (.time_lag) lookup table files and reads their respective JHC_headers (Ahead, Thead).
    • Creates the output file (.Switched) and copies Thead to Ohead (output header).
  3. Memory Allocation: Allocates memory for AngTim (Angle-Time depth data), Ang (beam angle data), Tim (time of flight data), and Out (output Depth-Across data).
  4. Data Reading: Reads the contents of ATFilename, .beam_angle, and .time_lag into AngTim, Ang, and Tim arrays, respectively.
  5. Conversion Loop: Iterates through each cell (i, j) of the output grid (Depth-Across space, dimensions from Ohead.dx, Ohead.dy):
    • Retrieves beam_angle from Ang[j * Ahead.dx + i] and time_of_flight from Tim[j * Thead.dx + i].
    • If both are valid, it uses interp_JHCarray_value() (an external function for 2D array interpolation) to find the corresponding depth value from the AngTim lookup table, given the beam_angle and time_of_flight.
    • Stores this depth value in the Out array at Out[j * Ohead.dx + i].
    • If either beam_angle or time_of_flight is invalid, the output depth is set to 0.0.
  6. Output: Writes the Out array to the .Switched file.
  7. Cleanup: Frees allocated memory and closes all files.

Output Files (JHC .r4 Array Format)

  • <svp_handle>.Switched: A 2D floating-point array (raster) representing depth values in Depth-Across space. Each cell contains the depth corresponding to an across-track distance and depth (derived from time of flight and beam angle). The output file contains a JHC_header followed by the raw float data.

Dependencies

  • support.h: For general utility functions.
  • array.h: For JHC_header structure and interp_JHCarray_value function.

Notes

The conversion from Time-Angle to Depth-Across space is a critical step in using ray tracing results for multibeam sonar processing. The output Depth-Across lookup table is then used to quickly and efficiently correct raw sonar measurements for sound velocity profile effects. This tool helps to create the necessary intermediate products for accurate bathymetric mapping.