We interrupt this program to share some totally useless information to anyone but the geodesy minded geeks.?ÿ
Our statewide RTN has recently updated the ellipsoid height each reference station broadcasts. The height adjustment amount from the 2013 adjustment varies from station to station and does not match the CORS ellipsoidal height exactly.?ÿ It uses IGS stations outside the state as a basis for the adjustment which as I understand it is the basis of CORS.?ÿ The question we had is what is the best tie to NAVD88, active height computed using the broadcast height and the most recent Geoid or passive height based on 1935 leveling? The answer at least in the area we checked is either one.?ÿ
We did 6 hour occupations on two passive 1st order NGS marks 5 miles SW and 12.5 miles NW of the project. These are two marks set on the first CGS level run through the state about 90 years ago.?ÿ We held the 2019 adjustment broadcast RTN ellipsoidal heights on 3 area RTN stations and let the passive mark height float. On the mark 12. 5 miles NW the observed height computed using Geoid 12B was 0.006 ft higher than published.?ÿ?ÿOn the mark 5 miles SW the observed height computed using Geoid 12B was 0.017 ft lower than published.?ÿ
I don't known about the rest of you but I'm going to call that close enough to say that either the 90 year old level run or the RTN using Geoid 12B is a correct tie to NAVD88 in the area.?ÿ
I mentioned that the 2019 adjustment does not match the CORS ellipsoidal height exactly.?ÿ To test the difference it makes we processed the two passive mark observations using OPUS. The OPUS reference station intervals are more than the RTN intervals but one station in both solutions are common.?ÿ Apparently the powers that be at NGS don't see the wisdom in accepting RTN stations at closer intervals.?ÿ The OPUS solution at the mark 12.5 miles away was 0.03 ft. lower than published. The OPUS solution on the mark 5 miles away was 0.06 ft. higher than published. So while the comparison to published is fine it is not as good as the RTN results using the 2019 adjustment. To me this is an indication why it may not be a good idea to constrain RTN reference station heights to CORS stations heights even if as in this example both solutions are good enough for most applications.?ÿ?ÿ
Now back to regular programming.?ÿ
