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Transforming NAD 83 to present day GPS

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ashton
(@ashton)
200+ posts Member

I'm an aerospace education officer and involved in search and rescue with the Civil Air Patrol. I have one member who claims drones can achieve accuracy in the millimeters. Maybe the high-end setups with multiple-frequency receivers and real-time corrections against one or more reference stations could do a lot better than the typical hand-held hiking GNSS unit, if not millimeters, but certainly not the setup he's using.

We agreed that we would observe some NGS monuments to establish what our equipment is capable of. Equipment includes

  • drones (that I'm not very familiar with)
  • Garmin eTrex (yellow, so old it doesn't have a number)
  • Garmin eTrex 20 (personal equipment)
  • Nikon D7100 with Solmeta GPS attachment
  • various personal smart phones with cameras
  • Motorola APX 4000 radios with build-in GPS that can display lat/long on the handheld radio.

My question: I know that the NAD 83 NGS datasheet positions shouldn't be directly compared with GPS positions, and they can be transformed using NGS's HTDP software. One mark, OD0251, has the following horizontal position line:

OD0251* NAD 83(2011) POSITION- 43 29 32.16709(N) 072 58 22.77824(W) ADJUSTED

This is available directly as the "from" reference frame in HTDP. Assuming the GPS observations are made this year, what should the "to" reference frame be? The closest I could find was ITRF 2014.

I realize the correction will only be on the order of a meter, which is on the same order of magnitude as the uncertainty of position of the best passive marks. But I want to be able to show the program output to anyone who is claiming millimeters.

This, of course, is academic. One doesn't need accuracy to millimeters to find a crashed plane or a lost person.

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Topic starter Posted : September 19, 2021 8:29 am
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Mark Mayer
(@mark-mayer)
1,000+ posts Member

There isn't really a question here.....

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Posted : September 19, 2021 9:01 am
Rover83
(@rover83)
500+ posts Member
Posted by: @mark-mayer

There isn't really a question here...

I don't see a question either...

But if someone is looking to use the above referenced NGS mark for what (I think?) is a test of coordinate accuracy, they're going to have a hard time proving millimeters when the NGS datasheet clearly states that network and local accuracies of the published coordinates are well above a centimeter:

 OD0251  Accuracy and standard deviation values are given in cm.
 OD0251
 OD0251  Type/PID  Horiz  Ellip  Dist(km)  SD_N   SD_E   SD_h         CorrNE
 OD0251  -------------------------------------------------------------------
 OD0251  NETWORK   1.37   1.88             0.63   0.46   0.96    +0.17867556
 OD0251  -------------------------------------------------------------------
 OD0251  LOCAL (002 points):
 OD0251  OD1249    1.34   1.82     5.04    0.62   0.44   0.93    +0.21980468
 OD0251  OD1796    1.36   1.76    28.29    0.62   0.47   0.90    +0.07366315
 OD0251
 OD0251  MEDIAN    1.35   1.79    16.66
 OD0251  -------------------------------------------------------------------
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Posted : September 19, 2021 9:33 am
Jp7191 and Lurker liked

ashton
(@ashton)
200+ posts Member

@mark-mayer The question is, what reference frame should be the "to" reference frame in HTDP?

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Topic starter Posted : September 19, 2021 9:33 am
ashton
(@ashton)
200+ posts Member

@rover83 Agreed, if one had a GNSS receiver that under the right conditions was capable of millimeters, one couldn't prove it by making observations on this mark. But if someone claims that the eTrex teenager is capable of a few meters and the measurement is 80 meters off, the claim is disproven.

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Topic starter Posted : September 19, 2021 9:39 am
Mark Silver
(@mark-silver)
500+ posts Vendor

I wrote an American Surveyor article that was published in March 2015. Unfortunately it may be one of these evergreen articles that can be used over and over. Like in a week or two...

In any case it shows exactly how run HTDP. Here is a link to the archived PDF:

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Posted : September 19, 2021 9:54 am
ashton, Gary and mattsib79 liked

Rover83
(@rover83)
500+ posts Member

@ashton 

I was under the impression that this was about drones and millimeters rather than Garmins and tens of meters.

To answer your question about datums, you probably want to use ITRF2014 and current date for output datum & epoch. ITRF2014 and WGS84 (at least, equivalent realizations at same epoch) are effectively identical, and are what pretty much all recreational-grade receivers are outputting. There may be a couple that can do on-the-fly transformations but I can't think of any off the top of my head.

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Posted : September 19, 2021 10:41 am
Mark Mayer
(@mark-mayer)
1,000+ posts Member

@ashton 

I'm not at all certain about this.... but autonomous positioning from GPS will be WGS84, the latest realization of which is WGS84(G1672). So that would be your "from" for an autonomous position.  The "to" is the North American plate realization of NAD83(2011). Or maybe you are looking to do the reverse? Nevertheless, the shift between those two will be trivial compared the error in autonomous positioning.

image

But it seems to me that you are trying to fix stupid. That can't be done. When you wrestle with that pig you both get dirty. The difference is that the pig likes it. 

Noting that the NGS Control points have stated errors well above the millimeter level would seem, to me, to be sufficient proof that positioning to the millimeter is not a simple task.

 

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Posted : September 19, 2021 11:53 am
Bill93
(@bill93)
5,000+ posts Member
Posted by: @rover83

@ashton .

I don 't know about current recreational units, but a decade ago all Garmins offered a choice of NAD83 but it was a null transformation which was what the earliest documents on GPS said. So you were still really in WGS.

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Posted : September 19, 2021 1:50 pm

MathTeacher
(@mathteacher)
1,000+ posts Member

Interesting problem. First, about the equipment, those old, old Etrex receivers came in two versions; one with WAAS and one without. The one I have is without WAAS, so it consistently under-performs newer receivers. Also, the Garmin output is rounded to tenths of a second, so a perfectly correct internal reading will display with roughly +/- 5  feet accuracy due to rounding. You can get more decimal places by downloading the reading to something like ExpertGPS, but inaccuracies caused by the limitations of the receiver will not be eliminated.

For HTDP, I would convert NAD83 to WGS84. Your data sheet says that the NAD83 coordinates were determined in June, 2012, so I used 6-15-2012 as the input date. Then  used today's date, 9-19-2021 as the output date. That way, I have computed the value that I expect the receiver to show. The result is below:

image

Note that, rounded to tenths of a second, there is no change, so the Garmins won't detect any difference other than their own error.

Your member should do some math with a simple spherical earth to verify the length of a second of arc in feet or meters. That fifth decimal place in the NGS figures is about 0.3 mm which he can verify with minimal effort.

Even so, I would definitely go on with the experiment.

 

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Posted : September 19, 2021 3:04 pm
ashton
(@ashton)
200+ posts Member

@mathteacher I see that the GPS observations and adjustment was done June, 2012. But I also see that the epoch is 2010.00. I don't know whether the adjustment process made a best estimate of where the mark was on January 1, 2010, or June 27, 2012. Of course, the difference is probably very small, but it would look neater to use the right date.

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Topic starter Posted : September 19, 2021 3:28 pm
MathTeacher
(@mathteacher)
1,000+ posts Member

The point is continuously moving southeast and sinking with about 3mm per year displacement, so the start date does matter if one expects millimeter-level accuracy.

A little further research turned up this quote: "The epoch date indicates that the published coordinates represent the location of the control stations on January 1, 2010 — an important consideration in tectonically active areas (such as the western U.S.). In this way, the CORS coordinates (and thus the passive marks constrained to the CORS) are consistent across both space and time. Additional information on the MYCS1 realization of NAD 83 is available on the NGS CORS Coordinates web..."  from The National Adjustment of 2011 Project on the NGS web site.

So it looks like the proper start date is 1-1-2010. Additionally, you should heed Mark Mayer; the network accuracy is in centimeters, not millimeters, so even NGS would not state accuracy to the level that your member claims.

This post was modified 4 weeks ago by MathTeacher
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Posted : September 19, 2021 4:40 pm

Bill93
(@bill93)
5,000+ posts Member
Posted by: @mathteacher

The point is continuously moving southeast 

Points in the US are moving West or NW, so the same WGS coordinate numbers will refer to a point on the ground SE of what they previously did.

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Posted : September 19, 2021 5:02 pm
MightyMoe
(@mightymoe)
5,000+ posts Member

The only GPS actually measuring to millimeters globally will be fixed stations like CORS.

I've encountered many who believe outlandish accuracy about GPS. Its like chashing cats. I simply ignore it, that's my advice.

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Posted : September 19, 2021 5:30 pm
MathTeacher
(@mathteacher)
1,000+ posts Member

@bill93 

Sorta. If you look at the velocities on the HTDP output, North is negative, East is positive, and Up is negative. I interpret that to mean that the vector is Southeast and Down.

 

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Posted : September 19, 2021 5:37 pm

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