I have faced a problem with tidal observe data and the time series predicted data. If I made a time series from my observe data then there is no negative sign but, If I made prediction from tide time series then there found both positive and negative value.  Here I would like to attach screen shorts of both two time series.

I think, the problem is my observed data is excluded the mean sea level value. My query, is it necessary to add mean sea level data with my observed data or is there any other process to made tide time series.
If it is possible to make a proper suggestion, then it will be very helpful for me.

Rupak Loodh.