calcRayDiff

Description

calcRayDiff is a specialized utility designed to compare two sound velocity profiles (SVPs) by calculating the depth differences predicted by ray tracing through each SVP. It takes two SVP files (typically from a BOT MVP100/200/XXX profiler), builds Time-Angle (TA) lookup tables for each, differences their predicted depths, and then reprojects these differences into Across-track/Depth (XZ) space.

The primary purpose of this tool is to provide a graphical output (currently a PGM image) that quantifies the absolute scale of depth anomalies at every depth-across track position, which can be encountered by a shipboard sonar. This helps users understand the impact of using one SVP over another on multibeam bathymetry.

Usage 

calcRayDiff -pro1 <file1> -pro2 <file2> -outimage <image_file> [-tdcr_dep <val>] [-maxangle <val>] [-v]

Arguments

Option Description  
-pro1 <file1> Required. Path to the first SVP file (MVP format).  
-pro2 <file2> Required. Path to the second SVP file (MVP format).  
-outimage <image_file> Required. Output PGM (Portable Graymap) image file, showing depth differences.  
-tdcr_dep <val> Transducer depth (meters). 2.5
-maxangle <val> Maximum beam angle to trace (degrees). 75.0
-v Enable verbose output.  

How It Works

  1. Initialization: Parses command-line arguments to get input/output filenames and ray tracing parameters (transducer_depth, maxangle).
  2. SVP Reading (read_MVP()):
    • Opens the two input SVP files (-pro1, -pro2).
    • Calls read_MVP() to parse the MVP-formatted SVP files. This function reads depth-velocity pairs (ZVpair) and stores them in raw_zv1 and raw_zv2 arrays. It skips initial header lines from the MVP format.
    • Closes the SVP files.
  3. Water Column Initialization (jWC_init()):
    • Initializes two watercolumn structures (wc1, wc2) using jWC_init() (an external function). These structures will hold the processed SVPs and ray trace results.
    • Determines max_depth and max_range based on the input SVPs and maxangle.
  4. SVP Interpolation (jWC_interp_zv()):
    • Interprets the raw SVP data within wc1 and wc2 into a finely layered model (e.g., constant velocity layers). This prepares the water column for ray tracing.
  5. Ray Tracing (jWC_trace()):
    • Performs ray tracing through wc1 and wc2 to generate internal Time-Angle (TA) lookup tables for depth.
  6. Time Series Resampling (jWC_resample_timeseries()):
    • Resamples the ray trace results into a uniform time-angle grid for easier comparison.
  7. Across-track/Depth (XZ) Interpolation (jWC_interp_XZ()):
    • Interpolates the ray trace results into an XZ grid (Across-track/Depth) for wc1.
  8. Difference Calculation (AT_difference()):
    • Creates a new farray (AT_diff) to store the differences in depth between wc1.AT_depth and wc2.AT_depth in Time-Angle space.
  9. Reprojection to XZ Space (AT_to_XZ_reproject()):
    • Creates a new farray (XZ_diff) to store the depth differences in Across-track/Depth space.
    • Uses the XZ-interpolated time and angle grids from wc1 (wc1.XZ_time, wc1.XZ_angle) to map the AT_diff values into the XZ_diff grid.
  10. Output to PGM (write_out_PGM()):
    • Writes the XZ_diff array as a PGM (Portable Graymap) image, where pixel intensity represents the magnitude of the depth difference.
  11. Cleanup: Frees all allocated farray and watercolumn memory.

Output Files

  • <image_file>: A PGM (Portable Graymap) image file quantifying depth anomalies.

Helper Functions

  • read_MVP(FILE *grazfile, ZVpair *zvp, int *novals, int vflag): Reads depth-velocity pairs from an MVP-formatted SVP file.
  • getc_line(FILE *file, char *string, int printit): Reads a line from a file.
  • AT_difference(farray *at1z, farray *at2z, farray *atdiffz): Calculates the difference between two farray structures.
  • AT_to_XZ_reproject(farray *xzdiff, farray *atdiff, farray *xztime, farray *xzangle): Reprojects data from Time-Angle space to Across-track/Depth space.
  • write_out_PGM(farray *fa): Writes an farray to a PGM image file.

Dependencies

  • support.h: For general utility functions.
  • j_watercolumn.h: For watercolumn structure and ray tracing functions (jWC_init, jWC_interp_zv, jWC_trace, jWC_resample_timeseries, jWC_interp_XZ, jWC_free).
  • j_farray.h: For farray structure (a generic 2D floating-point array structure with associated metadata).

Notes

calcRayDiff is a specialized tool for quality control and validation of sound velocity profiles. It provides a powerful visual aid for understanding the impact of SVP uncertainty or choice on multibeam depth measurements. The output PGM image directly highlights areas where significant depth anomalies are predicted due to differences in SVPs, which can guide further investigation or processing strategies.