topic Re: Which version of NAD83 is assumed in geographic transformation? in Coordinate Reference Systems Questions

Which version of NAD83 is assumed in geographic transformation?

FrancesBiles — Fri, 08 Jan 2016 01:05:21 GMT

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?

Re: Which version of NAD83 is assumed in geographic transformation?

DanPatterson_Retired — Fri, 08 Jan 2016 01:14:56 GMT

Melita Kennedy .... would be the person to address this to. I have flagged this post for her attention.

Re: Which version of NAD83 is assumed in geographic transformation?

NeilAyres — Sun, 12 Dec 2021 09:31:20 GMT

The factoryCode for NAD83 (1986) is 4269.

And WGS_1984 is 4326.

So to list the available transformations I did this

sr1 = arcpy.SpatialReference(4269)
>>> sr1.name
u'GCS_North_American_1983'
>>> sr2 = arcpy.SpatialReference(4326)
>>> sr2.name
u'GCS_WGS_1984'
>>> trans = arcpy.ListTransformations(sr1, sr2)
>>> len(trans)
298

But a great deal of those 298 are composite transformations and I treat those with deep suspicion.

So listing out only those without "+" in the name gives me :

>>> for tr in trans:
...     if "+" not in tr:
...         print tr
...         
WGS_1984_(ITRF00)_To_NAD_1983
NAD_1983_To_WGS_1984_5
NAD_1983_To_WGS_1984_1
NAD_1983_To_WGS_1984_4
NAD_1983_To_WGS_1984_3
NAD_1983_To_WGS_1984_2

So that first one might do want you need to do.

Re: Which version of NAD83 is assumed in geographic transformation?

DanPatterson_Retired — Fri, 08 Jan 2016 08:59:34 GMT

some interim links

a big list
- Spatial Reference List -- Spatial Reference
particularly as they apply to Canada
- Confluence Mobile - QPS
- The Canadian Spatial Reference System (CSRS) | Natural Resources Canada

Re: Which version of NAD83 is assumed in geographic transformation?

NeilAyres — Fri, 08 Jan 2016 09:50:27 GMT

But looking into the geographic_transformations pdf I see this.

WGS_1984_(ITRF00)_To_NAD_1983 108190 CF 0.9956 -1.9013 -0.5215 0.025915 0.009426 0.011599 0.00062

WGS_1984_(ITRF00)_To_NAD_1983_2011 108354 CF 0.9956 -1.9013 -0.5215 0.025915 0.009426 0.011599 0.00062

WGS_1984_(ITRF00)_To_NAD_1983_CORS96 108151 CF 0.9956 -1.9013 -0.5215 0.025915 0.009426 0.011599 0.00062

WGS_1984_(ITRF00)_To_NAD_1983_HAR 108282 CF 0.9956 -1.9013 -0.5215 0.025915 0.009426 0.011599 0.00062

These are all the same thing.

So now I am not so sure

Re: Which version of NAD83 is assumed in geographic transformation?

DanPatterson_Retired — Fri, 08 Jan 2016 11:24:35 GMT

They did say ... using a 7 parameter transformation... they failed to mention whether it was the same one ... hmmm ... what's a meter or so, province-wide

Re: Which version of NAD83 is assumed in geographic transformation?

MelitaKennedy — Fri, 08 Jan 2016 23:05:46 GMT

We do have some "named" realizations for NAD83 in the US - HARN, CORS96, NSRS2007, 2011. "NAD 1983" should only be used for the 1986 version, but some people use it for everything. When I can find an ITRF/NAD83 transformation from the NGS (National Geodetic Survey) CORS website, I put it, but also put "equivalents" where I include it for HARN and CORS96 or NSRS2007 and 2011, and possibly "1986."

WGS84 is a generic definition. We've been debating adding the specifically named versions, but there's very little public information about how to convert between them. Also, what do we do with the existing "to WGS84" transformations? Do they apply also to any of the specific WGS84 realizations?

Specifically for Alaska, I'm not sure I have a full solution for you. There was no HARN grid published to convert from NAD83 (1986) to NAD 1983 HARN that I'm aware of. GEOCON v2.0 will convert from the Alaska FBN re-adjustment of 1992 to NSRS2007 and then from NSRS2007 to 2011. That still leave a gap between 1986 and 1992.

You might try contacting the NGS state advisor for Alaska to see what she recommends.

Melita

Re: Which version of NAD83 is assumed in geographic transformation?

FrancesBiles — Tue, 12 Jan 2016 02:14:08 GMT

Thank you everyone for the useful information. First an FYI, and then a question.

Regarding the NAD83 version used for the WGS_1984_(ITRF00)_To_NAD_1983 transformation, I was able to find the answer from clues given by Neil and Melita. As Neil pointed out, the 7 parameter values are the same for all four ITRF00 to NAD83(1986), CORS96, 2011, and HARN transformations. I wondered which version of NAD83 these parameters are associated with. According to this NGS CORS page, the 7 parameters are specifically for ITRF00-to-NAD83 (CORS96).

Q1) I'm not sure what the + ITRF_2000_To_WGS_1984 is indicating in the WGS_1984_(ITRF00)_To_NAD_1983 + ITRF_2000_To_WGS_1984 composite transformation. Why is this needed/what is it doing? Could someone please explain?

-Frances

Re: Which version of NAD83 is assumed in geographic transformation?

DanPatterson_Retired — Tue, 12 Jan 2016 02:21:02 GMT

Some minor shifts... the wgs84 associations are listed here, but I sent this before

Confluence Mobile - QPS

Re: Which version of NAD83 is assumed in geographic transformation?

FrancesBiles — Tue, 12 Jan 2016 03:21:46 GMT

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?

Re: Which version of NAD83 is assumed in geographic transformation?

FrancesBiles — Tue, 12 Jan 2016 03:28:17 GMT

...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.

Re: Which version of NAD83 is assumed in geographic transformation?

MelitaKennedy — Tue, 12 Jan 2016 18:11:22 GMT

It's converting from NAD83 to WGS84, then WGS84 to ITRF2000. The 2nd transformation's parameters are zeroes. I call it a "bookkeeping" transformation. Even though the first transformation says WGS_1984_(ITRF00), it's between WGS84 and NAD83. The (ITRF00) is to clue that the WGS84 version is the approximately ITRF00 realization.

Melita

Re: Which version of NAD83 is assumed in geographic transformation?

FrancesBiles — Tue, 12 Jan 2016 20:24:10 GMT

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!

Re: Which version of NAD83 is assumed in geographic transformation?

MelitaKennedy — Tue, 12 Jan 2016 20:45:29 GMT

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

Re: Which version of NAD83 is assumed in geographic transformation?

JoelCusick — Wed, 13 Jan 2016 23:39:09 GMT

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

Re: Which version of NAD83 is assumed in geographic transformation?

FrancesBiles — Thu, 14 Jan 2016 01:47:49 GMT

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.

Re: Which version of NAD83 is assumed in geographic transformation?

DanPatterson_Retired — Thu, 14 Jan 2016 02:56:41 GMT

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.