What measurement must be used when assessing the angle of onset of HGN?

The assessment of the angle of onset in Horizontal Gaze Nystagmus (HGN) testing is expressed in degrees. This method involves measuring the angle at which a person's eye begins to exhibit nystagmus as it moves away from the center towards the side. The measurement in degrees allows for a standardized understanding of the degree of impairment observed during the field sobriety test, aligning with the scientific model used in HGN testing.

When conducting this assessment, it is essential for officers to systematically record the angle in degrees to maintain consistent and accurate results. Other units of measurement, such as radians, yards, or feet, are not applicable in this context and do not provide a relevant framework for analyzing the onset of nystagmus in the eye. Degrees are specifically suited for describing the angles of motion in a way that reflects the impairment due to potential alcohol consumption, making it the correct choice.