I just THOUGHT I had a good grasp of Map Projections. Theory is only half of it, though, and at some point the rubber must be introduced to the road.
Your data:
1. OPUS Solutions on a few Static Observations.
2. RTK Points using radio from a nearby Water District Base.
3. RTK Points using your own base.
4. Some Conventional Total Station shots under those trees over there.
5. Published HARN Point in the area.
6. Published Benchmarks a bit outside of the area.
OK, so we roll up our sleeves and fire up AutoCAD Civil 3D. First thing we do is select a Map Projection. Let's say we want things in State Plane. Lotsa choices here:
OK, the OPUS Solutions were "REF FRAME: NAD_83(2011)(EPOCH:2010.0000)", so we'll choose "NSRS 2011 Arizona State Planes, West Zone, International Foot" from the list. Apples to Apples, I say.
But wait. In my Data Collector I chose "State Plane". Which one? I don't remember being able to pick a Reference Frame. Does it matter? Better so some checking. I pull up Corpscon6 and "convert" between NAD83(86) and HPGN/HARN:
Oops! The coordinates differ by half a foot! The choice of Reference Frames in Corpscon is limited, and I don't see a "2011" anywhere, so who knows what that difference might be. At this point serious doubt enters my life, and I gnaw my knuckles in the still watches of the night.
It looks like I might have more to learn. I Google "Terrestrial Reference Frames" and find out, Oh Horrors!, that there are 4 different "Official" Reference Systems, each with several Flavors(Frame Tag/Epoch Time):
1. ITRF
2. IGS
3. WGS84
4. NAD83
I guess I need to find out:
1. What Reference System/Frame Tag is being used by the CORS bases that OPUS used?
2. What Reference System/Frame Tag is being used by the Water District Base?
3. Ditto for my base.
4. What Reference System/Frame Tag is my data collector using when I pick "State Plane"?
5. The Conventional Shots are the least of my worries, but are there any subtlties involved I should worry about?
6. What Frame Tag is the Published HARN Point using?
Big Deal Dave
> 1. What Reference System/Frame Tag is being used by the CORS bases that OPUS used?
> 2. What Reference System/Frame Tag is being used by the Water District Base?
> 3. Ditto for my base.
> 4. What Reference System/Frame Tag is my data collector using when I pick "State Plane"?
> 5. The Conventional Shots are the least of my worries, but are there any subtlties involved I should worry about?
> 6. What Frame Tag is the Published HARN Point using?
Dave -
OPUS reports results in two reference frames:
- NAD83(2011)
- IGS08
Since you want to work in State Plane coordinates, you want to use the NAD83(2011) values. At the risk of oversimplifying, IGS, ITRF, and WGS84 are all basically the same thing, and they differ from NAD83 by about 2 meters.
The CORS stations used by OPUS are in NAD83(2011), as is stated on the OPUS report. To find out what realization of NAD83 your Water District is using you would need to check with them. The published HARN point has probably been adjusted to NAD83(2011) but you would need to pull up the NGS datasheet and verify that.
As far as your RTK base... the definition of NAD83 doesn't change; there is just a variation in the actual coordinate values for the points based on the realization of NAD83 that they're derived from. So the short answer is you're in the NAD83 that you punch in coordinates for.
Different data collectors handle the geodetic part in different ways... with Trimble, if you pick the US State Plane 83 coordinate system, they aren't applying any transformations or shifts to the coordinates - what you punch in is what you get. Some people argue that this is not geodetically correct; I'll leave that discussion to the geodesists.
Actually the CORS used by OPUS are in IGS08 and all the processing is done in that ITRF08 at the current epoch. At the end, the results are transformed to NAD83 (2011).
One must be careful with some software. I am not sure if it is still the case, but in recent Trimble (TBC, Access) if you picked NAD83 (2011) it would apply a transformation. So, if you had NAD83 (2011) it would be the same as having ITRF08 and then applying the transformation twice.
Regardless of what reference frame they process in, OPUS results are published in NAD83(2011) as well as IGS08.
When Trimble released the US State Plane 83 (2011) coordinate system in the firmware, it created a lot of confusion - people saw it and said "2011, that's the current datum, that's what I need to be using". The biggest problem - at least that I encountered - was that people chose it when using the VRS network, which was already broadcasting NAD83(2011) corrections. So as you stated, the software was transforming something that was already in the right place.
The release of that coordinate system coincided (more or less) with the availability of the RTX service, which (as do WAAS, EGNOS, Omnistar) transmits global corrections. I've found that using that coordinate system for WAAS tightens it up considerably.
To eliminate some of the confusion, they renamed it "State Plane 1983 (ITRF To NAD83)" in more current releases of Access.
Glad to hear they changed the designation. I messed around a bit with the real time RTX (demo), but did not think to try the transformation.
Thanks, Lee. I'm still chewing on all this.
Dave
John:
"So, if you had NAD83 (2011) it would be the same as having ITRF08 and then applying the transformation twice."
So how would you know if it is applying the transformation twice?
Dave
You wouldn't, that is the problem. Except you would miss any NAD83 (2011) points you check into by 1+ meters.
To me it was a big mistake the way they named that, as Lee said they have apparently changed it in later releases.
The problem was that it of course sounded logical to use the NAD83(2011) frame, but they had the "hidden transformation" built into it.
I don't see that as Trimble doing anything wrong. The problem extends to other manufacturers as well. It's a holdover of the (grotesquely erroneous) idea that geographic coordinates are all WGS-84. Rather than dumb down the (correct) name of a system why don't we recognize that not all providers base their systems on WSG-84?