I finally found my last problem with the interpolation. There were two. But last one was the most vexing. It was my calculation of fp seconds from datetime.timedelta. I had put this off in its only routine and had written: dt.seconds + dt.microseconds * exp(-6). Obvious when you see it: exp is exponent of base e not base 10. The correct expression is: dt.seconds + dt.microseconds * 10**(-6).

I really like the Python matplotlib plotting routines and interface for viewing, As you can see, I was able to pinpoint my problems thanks to the ease of this interface and its ability to create images.

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I went ahead and hacked my tidePlot.py which will do comparisons of time based data (using the timedate class). Using it to visually analyze my interpolation (added as an option to tideconvert), I see that it still needs work. The slope isn't right, and it doesn't catch all gaps in the data (right side of image).

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