Which version of NAD83 is assumed in geographic transformation?

[FrancesBiles]

I have some points in NAD83 (1986).  In fact, A LOT of our GIS data is in NAD83 (1986). I am using Desktop ArcMap v 10.2.2.3552. I want to transform the points to ITRF00.

If I choose: WGS_1984_(ITRF00)_To_NAD_1983 + ITRF_2000_To_WGS_1984 for the geographic transformation method, which flavor of NAD83 is being assumed?  Everything I found suggests that NAD83 (CORS96) is being assumed.  If that is the case, how does one transform between NAD83 (1986) to ITRF00? To ITRF08? (Or the most current version of WGS84?). The GIS data covers southeast Alaska.

From what I have read, it looks like depending on what version of ArcMap one is using, the name "NAD83" could refer to completely different versions of NAD83. For example, in earlier versions of ArcMap, the transformation NAD_1983_To_WGS_1984_5 assumed that coordinates were in NAD83 (1986), but in current versions of ArcMap (10.x) NAD_1983_To_WGS_1984_5 assumes that coordinates are in NAD83 (CORS96).  Is that correct?  If so it would be VERY HELPFUL if datum names/labels always had their "version" attached.  Does WGS84 in that transformation refer to the original WGS84 or WGS84(G1674), or?

Is there a comprehensive "cheat sheet" that crosswalks ArcMap versions with the datum versions assumed for all datum and geographic transformation names that involve NAD83, WGS84, ITRF reference frames?

[FrancesBiles]

Thank you Melita.

I’m still a bit confused.

To be specific, you are saying that the transformation method WGS_1984_(ITRF00)_To_NAD_1983 +
ITRF_2000_To_WGS_1984  is doing a conversion from NAD83 (CORS96) to WGS84 only (which WGS84? G1150?), correct?

However, the 7 parameter values used in the ESRI transformation match the values on the NGS website​
for converting between NAD83(CORS96) and ITRF00. Based on this I would think the ESRI transformation WGS_1984_(ITRF00)_To_NAD_1983 + ITRF_2000_To_WGS_1984 is converting between NAD83 (CORS96) and ITRF00, not NAD83 (CORS96) and WGS84(Gxxxx).

Am I not interpreting this correctly? Any clarification is appreciated!

[MelitaKennedy]

Okay, weirdly enough, for those transformation parameters, I added

ITRF_2000_To_   NAD 1983 HARN | CORS96 | 2011

Why did I omit NSRS2007? I dunno. Trying to reconstruct my thinking, I believe I decided that I shouldn't add ITRF2000 to NAD 1983 because the transformation wasn't designed for that, but for CORS96 and, to a lesser degree, HARN. I did put it in with WGS84 as an approximation to ITRF2000 and converting to NAD 1983, which is another approximation.

In your particular case, you're starting out with NAD 1983, so the "most accurate" transformation or pair of transformations is the one you have, rather than a direct one which as zero parameters, so even more of an approximation.

Technically, I would move NAD 1983 (1986) to HARN (NADCON/HARN transformation), assume it's relatively close to CORS96 (and redefine it as NAD 1983 CORS96), then use ITRF_2000_To_NAD_1983_CORS96 to convert to ITRF00.

Sorry, rather than getting simpler, geodetic transformations are just getting more complicated with time!

Melita

[FrancesBiles]

Hi Dan -

Thank you for your response.

I had tried the Confluence Mobile link you posted before but the page was blank. I was using IE, so just tried another browser and a page for "height aiding results" opened, which didn't look related to my question.

Was your response "some minor shifts" in answer to my last question? Can you provide more details as to what the + ITRF_2000_To_WGS_1984 is doing in the

WGS_1984_(ITRF00)_To_NAD_1983 + ITRF_2000_To_WGS_1984 transformation? I am converting from NAD83 to ITRF00. The ESRI geographic_transformations pdf shows the 7 parameter values used in the ITRF00_To_NAD83 part of the transformation, so I get that.  But why is the "+ ITRF_2000_To_WGS84" part needed? What is it doing?

[FrancesBiles]

...I also meant to add that the NAD83 page on the Confluence site has a great spreadsheet resource of datum conversions for ITRF00, WGS84, and NAD83.  Thought I would post the link in case other found it useful.

[JoelCusick]

Dan, Frances,

Wonderful link pointed out today.  I did some quick checking against my shifters (embedded in Trimble postprocessing software and Blue Marble) and this is a very good site that exposes the 7 parameters with WKID's.

Frances, ITRF_2000_To_WGS_1984 is molendesky, 3 parameter, 0,0,0 bookkeeping transform. It merely is being used to Project, without projecting so another datum switch could be employed.  Look at Pathfinder Office, coordinate system manager, datum tab, ITRF, and you'll see same shifters as expressed on Dan's link.  So, explicitly, if you use WGS_1984_(ITRF00)_To_NAD_1983 + ITRF_2000_To_WGS_1984 in your case,

1) Excuse the order, any transform responds to your projection defined.  So, your moving your NAD83 data, about 1.4 meters using WGS_1984_(ITRF00)_To_NAD_1983 which employs a 7-param. Helmert transform equivalent in Trimble as NAD 1983 (CORS96) and row 14 in the Excel spreadsheet that Dan Provided.  Note, that just like Melita stated, ITRF2000 is equivalent to "WGS84" in the language of Trimble, the language of ESRI, and apparently the ITRF Transformation parameters. 

2) Then, the second half of the composite shifter is really doing a "project define", by using 0,0,0 to "shift from WGS84 to your required destination definition of IRF_2000.

Datums are like going to the Dentist.  Wait too long, and the pain of a visit increases over time.  Brush often, and frequent check-ups (validation) is key to a healthy GIS (mouth).

Joel

[FrancesBiles]

Ok, thanks everyone. I believe I get it now! To summarize the responses and what I've read:

1) The first part of the transform, WGS_1984_(ITRF00)_To_NAD_1983, is converting from NAD83(CORS96) to ITRF00. ESRI considers ITRF00 identical to WGS84.  WGS84 in this case means WGS84(G1150).  WGS84(G1150) and ITRF00 are essentially identical (< 10cm difference).  However, b/c my points are in NAD83(1986) instead of CORS96, about a 1.4 meter error is introduced.

2) The 2nd part of the transform, ITRF_2000_To_WGS_1984, is a zero transformation (a.k.a., "bookkeeping" transformation). So, no transform is being applied, but this step basically performs the job of a "define projection" to ITRF00.

If I haven't mischaracterized anything I would mark this as answered. But everyone contributed to the answer - I don't think I'm able to "correct answer" everyone.

[DanPatterson_Retired]

Frances... you should be able to change it to a discussion or mark it as assumed answered since youo are the author and feel that there is more than one answer.  At least people will know that there is some closure and pertinent information in the threads...otherwise people may skip an unanswered thread as containing no useful information.