mergeAltitude

Description

mergeAltitude is a utility designed to merge external ASCII altitude data (e.g., from an aerial survey or kinematic GPS) into OMG-HDCS merged files, specifically linking altitude measurements to the geographic positions of individual multibeam soundings.

The tool interpolates altitude values from the ASCII altitude file to match the timestamp of each beam’s bottom strike in the merged file and outputs these merged geographic and altitude data to an ASCII file.

Usage 

mergeAltitude -in <merged_file(s)> -alt <altitude_file.ascii(s)> -out <ASCII_filename> [-v]

Arguments

Option Description  
-in <merged_file(s)> Required. One or more paths to input OMG-HDCS merged files.  
-alt <altitude_file.ascii(s)> Required. One or more paths to ASCII altitude files. Each file is expected to contain two columns: timestamp (double) and altitude (double).  
-out <ASCII_filename> Required. The path for the output ASCII file, which will contain merged geographic (lat/lon) and altitude data.  
-v Enable verbose output.  

How It Works

  1. Initialization: Initializes global variables, including verbose_flag, file handles, and counters.
  2. Argument Parsing: Parses command-line arguments to get input merged filenames, altitude filenames, and the output filename.
  3. Altitude Data Reading:
    • Iterates through each specified ASCII altitude file (-alt).
    • Reads timestamp and altitude values from each line and stores them into a 2D array alt[2][...].
    • Calculates the mean altitude (mean_alt) from all altitude data.
  4. Merged File Processing: Loops through each input merged file (-in):
    • Opens the merged file for reading (r+).
    • Reads the summary header.
    • Profile and Beam Iteration: Loops through each profile (ping) in the merged file, and then through each beam within that profile:
      • Reads the current profile header and raw beams.
      • If beams[k].observedDepth is valid:
        • Timestamp Calculation: Calculates a precise timestamp (pasted) for the current beam’s bottom strike using profile.laser_timestampRef and beams[k].timestampOffset.
        • Geographic Positioning: Locates the beam’s geographic position (lat, lon) using locate_beam.
        • Altitude Interpolation/Lookup: Iterates through the loaded altitude data (alt array) to find the two altitude records that bracket pasted.
        • Output to ASCII: If bracketing altitude records are found, it prints the lat, lon, and interpolated/closest altitude (alt[1][n]) to the output ASCII file.
  5. Cleanup: Closes all open files.

Output Files

  • <ASCII_filename>: An ASCII file containing latitude, longitude, and altitude for each beam with a valid altitude lookup.

Dependencies

  • OMG_HDCS_jversion.h: For OMG-HDCS data structures.
  • support.h: For general utility functions and error handling.
  • j_proj.h: For coordinate projection functions.

Notes

This tool is used to integrate external altitude data with multibeam soundings, providing a comprehensive vertical reference. This is critical for applications that require precise heights, such as mapping features above or near the water surface using laser bathymetry. The interpolation ensures that altitude values are accurately synchronized with each individual sounding.