Data does not line up when the correct datum transformation is applied

Description

Consider a situation where there are two sets of data that are assumed to have the correct projection definitions, but when added to a map, do not align.

For example, the two sets of data are both in projected coordinate systems, NAD 1983 and WGS 1984, with units of feet or meters, but when we apply the correct datum transformation, WGS_1984_(ITRF00)_To_NAD_1983, the data are offset by about a meter.

Cause

In the 1980s, the Geographic Transformation between NAD 1983 and WGS 1984 in common usage was NAD_1983_To_WGS_1984_1, WKID 1188 (see page 51 of ArcGIS Pro Geographic and Vertical Transformation Tables).

Because of the state of geodesy at that time, NAD 1983 and WGS 1984 were defined as being identical. The transformation between them was a Geocentric Translation, and the three transformation parameters were 0, 0, 0. (see page 95 of the Geographic and Vertical Transformation Tables).

This was correct at the time, and a lot of data was transformed between NAD 1983 and WGS 1984 using this transformation. What was done previously was not wrong, but more accurate methods are now available, so this process will allow you to bring the accuracy and precision of your data to current standards.

As geodesy has advanced and become more precise, it has been recongized that NAD 1983 and WGS 1984 are not the same. More accurate datum transformations have been calculated, so it may be necessary to apply the following steps to back out of the previously applied transformation, compare the datums used, and apply a more modern and accurate transformation.

Solution or Workaround

The following steps describe how to identify whether the two sets of data use different datums:

1. To test, both sets of data must be in a geographic coordinate system, so we project the layer from NAD 1983 in the Projected Coordinate System (units of feet or meters) to a geographic coordinate system GCS_NAD_1983, with units of degrees. We can now compare the datums.
2. When we compare that new feature class or shapefile with the data on WGS 1984, we still have about a one meter offset.
3. The test is to redefine the data that is on GCS_NAD 1983 as GCS_WGS_1984. If we are correct in our assessment, the two data layers align. They also align when Data Reviewer checks are run for example.

Since the data did align, we then need to know why the offset occurred in the first place. This occurred because as stated previously, the standard transformation between NAD_1983 and WGS_1984 treated both datums as being identical, so that standard transformation, with 0 parameters was applied, but no changes were made to the data other than to rename it.