We had a couple jobs in 2021 and 2022 where we used RTK to collect coordinates on our control points using state plane coordinate system HTDP NAD83(2011) California Zone 3, NAVD88, Epoch 2010.0000
Recently we had a new job in the area and we took new observations on the old control points, using the same coordinate system. Our RTK base for this work was on a point with known coordinates (we'd used it recently for the base and had a post-processed coordinate value).
We shot 7 control points, and the new shots have a consistent shift to the southeast of ~10" from the old coordinates.
I can't explain why this shift occurred. Does anybody have a similar experience, and if so what was the issue?
What are you using HTDP for? If transforming between reference frames, recognize that it will not give good results if transforming NAD83 realizations (2011, HARN, etc). If transforming for time, this is either unacceptable (due to bug fixes?) or your ground has moved 10". Here's a revision history showing changes in late '21 and '22.
You need to know how the project was referenced to state plane in 2021. What was known in 2021? If a RTN was used has it had another realization? If ground control was used remeasure that. If you don't know you don't know. I've seen ground control reported to be on the same coordinate system and datum not be related by that much too many times to count. Too many possibilities to explain the difference to write about.
BTW why are you measuring in inches??
This utility might help you analyze the movement in your project area over time. It is California centric (mostly) and much more refined and updated than HTDP.
http://sopac-adj.ucsd.edu/scip/
Zone 3 includes the SF Bay Area, which is moving 5-6cm (more or less) northwesterly relative to NAD83 per year.
