Transforming NAD 83...
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Transforming NAD 83 to present day GPS

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500+ posts Member


Interesting...especially since by the time the HARN realization(s) was/were out in the 90s, NAD83 and WGS84 were already substantially different, by several feet. I guess the thought process was that since the recreational-grade receivers usually don't get you much better than 1-3 meters (and considerably worse before SA was turned off), any actual transformation wouldn't really be noticeable. Makes practical sense, but still odd.

Posted : September 19, 2021 5:43 pm
5,000+ posts Member


There is a way to get the best number. 

Two high quality tribrachs, tripod legs, and two high quality receivers. 

Make sure both tribrachs are adjusted as tightly as possible. 

Set one receiver over the point, collect 24 hours of data and save the file, measuring the HI to the proper point on the receiver in feet and meters. 

Repeat with the other tribrach and receiver. 

Wait a week or so for the best data to become available. 

Using a high end processor, process the point with data provided by surrounding CORS stations. 

Then adjust the two sets of numbers using a least squares program. 

Then you may have a snapshot of that point in that time frame to a few millimeters,,,,,,,,,,,,maybe. 

Be sure and find the latest possible updated coordinates for each CORS station.

You may also send the files to OPUS but it's my understanding that OPUS uses datasheet numbers for their processing (I don't care about that at all, but you seem obsessed with it).

That's how it's done, Good luck. 

Understand that all the adjustment programs are really good, but they are not based on updated observations for the point. 

Posted : September 20, 2021 6:39 am
200+ posts Member

@mightymoe I've encountered folks who stick to this or that belief, and I realize it's typically a waste of time to try to change their mind. But the member in question is enthusiastic about making observations of NGS marks and seeing what the results really are. I think this is a case of a person who only heard the myths and has never been exposed to rigorously developed information. The technical education I know of for this member is the pilot licensing process.

Topic starter Posted : September 20, 2021 6:48 am

5,000+ posts Member


I will tell you to let go of finding that perfect number. Use the 2012 number it will work for what you need. 

I got into a silly argument over the top of the Washington Monument with a "senior engineer" some years ago. He insisted that the unit on the monument was accurately measuring the monuments position from satellites. I tried to explain that it needed to be tied into the NGS system of points to get an accurate number. He actually got offended that I would even suggest such a thing.

It opened my eyes to how much disinformation is out there and how time-consuming it can be to correct.  


Posted : September 20, 2021 8:30 am
Jaccen and dmyhill liked
1,000+ posts Member

Personally, I encourage observing NGS marks with whatever equipment you have. Visit the same mark on different days and see how the recreational GPS readings vary. In fact, it's instructive to see how the readings vary over 10 minutes or so. Just place the receiver on the mark and watch the coordinates bounce around for 10 minutes. These receivers were not designed for precise work.

The math teacher in me requires an analytical problem. Consider three NGS marks arranged like this:


1) Use the State Plane coordinates on the data sheets to calculate the lengths of the sides of the triangle and the Law of Cosines to calculate the angles.

2) Use the NGS Geodetic Tool Kit to calculate the same quantities. Compare the results with those in (1). This will illustrate the difference between coordinates on a plane and their corresponding coordinates on an ellipsoid.

3) Repeat Geodetic Tool Kit exercise, but round all coordinates to coordinates to one tenth of a second. This will illustrate one of the problems that drones and ground-based surveyors have to contend with.

Tell him if he doesn't do these exercises, he's getting a lump of coal in his Christmas stocking. Then tell him that he gets an additional Almond Joy in his stocking if he also completes this one:

4) Go to the FCC database and find the coordinates for three communications towers near him. Repeat Exercise (2) for the triangle these points make. Estimate the error in distances caused by insufficient significant digits in the FCC coordinates.




Posted : September 20, 2021 8:37 am
1,000+ posts Member


A .pdf file outlining what the ITRF "is" and where it comes from (and where it is going).

Posted : September 20, 2021 9:56 am

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