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This Content Originally Published by a land surveyor to Land Surveyors United Network

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Very good question. The ellipsoid  is a mathematical model of the earth, that's it, nothing physical about.

NAD83 SPC use a GRS80 Ellipsoid, I am pretty sure the difference is exactly what you have mentioned, and the EPOCH or date that the adjustment was made.

HARN and HPGN is an adjustment made by the NGS utilizing GPS to improve the accuracy of NAD83 coordinate system.

The ellipsoid is just a mathematical tool to hang geometric coordinates on it hasn't changed much since 1866. It is akin to a paper map, you can measure a 300 mile road trip with confidence, but a 3000 mile airplane trip is going to need a globe to accurately measure. In the same way the various adjustments like NAD 27, NAD 83   (HARN) and NAD 83/2011 reflect refinements in the geoid (gravity) model, more precise measurements from global GPS and the understanding that the tectonic plates are moving the land beneath us constantly at different speeds. Eventually NAD 83 will be semiretired, but the ellipsoid will continue to be a constant.

A datum has a lot of as-built aspects to it, various biases and mistakes are forced into all the coordinates over time, as you by necessity build up the network by tying into older and imperfect control.

Eventually NGS gets annoyed by all the inaccuracy and shakes the carpet, getting better coordinates for the fundamental control and recomputing/adjusting all the vectors thereto. The result is overall pretty similar to before, but in a project area you may notice biases (everything changed by 1 cm to the north, 2 cm east, 3 cm up) So for the most accurate uses of NAD83, folks like to track which adjustment of NAD83 was used.

The NGS tool NADCON was originally built to transform between NAD27 and NAD83, but now incorporates the GEOCON grids for gridded transformations between flavors of NAD83. See NEW! BETA https://beta.ngs.noaa.gov/gtkweb/ and click "choose an input / output datum" to see supported adjustments.

The EPOCHs are a slightly different issue,GPS is accurate enough now to see the deformation of the continent over time. So WHERE also requires a WHEN; NGS tries to de-deform all these decades of measurement so they provide a single coherent snapshot, circa 2010.0 with the latest adjustment. You can use their HTDP tool to move those coordinates back and forth to any epoch. </joe>

So it sounds like when a transformation is done to an ellipsoid, you are actually transforming the adjustments along with it, like transforming the datum "realization" ?

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