Determination of the
Error on the Velocity Measurents by the ORE 7300 at 3-Mile Slough
( | )

One way of estimating the error on the ORE velocity measurements is to compare single measurements with averaged measurements. Single velocity measurements (vi) from a fifteen-minute period are combined to give average velocity measurements (vav). A single value of vi is probably the result of one period of about 30 seconds of sampling, which itself may use more than a single timing. For the moment, however, we assume each value to be a single, independent measurement.

The best value to compare with vi is a smooth interpolation of vav. Individual values of vav correspond in time to the center of a fifteen-minute interval, and so an interpolation must be done. We have tried linear interpolation, and also cubic interpolation. In the case of linear interpolation, we calculated the average (vav(i) + vav(i+1))/2. For cubic interpolation, we calculated (-vav(i-1)+9*vav(i)+9*vav(i+1)-vav(i+2))/16. Figure interp1 shows the offset of vi and vav values, and the agreement of interpolations of vav with vi.

In order to see if cubic interpolation was in fact necessary, we have compared the linear and cubic interpolations. Figure interp2 shows the difference between the two interpolation methods. The improvement with cubic interpolation is slight.

Figure interp2 also shows the difference between single-sample velocities and the interpolated values. There is no clear evidence for cycle-jumping.