All, I am trying to get a measure of minimum depth in our pen - a new, deeper keel is coming and the obvious question about low tide clearances springs to mind - no puns intended.
So I ran the instruments across time while in the pen, looked at solstice low tide depth, and the average depth under the keel is 0.45 m across the lowest tide 4 minutes; so good so far.  However the variation across the same time is from 0.85m to 0.17m, not so good if true.  A standard deviation of approx 0.3m, so the whole range from utterly safe to firmly parked.
The recording rate is 1 Hz.  Is at Is this variation normal in a depth sounder, or have I something else to think about?  The bottom would be classic slightly silted protected pen bottom.