In the early years of GPS, the DOT would do surveying control networks for projects using crews with specially equipped trucks. Cables, power connections, metal framed backpacks carrying Ashtech units. It was a production. I was able to watch them reducing the field data from .dat files to a final product and that was a mysterious process.?ÿ
We would then use the monuments which were aluminum caps imbedded in concrete placed about 1/4 mile apart. From there we occupied the monuments and did our thing using T2's with distance meters and Topcon Total Stations to gather data and set corners.?ÿ
Fast forward almost 30 years and I get a call from a surveyor. He says he is occupying some of the monuments which are recorded on a ROS at the county. He is missing them by +-2'. These are metric points as the job was early 1990's in the middle of DOT and metric conversion projects. So conversion is not an issue.
The ROS states that the control is NAD83 xxxZONE.
I explained that the coordinates?ÿ for the monuments positions are correct, think of the date.
There was only one EPOCH available at the time, NAD83(86). It wasn't SOP to declare an EPOCH cause at that time there was only one. It took some time after 1993 for the NAD83(93) to become available so it wasn't used until some years after 1993. Of course back then we were only using conventional and didn't care if a future EPOCH would shift everything. ?ÿ
So,,,,,,,occupy a local HARN point, apply the NAD83(86) numbers, run a session on the ROS monuments and there you are, no doubt they will check very close. I'm not going to do it but he can if he wants.?ÿ
Like so many things dealing with surveying you have to know the history.?ÿ
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2 feet is about the shift from 83(86) to 83(91) here in PDX as well.
You nailed it.?ÿ You must consider the time frame on everything all the way down to when a certain item could have been set for a monument.?ÿ Finding a PK nail that was allegedly set/found prior to the existence of PK nails is one example. It might be some other kind of nail but not a PK.?ÿ I recall Kent McMillimeter's study of rebar and the markings as an example of determining the time frame before which something could not have been placed.
It's kind of like saying 'World War 1' in 1925. Nobody did it, except Grandpa Walton of course...
In the original post in this thread, I think we're talking about different "realizations" rather than different "epochs." Early NAD 83 realizations didn't have explicitly stated epochs, unlike the more recent realizations. For example, think of NAD 83(2011) epoch 2010.00. The "(2011)" specifies the realization, and the "epoch 2010.00" specifies the epoch. For much of the US, the epoch is of small importance compared to the realization, i.e. those parts of the country that have small NAD 83 velocities.
@thebionicman maybe he was a prophet, the recipe effects different people in different ways.... on a side note - I just finished watching a series on Amazon called World War I- in the last episode at the signing of the armistice- Foch reportedly said, “this isn’t the end of the war””. It’s the beginning of a 20 year armistice” - He was off by 65 days.
For much of the US, the epoch is of small importance compared to the realization, i.e. those parts of the country that have small NAD 83 velocities.
That is true. However, California had to produce a NAD83(2011) Epoch 2017.50 because epoch 2010.00 positions no longer fit true of date measurements, especially in the San Francisco Bay area. Because of the delay in the delivery of the NSRS Modernization Project, we may have to produce one more epoch of NAD83 to bridge the gap between 2017.50 and 2024/2025.
Always nice to find an excuse to use NCAT. Calculates the change and the uncertainty and demonstrates the idea, but you have to have faith in black boxes.
QW0149 DESIGNATION - G 483 QW0149 PID - QW0149 QW0149 STATE/COUNTY- MT/YELLOWSTONE QW0149 COUNTRY - US QW0149 USGS QUAD - BILLINGS WEST (2017)
I don't consider NCAT to be a "black box" in the normal sense of the term.?ÿ You can easily extract horizontal and/or vertical stations from the NGS database that have the same datum/realization you're dealing with and compare their changes with those predicted by NCAT, NADCON/VERTCON for any area of the country.?ÿ When I think of the term Black Box in relation to datum transformations I think of the hundreds that are in virtually every GNSS receiver and GIS software that originally came from the Dept. of Defense WGS 84 manual for datums around the world where no background data has been provided, just an dX, dY, dZ.
