Which ArcGIS 10.7 transformation to convert WGS84 data to Washington State Plane North?

Question

A bulk of my GIS datasets reference Washington State Plane North FIPS 4601 (US Feet). Often I get data in WGS84 that I need to transform to FIPS 4601. Ten years or so I worked though many of the default transformations in ArcGIS to prove to myself that the NAD_83_To_WGS84_5 (CONUS) transformation was in fact the best transformation for minimizing any shifting in the datasets. I do not remember the shift in the different transformation from back then but I have been relying on NAD_83_To_WGS84_5 since that time.

I am now running ArcGIS 10.7.2 and noticed a shift of 1.28 meters in data using the NAD_83_To_WGS84_5 transformation.

I have rerun my experiments using some newly created (AKA clean) point data in WGS84. I have experimented with the NAD_83_To_WGS84_5 (CONUS) and the default ITRF00_To_NAD83 default transformation in the reprojection tool and see that this adds about 1.28 meters of shift in my data.

I also ran a transformation using NAD_83_To_WGS84_1 (CONUS, Canada, Alaska) and see that my data less that 0.000 meters away.

Looking at the ESRI publication ArcGIS 10.7.1 and ArcGIS Pro 2.4 Geographic and Vertical Transformation Tables I see that there is now a newer NAD_1983 to WGS84_OR_WA_41 transformation. This transformation causes about a 0.2 meter shift in my data.

Interestingly, I duplicated this transformation experiment in QGIS 3.4 and see that QGIS returned less than a 0.000 meter shift in the data without ever being prompted for a transformation type.

My question is: Why is the more generalized NAD_83_To_WGS84_1 returning better data than the more focused transformations and is there a better transformation that I should be using?

Answer 1

I wonder if you're misinterpreting your results.

If you use the Project Tool to convert data between two coordinate reference systems, the tool lets you set a geographic/datum transformation if the geographic coordinate reference systems are different. If you then add the input and output datasets to ArcMap, you must set the same transformation. ArcMap will not automatically set a geographic/datum transformation.

If you don't set the transformation, ArcMap will reproject all layers to the data frame's coordinate reference system, but ignore the differences in the geographic coordinate reference systems. Offsets there should reflect the missing transformation.

The NAD_1983_To_WGS_1984_1 transformation has zeroes for the parameters. It assumes that NAD 1983 and WGS 1984 data are equivalent within a certain accuracy. When both were created in the mid-1980s, they were considered equivalent. Over the years, both have had re-adjustments / realizations released that improved the accuracy of the reference frames. They've also been drifting apart because of how they're defined. WGS 1984 is tied to the ITRF (International Terrestrial Reference Frame) while NAD 1983 is tied to the North American plate.

NAD_1983_To_WGS_1984_5 models the difference between NAD 1983 and WGS 1984 at approximately the mid-1990s/early 2000s. By that point, most of the US had had HARN re-adjustments done which greatly improved the NAD 1983 network. That's the majority of the current difference between current realizations: NAD 1983 (2011) and WGS 1984 (G1762).

WGS_1984_(ITRF00)_To_NAD_1983 and NAD_1983_To_WGS_1984_5 have similar parameter values, but not exact. The first one is more recent: ITRF00 versus ITRF96.

Note: Esri uses "coordinate system" where ISO (International Standards Organization) 19111 (spatial reference data model) uses "coordinate reference system".

Disclosure: I work for Esri.