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Jun 02 2011

The Vertical Side of Things

A vertical datum is defined by the measured gravity anomalies between ellipsoid and geoid. But it’s not quite that simple. Between understanding mean low water versus mean high water versus mean low low water versus mean sea level, this area can get very dangerous. On top of that you have other things to complicate the “where” of the vertical such as subsidence (in California’s Central Valley) and busted benchmarks. There is also the all too often reluctance of some agencies or cities or other entities to make the painful move to a new established datum, leaving a mess of confusion and poor assumptions about conversions. We once worked on a project looking at the feasibility of shifting a data set (over an area of a few hundred square miles!) from NGVD29 to NAVD88 and quickly found, with the help of the client, that it’s not as simple as opening up Corpscon and doing a simple Vertcon conversion.

 

As an engineer, I need to understand some of those dangers but it likely won’t be within my range of work to deal with them directly. In my previous post discussing the concepts of the geoid and ellipsoid, I make an attempt to walk through orthometric height versus ellipsoid heights and the difference, and how this comes into play depending on the type of GPS receiver you are using and what elevations you are actually seeing.

The 2 Most Prominent Datums

NGVD29: National Geodetic Vertical Datum of 1929

NAVD88: North American Vertical Datum of 1988

Everyone should be using NAVD88 by now, it’s a better and more accurate system built on more measurements and GPS. However, this is clearly not the case. We still have clients asking for projects in NGVD29 mostly because we’re tying into older data sets or plans that were originally in NGVD29. This is where problems start to arise because NGS control is in NAVD88 now with less and less reliable NGVD29 elevations. But, wait, you say, can’t we just use Vertcon for the conversion? I would say to proceed with caution and consult your surveyors. In my experience, I trust actual benchmarks for the conversion more than the program based on the general math of original NGVD29 benchmark data.

What is your experience with vertical datums? Have you run into problems going from NGVD29 to NAVD88 (or vice versa)?

Sources:

“Datums, Heights, and Geodesy” by Daniel Roman

“Vertical Datum Transformation” from NOAA

“Vertical Datum” from FEMA

Vertical Datums, Elevations, and Heights from USNA

2 comments

  1. Jim

    Can you tell me when California, or specifically Caltrans changed from NGVD29 to NAVD88? Thanks for your help.

    1. Anthony Alvarado

      I’m not sure. For most agencies, it should have been the early 90s. However, many groups stuck with NGVD29 into the 2000s. I know one group that didn’t fully make the switch to NAVD88 until even a few years ago.

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