Assessment of acoustic Doppler current profiler heading errors on water velocity and discharge measurements

Boat-mounted acoustic Doppler current profilers (ADCPs) are commonly used to make both water velocity and discharge measurements. Most ADCPs measure the direction of travel of the water and boat using an internal compass. The effect of the heading on measurements made with an ADCP depends on the type of measurement and the boat velocity reference. When bottom track is used for the boat velocity reference a heading error will cause a rotational error in the measured water velocity, but the magnitude of the velocity and the computed discharge is unaffected. When an external boat velocity reference such as a global positioning system is used, the effect of heading errors can be substantial. Potential heading errors include errors in the compass reading caused by distortion in the earth magnetic field due to local objects on the boat and errors in determining the magnetic variation for a specific location. When using an external boat velocity reference heading errors will affect both measured water velocity and discharge. Analytical assessment of the heading errors shows the effect of these errors on velocity and discharge is directly proportional to the ratio of the boat speed to water speed. Therefore, maintaining a slow boat speed is imperative to accurately measuring water velocity and discharge when using an external boat velocity reference.