The speed of a vessel may cause it to ride lower (squat) or higher (lift) in the water than usual. For some vessels this effect is large enough to introduce significant errors into the soundings if it is not compensated for.
The Squat Model component (formerly called Dynamic Draft) lets you enter a table of speed-squat value pairs in the right side of its Attributes window. During the Georeference Bathymetry process, HIPS and SIPS gets the instantaneous vessel speed from the navigation system. It then interpolates an appropriate squat value for that speed from the table of squat-speed pairs. Finally, the Georeference Bathymetry process uses that squat value to correct the observed depth and obtain the corrected depth with the following formula: Depth = Observed Depth – Waterline + Squat
The Squat Model component has the following attributes in the Attributes window:
• Date: The year and Julian day of the time stamp for the squat table (see Component Time Stamps).
• Time: The hour and minute of the time stamp for the squat table (see Component Time Stamps).
• Apply: Defines whether to apply the squat table during Georeference Bathymetry.
• Comments: A text field for your use.
The table in the right side of the Attributes window contains the squat-speed pairs for the currently selected time stamp:
• Squat: The distance that the vessel squats at the associated speed.
When setting the Squat value, keep in mind the following:
• Squat is considered positive towards the seafloor. Therefore, a positive Squat indicates that the vessel is sitting lower in the water while a negative Squat indicates it is sitting higher in the water.
• All squat values should be relative to the same reference, but the actual reference is not important.
• The first speed-squat pair must correspond to a state where no squat correction is necessary.
• Speed: The vessel speed at which the associated squat takes effect.