Tuesday, February 5, 2013

Final Word on NAD 83

The National Geodetic Survey has recently completed its final realization of NAD 83 until they move us to a new datum in 2022. This latest realization, or adjustment to be technically accurate, is identified as NAD 83/2011 Epoch 2010.00 for those operating in the Continental United States, Caribbean and immediate surrounding areas. This adjustment yields some extremely small shifts in positions compared to its predecessors, but also adds a couple very valuable and critical benefits.

  • The first of these benefits, and in my eyes the most important, is that finally NGS CORS stations and all the physical monuments we use to “check into” are finally synchronized together, instead of being on two different adjustments, and never brought together to a common reference frame with each other.
  • The next critical element, which few people realize, is that the new NAD 83/2011 Epoch 2010.00 can only be used with new Geoid 12A geoid model, due to technical reasons beyond the scope of this article. Older geoid models such as 2009 or earlier do not share a common system of integration and will yield issues in both software and physical measurements on the ground.

  • The third major benefit of the new adjustment is it coincides and is integrated into a revised NGS Datasheet, which includes much more detailed information, not only adjusted coordinates, but also ways to explore what other points are related geodetically to the point on the datasheet, as well as pictures, accuracy predictions and ways to more accurately assess the usefulness of the point for your needs.

The new datasheet does not account for tectonic motion, commonly referred to as velocities, and it is important to realize that the epoch date included in the name of any coordinate system tells you that those coordinates for the point you are standing over is where it was on that date, not necessarily where it is now. A few millimeters a year over a couple decades adds up quick. The need to use a technically competent and up to date professional services consultant is becoming more important every day.