Jim McLefresh submitted a new blog post
A vertical datum is technically, a surface of zero elevation to which heights of various points are referred in order that those heights be in a consistent system.
Howdy,
Enjoyed your article. Good summary and tabulation. In some ways, the hardest part of modern surveying is keeping track of what each number means and how it is related to others. Looking at CO-OPS data and NGS data for points in both databases is revealing.
I had hoped there would be some discussion about the linkage of the geoid to these datums. After all, the new vertical datum will be based on GNSS-derived ellipsoid heights and a high-resolution geoid model.
The relationship: ellipsoid height (h) - orthometric height (H) - geoid-ellipsoid separation (N) should sum to zero (h-H-N should equal zero (0). While some want to rely on the legacy network of monumented points, benchmarks are increasingly endangered by man-made and natural forces and have not been revisited to verify their accuracy, in most cases, in decades. There are fundamental problems with NAVD88 including a cross-continent tilt.
Remember the NGS definition of the geoid: "The equipotential surface of the Earth's gravity field which best fits, in a least squares sense, global mean sea level." The equipotential part is especially important because it captures the essential task we seek to achieve with a system of heights "the determination of fluid flow."
The NGS site has a number of presentations on their web site discussing and illustrating deficiencies in the current and past vertical networks.
Annoyingly,
DMM
