Hi.?ÿ Newbie here.?ÿ I've been chipping away at learning about the different datums and reference systems used in the US for survey work (I'm looking at going into surveying).?ÿ I was learning about how NAD83 has changed over the years and how it has been realigned at different epochs.?ÿ I'm pretty confused about what a realignment actually means and how it relates to Epoch.?ÿ I think that NAD83 moves along with the NA continent, so as the years go by, monuments keep their coordinates, unlike with WGS84 and ITRF systems.?ÿ However, periodically NAD83 has gone through different iterations and updates, usually relating to some year epoch.?ÿ My question is; when one of these realignments happen, are the coordinates of the system itself all reset to what they were back in epoch 1984 when the coordinates looked more like a global system like WGS and come back into alignment with those systems.?ÿ Or, does NAD83, continue to remain aligned with the continent and remain out of alignment with WGS 84??ÿ I imagine the coordinate system either statying with the continent or constantly being reset to the planet as a whole.?ÿ What other major changes effect how the coordinates change at each realignment? I hope I'm making my questions clear.?ÿ Thank you for any discussion!?ÿ
The realizations of NAD83 remain mostly tied to the tectonic plate and the difference between them and ITRF changes continuously over time.?ÿ WGS84 has been updated several times and each update is a snapshot of ITRF at some GPS week. The unfortunate US folks on and near the Pacific plate have bigger problems.
In the 1990's there were the HARN realizations of NAD83 which sought to make each state as self-consistent as possible, but had discrepancies at the borders. Later realizations were national.
Each realization holds selected points for which there is good data, and so the difference from one to the next is not exactly constant over the country.
NGS has some good reading material on datums and realizations, as the prepare for a major change in the next few years.
My question is; when one of these realignments happen, are the coordinates of the system itself all reset to what they were back in epoch 1984 when the coordinates looked more like a global system like WGS and come back into alignment with those systems.?ÿ Or, does NAD83, continue to remain aligned with the continent and remain out of alignment with WGS 84??ÿ I imagine the coordinate system either statying with the continent or constantly being reset to the planet as a whole.?ÿ What other major changes effect how the coordinates change at each realignment?
Remember that several of the NAD83 realizations were done in order to include (many) additional observations, which meant that some of those positions were more precisely aligned with the updated reference frame than before, so they might have moved in a way that doesn't necessarily describe the movement of the tectonic plate.
Also remember that the original NAD83/86 reference frame was nowhere near as precise as the next iteration of HARN and FBN datums; as GPS measurements began to be included, you wouldn't see those updated positions "move with" or be "reset" to the original datum, because the original positions just weren't as precise to begin with.
In theory the CORS held fixed in each adjustment will be moving with the North American plate, but again the incorporation of new/additional observations certainly can affect "where they are" in the new realizations as well.
Finally, remember that local movement can and does affect the motion of coordinates, especially on the West Coast and Alaska...
The NGS has a ton of publications on the differences between reference frames and how they were arrived at.
Thank you for the discussion, that is a huge help.?ÿ I do in fact, live in the SF Bay Area and can attest that our monuments don't lineup well when checked by GNSS against NGS datasheets!?ÿ I've had some trouble there in a city called Alameda which is out in the water, past the San Andreas fault.?ÿ Hence one of the reasons I had this question in mind.?ÿ This is my first inquiry on the forum and I'd like to thank you for taking the time.?ÿ Cheers!
