bi-directional Geographic Datum Transformations

[Craig_Eissler_Iceman]

Hello, I understand that Geographic Datum Transformations are "bi-directional".

But when I review the very long list of these Transformations in that separate PDF document, I see, for example:

NAD_1983_To_WGS_1984_1
and
WGS_1984_(ITRF00)_To_NAD_1983.

...What is the difference?
...Or, why isn't it written the other way around?

Thanks
-CCE

CCE

[HaydenWelch]

Edit: Here's a good post about this

These are just two separate transformations. They are bi-directional, but ITRF00 is more accurate. The flipped names is just because that's how they were written by whoever made them, here's a side by side git diff of the details for each:

Left: NAD_1983_To_WGS_1984_1 

Right: WGS_1984_(ITRF00)_To_NAD_1983

As you can see when the transformation is set in ArcPro, it uses the same for both directions:

In that second transformation it even uses the ITRF00 translation twice to translate between the different NAD systems because going through WGS84(ITRF0X) is more accurate (see the Accuracy at the bottom of the diff). Here's that dual layer transform for NAD83 -> NAD83 (2011):

  Accuracy	0.03 Meters
    Right	-63.88°
    Left	167.65°
    Bottom	14.92°
  Area of Use	USA - CONUS and Alaska; PRVI
	
    Scale Difference	0.00171504 Parts Per Million
    Z Axis Rotation	0.01159935 Arcseconds
    Y Axis Rotation	0.00942645 Arcseconds
    X Axis Rotation	0.02591467 Arcseconds
    Z Axis Translation	-0.52655 Meters
    Y Axis Translation	-1.90331 Meters
    X Axis Translation	0.99343 Meters
  Method	Coordinate Frame
	
  Target GCS	GCS NAD 1983 2011
  Source GCS	GCS WGS 1984
  Authority	Esri
  WKID	108363
  Direction	Forward
Transformation Step 2	WGS 1984 (ITRF08) To NAD 1983 2011
	
  Accuracy	0.1 Meters
	
    Right	-63.88°
    Left	167.65°
    Bottom	14.92°
    Top	74.71°
	
  Method	Coordinate Frame
    X Axis Translation	0.9956 Meters
    Y Axis Translation	-1.9013 Meters
    Z Axis Translation	-0.5215 Meters
    X Axis Rotation	0.025915 Arcseconds
    Y Axis Rotation	0.009426 Arcseconds
    Z Axis Rotation	0.011599 Arcseconds
    Scale Difference	0.00062 Parts Per Million
	
  Area of Use	USA - CONUS and Alaska; PRVI
    Top	74.71°
  Direction	Reverse
  WKID	108190
  Authority	Esri
  Source GCS	GCS WGS 1984
  Target GCS	GCS North American 1983
	
Output Geographic Coordinate System	GCS NAD 1983 2011
	
Transformation Step 1	WGS 1984 (ITRF00) To NAD 1983
Input Geographic Coordinate System	GCS North American 1983
Transformation Path	WGS 1984 (ITRF00) To NAD 1983
 + WGS 1984 (ITRF08) To NAD 1983 2011

[Craig_Eissler_Iceman]

Interesting, Hayden -- thanks!
...May I ask just a couple more related questions...

Am I correct that the WGS84 (ITF00) - NAD83 is the "default"?
If so, is that why it's listed in the Map Properties > Transformations section?

I don't see any others listed.  ...So, I'm not sure how to relate those Map Property Transformations Settings to the Help Doc statement below when it says "a best Transformation is suggested".

[DanPatterson]

read .... best for your "area" (long/lat)

[Craig_Eissler_Iceman]

Yes, thanks, I understand. What I'm really trying to ask is, when, where, and how, does this occur?

In other words, is this "suggestion" only referring to using the 'Project' tool? ...I see them there.

Or, does this "suggestion" also occur during Projection on-the-fly; ...but I just don't see it?

And, "what is the purpose of the Map > Properties listed Transformations"? 
...and does it have anything to do with the "suggestions"?

[DanPatterson]

Project tool.... this should be the only avenue to use if you want to control your output.  You can select from suggestions or follow your own path if you have a particular reason or knowledge

Project on the fly .... IMHO  the worst thing introduced to the GIS world, It tries to do the right thing but ultimately the responsibility is the user, so I would even expect it to have suggestions

Map Properties  .... again you can do what you want if you want to mix projected and geographic data into one map because you have to , or choose to.  It gives an option to take some control over what to use if anything is suggested.

[Craig_Eissler_Iceman]

Thanks, but yes, I know all that already.
I'm sorry my questions aren't making sense..
Let's see if someone else can decipher my very specific questions.
I respect your knowledge, and thanks again for your time.

[Craig_Eissler_Iceman]

OK, I think I now understand that:

• A New Map will NOT have any Datum Transformations listed in the Map Properties > Transformations section.
• However, when Projection on-the-fly occurs when adding a Layer to the Map, ArcGIS will use a "default" Datum Transformation. Then, that specific Datum Transformation will be Listed in the Map Properties > Transformation section.
• Another option is for the User to Add a different Transfomation conversion method for this specific path situation (like WGS 84 to NAD 83) BEFORE adding that Layer to the Map in order to override the  default. ...Or, AFTER the Layer has been added, the User can change the Transformaiton Listed, to override the default, which will apply the re-projection on-the-fly immediately.
• Finally, if Transformations are not currently being used on the Map because they've been removed form Contents, it will be dropped down and now listed in the 'Additional transformations' sub-section where it can be kept for potential later use or deleted. 
• For the Project tool, things are more straight-forward. The default Transformation listed in the tool is more of a suggestion -- a "default suggestion". In other words, you have the opportunity to change it BEFORE running the tool.
• So, for both situations (Projection on-the-fly and the Project tool), the "default/suggested Datums" are the same and pulling from the same ArcGIS Index listing of Datum Transformations.