Datum Conversion

Yes, but I get it wrong every time I say it. Loyal is the guy on this one. There is a way to move from realization of NAD 83 to another realization, but HDTP is NOT it. 🙂

I'm sure Loyal will be along soon to enlighten us. He always does.

I thought it was HDTP too but at 350 pm i couldn't tell if I was missing something due to late afternoon survey brain or its just not the right utility.... lol

HDTP is for velocity changes in time, not static realizations. Loyal will smack my head if I got it wrong. 🙂

I'm not Loyal but, I do have an idea on how to handle this.

In my opinion the best way to do that would be to do some OPUS-RS occupations on several stations with NAD83/1992 coordinates. You will then have multiple stations with both NAD83/2002 coordinates and NAD83/1992 coordinates and you can then compute transformation parameters between the two datums. Then apply these transformation parameters to the station that has the single NAD83/2002 coordinate.

Peter Lazio

Peter

That method will work for just about anything if you keep it small enough, say a county or two. I thought there was a utility that would transform between the different realizations of NAD83.

Howdy,

The following was copied from the NGS web site. I changed the key statement to bold. To the best of my knowledge this is still current.

Page describing differences between NAD83(NSRS 2007) and NAD83(CORS 96)

NGS has adopted a realization of NAD 83 called NAD 83(NSRS2007) for the distribution of coordinates at ~70,000 passive geodetic control monuments. This realization approximates (but is not, and can never be, equivalent to) the more rigorously defined NAD 83( CORS 96) realization in which Continuously Operating Reference Station ( CORS ) coordinates are distributed. NAD 83(NSRS2007) was created by adjusting GPS data collected during various campaign-style geodetic surveys performed between the mid-1980's and 2005. For this adjustment, NAD 83( CORS 96) positional coordinates for ~700 CORS were held fixed (predominantly at the 2002.0 epoch for the stable north American plate, but 2007.0 in Alaska and western CONUS) to obtain consistent positional coordinates for the ~70,000 passive marks, as described by Vorhauer [2007]. Derived NAD 83(NSRS2007) positional coordinates should be consistent with corresponding NAD 83( CORS 96) positional coordinates to within the accuracy of the GPS data used in the adjustment and the accuracy of the corrections applied to these data for systematic errors, such as refraction. In particular, there were no corrections made to the observations for vertical crustal motion when converting from the epoch of the GPS survey into the epoch of the adjustment, while the NAD 83 ( CORS 96) coordinates do reflect motion in all three directions at CORS sites. For this reason alone, there can never be total equivalency between NAD 83(NSRS2007) and NAD 83( CORS 96).

Note: NGS has not computed NAD 83(NSRS2007) velocities for any of the ~70,000 passive marks involved in this adjustment. Also, the positional coordinates of a passive mark will make reference to an “epoch date”. Epoch dates are the date for which the positional coordinates were adjusted, and are therefore considered “valid” (within the tolerance of not applying vertical crustal motion). Because a mark's positional coordinates will change due to the dynamic nature of the earth's crust, the coordinate of a mark on epochs different than the listed “epoch date” can only be accurately known if a 3-dimensional velocity has been computed and applied to that mark.

I agree with the comments above.

I would also like to expand a bit on the HPGN/HARN coordinate estimates (1992 for Arizona).

IF (big IF maybe) you have a suite of “original” 1992 Arizona HARN sites in your area of interest, then by all means obtain some CURRENT OPUS (or OPUS_RS) solutions on as many as practical. Do NOT mix NAD83(1992) estimates from the original adjustment (1992) with NAD83(1992) estimates from the FBN adjustment (GPS-1347, 1999), they are NOT necessarily the same critter, and the different estimates can vary by several centimeters.

The “original” HARN adjustment can (and probably should) be considered a local realization of NAD83 LIMITED to the stations embraced within THAT adjustment. The same can also be said for the FBN (GPS-1347) adjustment, even though BOTH adjustments are tagged as NAD83(1992).

Remember, IF you are trying to “fit” existing Control that was constrained to NAD83(1992) [either original or FBN], then you MUST duplicate (to the extent possible) the original constraints. This MIGHT be rather difficult, if you have differential movement between the HARN stations in your area (which you very well may have). HTDP can HELP you resolve some of this, BUT it isn't perfect, and you will need to be CAREFUL when applying the corrections for the modeled velocity.

IF (possibly big IF again) the CURRENT (OPUS) spatial relationships between the HARN/FBN stations are “reasonably” consistent with the “superseded” NAD83(1992) estimates, then you're in like Flint. If they are NOT, then you should probably start playing with HTDP a little, and see IF that will resolve some of the spatial issues between the various stations that you need to use as NAD83(1992) constraints.

Now this is where I will probably get into trouble....

If it where ME...I would probably transform the NAD83(1992) estimates to ITRF-2005 Epoch 1992.0 XYZ. Then I would transform the OPUS derived ITRF-2000 Epoch 2011.xxxx XYZ to ITRF-2005 Epoch 2011.xxxx XYZ (same epoch) and START there. Look at the solved vectors between your stations, and see what you see. Then transform the ITRF-2005 Epoch 2011.xxxx “back” to ITRF-2005 Epoch 1992 (using HTDP v3.0), and look at the solved vectors between your stations again. If that IMPROVES the spatial agreement between what you observed “today,” and what was published back in the 1990s, then than you are off to the races. If not, then you will probably need to proceed from another angle. I should probably note here....nearly 20 years of crustal movement in Arizona (relative to the ITRF) will probably show up as a small rotation, so the solved DISTANCES between the HARN sites should be the key to your analysis. The reason that I would use ITRF-2005 (and not NAD83) is that HTDP v.3.0 uses a "ZERO vertical velocity" model expressed in ITRF-2005! When you make the 14 parameter transformation to NAD83, that imparts about a -1mm year vertical velocity relative to NAD83 (in Arizona).

It will PROBABLY never be perfect, so exactly where you draw the line (set the threshold) is up to you. The SMALLER your area of interest is, the better your results will (probably) be. In all likelihood you will probably find that a simple (and locally unique) aX, aY, aZ or aN,aE,aU adjustment will get you “back” to what you want to be. BUT, bear in mind, that whatever you come up with today, may be (and probably WILL be) obsolete when the Multiyear adjustment and NAD83(CORS96a) replaces NAD83(CORS96) in a few months.

Hope that made some sense,
Loyal