Thank you!!! I didn’t realize that the rinex header setting (rtkpost > options > positions: Base Station) was giving RTKPOST the CORS coordinates in NAD83, I thought it was referencing the xyz location. Changing this setting to Lat/Lon/Height and inputting the converted coordinates (using the same HTDP tool I linked above) for the base station did the trick! Now my coordinates are plotting within a cm of the marker, which is well within the uncertainty that comes from using a less-than-survey-grade tripod.
For others who might stumble upon this post looking for answers to a similar problem, here’s a brief outline:
-I collected observations for ~25 minutes over a known location with an emlid Reach RS system.
-emlid outputs location data in reference to WGS83 datum.
-The known location (PSU1b) is reported in NAD83.
-The CORS data I got for post processing has, in the header, a lat/long/h which is in reference to NAD83.
-In post processing if you allow RTKPOST to get the base’s location (the cors station in this case) from the rinex header, it references the NAD83 location.
-This means you’re post processing data which references WGS84 against data which references NAD83. Resulting error will be on the order of meters.
Solution: Use Lat/Lon/Height setting instead of rinex headers if you need to perform an datum transform on the reference station location. Use a tool like HTDP - Horizontal Time-Dependent Positioning to perform the transformation. This will also adjust the reference station coordinates for plate velocities, which is important if you’re hoping for cm or better accuracy.