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Vertical datum conversion11/1/2022 ![]() ![]() Let’s say that you download data in the Astoria station datum and you want to convert the data in NAVD88. In recent years, NOAA has been updating its reference tables with excellent graphs showing how the different datums relate to each other (like the one below for Astoria, OR – -, where all elevations presented are related to the station datum, which is the Columbia River Datum or CRD). Vertical elevations can refer to NAVD88, the station datum, Mean High Water, Mean Low Water, Mean Range of Tide, and a variety of other datums. NOAA’s VERTCON program can be used to compute the conversion factors from one datum to the other. are the North American Vertical Datum of 1988 (NAVD88) and the National Geodetic Vertical Datum of 1929 (NGVD29). The two most common vertical datums used in the U.S. Let me step back a moment and I’ll explain why. And I recently had a “eureka” moment when I finally realized that the sentence “NAVD88 is higher than NGVD29 (for most of the locations we work on)” is actually wrong. ![]() I need pen and paper and a little numbering conversion to make sure that I’m doing things right when moving from one vertical datum to the next. However, I have to confess this: datum adjustments are always a little bit puzzling to me. RAS lets you adjust the vertical datum of individual cross sections (under “Geometric Data”, “Cross Section”, “Options”, “Adjust Elevations”) or portions of the model (“Geometric Data”, “Tools”, “Datum Adjustment”). If you have built enough HEC-RAS models, you’ve probably discovered that having data in different vertical datums is a bit of a pain. Written by Chris Goodell | February 10, 2014 ![]()
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