NAD 1983 CSRS and transformations...

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06-07-2021 11:30 AM
N_W_S_E
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Hello my fellow Canadians!

Lately I have been researching more and more about transformations from NAD 1983 GCS to NAD 1983 CSRS GCS, and then NAD 1983 CSRS into WGS 1984 Web Mercator (auxiliary sphere), and the more I read, the more confusing it gets.

There are a few articles I have found, but they seem to be dated. This article is absolutely fantastic (shoutout to all of the ESRI Blog Creators, your information is invaluable, and has helped me immensely), but I don't think it has been updated to fit 2021 and all the changes that has happened to NAD 1983 CSRS.

 This article states:

Please note that different Geographic systems need specific transformations. As of 2018, CSRS v6 and v7 are becoming very popular - however there is no specific transformations available at this time. If using ArcGIS 10.6x, the newer v6 and v7 Geographic systems are available

With that being said I am using both ArcGIS Desktop 10.8.1 and ArcGIS Pro 2.8.

From the first article I linked, I downloaded the ON83CSv1.gsb from LIO / Geohub and created a custom transformation that transforms data from NAD 1983 ORIGINAL to NAD 1983 CSRS. This works perfectly to align data that has been projected in NAD 1983 to transform on-the-fly into NAD 1983 CSRS:

N_W_S_E_0-1623089596774.png

My confusion comes from the latter part of the article where Ms. Sibbet talks about doing a transformation to WGS 1984 Web Mercator (auxiliary sphere) in order to upload to ArcGIS Online (which I do a lot, hence I want to get this right).

Generally. she states that if transforming from NAD 1983 CSRS to WGS 1984 Web Mercator (auxiliary sphere), one should use the transformation: NAD_1983_CSRS_To_WGS_1984_2.

In her example, she uses her custom transformation followed by NAD_1983_To_WGS_1984_2:

N_W_S_E_1-1623089915400.png

After all this background info, I am finally getting to my question. Currently ArcGIS Pro is defaulting to the following:

N_W_S_E_2-1623089993808.png

And it aligns perfectly.

For curiosity sake, I tried this:

N_W_S_E_4-1623090461119.png

And it resulted in slight misalignment.

I am wondering why 'we' are now using WGS 1984 (ITRF00) to NAD 1983. The following transformations in the photo BELOW work perfectly to align everything precisely, but I am confused about the deviation from NAD_1983_To_WGS_1984_2 and the implementation of WGS 1984 (IRF00) TO NAD 1983

N_W_S_E_3-1623090307103.png

I understand the common practice is to avoid projecting on the fly, but this doesn't just apply to that, it also applies to transformations when reprojecting data, so I'm trying to understand it all!

Thanks in advance! I look forward to hearing your thoughts.

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