
How NAD are your NAD83 Coordinates?
On a project that I’ve been working on for over a week as I sort out roughly a century’s worth of boundary confusions in a subdivision laid out in 1910 in Austin, Texas, one of the more persistent apprehensions of various commenters was that they were freaked out that anyone would seriously consider connecting a project that was less than 10km from a major CORS site such as TXAU (which I actually connected to by vectors from that CORS site) by actual vectors from that CORS site. There were various schemes suggested, but the consensus was that just downloading the observation files for the CORS site and solving the vectors to various stations on the network was RISKY BUSINESS. Evidently, someone knew someone why had sat through a seminar somewhere in which the salesman/presenter explained that this was a VERY BAD DEAL and was to be AVOIDED AT ALL COSTS.
I suggested that any surveyor who had such anxiety issues could easily deal with them by the simple expedient of solving the position of the CORS antenna via OPUS Static. To test the truth of this, I downloaded 6hr files logged at 30second epochs at the CORS site in question, TXAU, on the same three days when I’d surveyed vectors to various control points on the project, and submitted them to OPUS to get solutions for the three different days for the CORS antenna.
So, how did the NGS published coordinates for TXAU compare to the OPUS solutions? As one would suspect, the results were miles apart. In the image below, TXAU_NGS represents the coordinates of TXAU published by NGS and TXAU represents the least squares estimate of the mean of TXAU_343, TXAU_336, and TXAU_334. Note that TAXU missed the coordinates published by NGS by a HUGE amount that was :
0.006 ft. Horizontally and
0.009 ft. Vertically.However, considering that the uncertainty components in the mean off those three days were
0.009 ft. Horizontally (95% confidence) and
0.0100 ft. Vertically,someone might say that the OPUS positions were actually perfectly consistent with the published position of TXAU. The published position won, which was good to know when the survey identified some 2cm problems with some Network RTK results by others within the project limits.
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