On several of our stations we have more than one water level sensor; the "main" water level sensor (the one we trust most) is always the "primary water level" sensor.

Water elevations are measured at each station relative to a zero point (called the station datum) chosen such that none of the water level measurements will be below the zero point. Each station has its own station datum unrelated to the others, and we keep a series of deep-driven-rod bench marks to maintain the station's zero point over time and to determine the conversions to other vertical datums.

Most of our stations use an Aquatrak sensor to compute "primary water level". Each primary water level reading is computed by collecting 181 1-second water level measurements, computing the standard deviation of those measurements, discarding any "outlier" measurements more than three standard deviations from the mean, and reporting the mean of the remaining samples as the primary water level elevation for that period.