layout: default title: applyOffset parent: OMG Tools nav_order: 77 —

applyOffset

Description

applyOffset is a utility designed to adjust the geographic positions of profiles (pings) within OMG-HDCS merged files. Its primary use is to correct for the offset between the navigation antenna and the multibeam transducer, particularly if this correction was not applied during the initial data merging process.

The tool can apply a fixed offset based on user-defined ahead and right values relative to the vessel’s heading, or it can apply corrections from an external offset file (-offsetfile) combined with an ElltoCD correction.

Usage 

applyOffset <datafile(s)> [OPTIONS]

Arguments

Option Description  
<datafile(s)> Required. One or more paths to OMG-HDCS merged files to be processed. These files are modified in place.  
-ahead <val> Specifies the offset distance (in meters) of the antenna ahead of the transducer. 0.0
-right <val> Specifies the offset distance (in meters) of the antenna to the right of the transducer. 0.0
-offsetfile <filename> Specifies an external file containing custom offsets. This option is used in conjunction with -elltocd.  
-elltocd <val> Used with -offsetfile. This value is added to a oheight value read from the offset file and applied as a depth correction. 0.0
-v Enable verbose output.  

How It Works

  1. File Processing: The tool iterates through each provided merged file.
  2. Offset File Handling (-offsetfile): If -offsetfile is specified:
    • It expects a single input merged file.
    • It reads ocount, otime, oheading, oroll, opitch, oheave, and oheight from the offsetfile.
    • For each ping, it modifies the observedDepth of all beams by subtracting profile.vesselHeave and (oheight + ElltoCD) * 1000.0. This means it applies an external heave-like correction to the depths.
  3. Fixed Offset Handling (Default): If -offsetfile is not specified:
    • For each profile (ping) in the merged file:
      • It reads the profile header.
      • Determines the transducer’s current geographic location (lat, lon) using locate_transducer.
      • Apply Antenna Offset: If ahead or right offsets are provided, it calculates lat_offset and lon_offset based on the vessel’s heading (rad_heading). These calculated offsets are then added to the current lat and lon.
      • Update Profile Header: The vesselLatOffset and vesselLongOffset fields in the profile header are updated with the new (offset-corrected) latitude and longitude.
      • Update Beam Offsets: For each beam within the profile, it recalculates beams[i].latOffset and beams[i].longOffset based on the new vessel heading and the beam’s acrossTrack distance. This effectively recalculates the individual beam geographic offsets from the transducer position.
  4. In-Place Modification: The merged file is modified in place, updating either beam depths (with -offsetfile) or profile/beam geographic offsets (with ahead/right).

Output Files

The input merged files are modified in-place.

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

The tool modifies the merged file in place, so it’s advisable to back up files before processing. The ahead and right offsets are crucial for precise georeferencing of multibeam data, especially when the navigation antenna is not directly over the sonar transducer.

How It Works

  1. File Processing: The tool iterates through each provided merged file.
  2. Offset File Handling (-offsetfile): If -offsetfile is specified:
    • It expects a single input merged file.
    • It reads ocount, otime, oheading, oroll, opitch, oheave, and oheight from the offsetfile.
    • For each ping, it modifies the observedDepth of all beams by subtracting profile.vesselHeave and (oheight + ElltoCD) * 1000.0. This means it applies an external heave-like correction to the depths.
  3. Fixed Offset Handling (Default): If -offsetfile is not specified:
    • For each profile (ping) in the merged file:
      • It reads the profile header.
      • Determines the transducer’s current geographic location (lat, lon) using locate_transducer.
      • Apply Antenna Offset: If ahead or right offsets are provided, it calculates lat_offset and lon_offset based on the vessel’s heading (rad_heading). These calculated offsets are then added to the current lat and lon.
      • Update Profile Header: The vesselLatOffset and vesselLongOffset fields in the profile header are updated with the new (offset-corrected) latitude and longitude.
      • Update Beam Offsets: For each beam within the profile, it recalculates beams[i].latOffset and beams[i].longOffset based on the new vessel heading and the beam’s acrossTrack distance. This effectively recalculates the individual beam geographic offsets from the transducer position.
  4. In-Place Modification: The merged file is modified in place, updating either beam depths (with -offsetfile) or profile/beam geographic offsets (with ahead/right).