Vertical Datum

Elevation measurements must be in reference to a vertical (sea level) datum. In the US most are tied to NGVD 29 or NAVD 88 (the numbers represent the years 1929 and 1988). Most elevations measured and reported today use NAVD 88. Unfortunately, the USGS still uses NGVD 29 as the base datum for Carnation and Duvall gages.

In the Snoqualmie Valley NGVD 29 elevations are 3.576 ft "lower" than NAVD 88 elevations according to this conversion tool. In other words, add 3.576 ft to NGVD 29 elevations to convert them to NAVD 88 elevations.

We use NAVD 88 for the SVPA gages.