When marking targets visible on sidescan data from the Humminbird/Lowrance sonars, it is important to keep in mind that the boat heading (and normally the transducer heading) is not necessarily the same as the "Course Made Good" (CMG) based on GPS positions.

If the boat is pointing into the wind or strong current in order to stay on track, and the marked targets are far away from the track, this error may be large. The image below illustrates this:

Angle Errors

  • The boat (and the transducer) is pointing to starboard by 20 degrees relative to CMG.
  • The Brown image is calculated based on the assumption that the transducer is aligned with CMG.
  • The Blue images are based on a "pointing angle" of 20 degrees (i.e. you point the vessel 20 degrees into the wind/current in order to stay on track of the survey line)

In essence: When marking a target, keep in mind the heading of the boat relative to CMG and remember that the position of the targets may need to be adjusted for this heading error. If making the marks on the sonar during the survey, it is important to also make a mark at the track-line, since that is the "point of rotation" for this correction. The extent of the search area can then be increased to account for uncertainties in the transducer-angle.

If the boat is drifting while recording, the difference between CMG and the heading of the boat may be very large, and even completely flipped 180 degrees (...drifting backwards). In that case, if you targets are marked at a certain range, the marks may even be interpreted as being on the WRONG side, since there is no available information about the actual heading of the transducer. (i.e. everything is based on the CMG from the GPS)

The SonarTRX images can be corrected for a constant heading differences for a given batch of image-tiles. This is done from the Mosaic creation dialog.