OPUS Solution vs Older Benchmark

Hey all,

So I'm relatively new to all this. I am not a licensed surveyor. I'm a geologist with an engineering firm. And over the past couple years, I've been using a TOPCON GR3 when we need relatively accurate locations for our own internal projects. For example, we sometimes need to track the locations of groundwater treatment injection points over an active construction site. The survey nails and whiskers marking these locations get buried. Sometimes even monitoring wells get buried. So we've used the GR3 to establish our own relative field control points and then re-locate features over time. And it's been beyond awesome to see how well this equipment works-- awesome enough that I've been trying to read up more on GPS and geodesy and find ways to experiment with this equipment in the field when possible.?ÿ

I've recently been on projects near various benchmarks associated with bridges and such. So I've had a lot of fun running static sessions on these benchmarks. I use OPUS for post-processing. And the resulting coordinates have generally been within an inch of benchmark coordinates. Just this last week, however, I ran two separate 4+ hour static sessions on a benchmark with coordinates in NAD83 (1991.35). Our GR3 runs only in NAD83(2011). My understanding is that these coordinates can be transformed to NAD83(2011) with NGS NCAT using NAD83(HARN). And so I transformed the coordinates from NAD83(1991.35) to NAD83(2011) to compare against my OPUS solutions. My OPUS solution x,y are each about 5 inches off of the transformed benchmark coordinates. (The two OPUS solution x,y coordinates are about half an inch apart. Overall RMS is 0.016 meters for both, with peak x,y errors of about 0.008 to 0.013 meters. In the shorter session, 93% of observations were used. In the longer session, 88% of the observations were used.) I'm just curious: Is it common to see a horizontal difference of about 5 inches between an OPUS solution and coordinates associated with an older benchmark in an older datum realization??ÿ

I'm also curious about using older datum realizations with our GR3. The GR3 currently handles only NAD83(2011). In the case above, I wanted to reference our locations to this benchmark. So I transformed the benchmark NAD83(1991.35) coordinates to NAD83(2011) and started the base station with those transformed coordinates-- I ran the static sessions just because I was curious. My understanding is that there are probably small non-linear differences between different datum realizations, so if I wanted to run the GR3 with the NAD83(1991.35) coordinates, I should install that datum realization on our handheld FG-200, even though my understanding is that over short distances it almost certainly wouldn't matter. On the flip side, my understanding is that, technically, if I wanted to convert these coordinates (the ones acquired relative to the transformed benchmark) back to NAD83(1991.35), I should transform each separately due to potential non-linear differences, rather than calculate the offset at the benchmark and then apply it to each location-- even though, again, over a short distance, it almost certainly wouldn't matter? Does that make sense?

I've had a lot of fun reading these forums. And reading up generally on geodesy. It's fascinating stuff. And I apologize if some of what I wrote above is confusing or just embarrassingly wrong. I'm not a surveyor. But I seriously appreciate the opportunity here to pick so many brains and hopefully learn a bit more.?ÿ

Thanks!

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