CH Field Survey & Data Collection 2

Question 1

What is the primary purpose of a vessel's motion reference unit (MRU) during hydrographic surveys?

Accepted Answer

To compensate for pitch, roll, and heave effects on sonar data

Answer

An MRU measures vessel dynamic motion (pitch, roll, heave) so these movements can be removed from depth measurements to obtain accurate seafloor depths.

Question 2

During a side-scan sonar survey, what does 'towfish altitude' refer to?

Accepted Answer

The height of the towfish above the seafloor

Answer

Towfish altitude is the height of the side-scan sonar towfish above the seafloor, typically optimized at 10–15% of the total range to maximize coverage and resolution.

Question 3

What is 'layback' in the context of towed sonar operations?

Accepted Answer

The horizontal offset between the vessel's GPS antenna and the towfish position

Answer

Layback is the horizontal distance between the vessel's reference point (usually the GPS antenna) and the towfish, which must be accounted for in positioning.

Question 4

When establishing a horizontal control network for a hydrographic survey, which datum is most commonly used in the United States?

Accepted Answer

NAD 83

Answer

NAD 83 (North American Datum of 1983) is the standard horizontal reference datum used for hydrographic surveys conducted in the United States.

Question 5

What is the recommended minimum overlap between adjacent multibeam swaths in a hydrographic survey?

Accepted Answer

10%

Answer

A minimum of 10% swath overlap between adjacent lines is generally recommended to ensure full bottom coverage and detect any data gaps.

(CH) Certified Hydrographer Practice Test

(CH) Certified Hydrographer Practice Test

CH Field Survey & Data Collection 2