A geographic datum transformation is a calculation used to convert between two geographic coordinate systems to ensure that data is properly aligned.
Geographic coordinate systems describe how locations on the earth are placed on a hypothetical reference spheroid. They use angular units, such as degrees, to assign locations to coordinates on a reference spheroid. There is more than one geographic coordinate system because each is meant to best fit certain portions of the earth. This is necessary because the earth is actually a lumpy and slightly squashed sphere. The transformation is a calculation to convert the geographic coordinate system of the layers to match the geographic coordinate system of the map as the map draws so that everything is aligned. The data is not changed by a transformation. This real-time translation is sometimes referred to as projecting on the fly.
The best geographic coordinate system to use depends on where and how much of the earth's geography you are mapping. In ArcGIS Pro, the best transformation for your map is suggested based on the data and the extent, but you can specify a different one.
Some transformations that require files are not installed with ArcGIS Pro. You can download and install the ArcGIS Pro Coordinate System Data files from My Esri. The additional files are the EGM2008 and GEOID12b geoid models; VERTCON files; GEOCON v1; and three NTv2 files for Switzerland, XRail, and OSTN15.
In ArcGIS Pro, both maps and their layers have coordinate systems, and they are not always the same. Each coordinate system may be either geographic or projected. Projected coordinate systems always include an underlying geographic coordinate system. A projection transforms the angular coordinates (such as latitude and longitude) from the reference spheroid to distance units (such as meters) on a flat surface. For example, the projection may describe how the spheroid coordinates will map to a flat rectangle hypothetically wrapped around the reference spheroid as a cylinder. See a list of the map projections supported in ArcGIS Pro.
Transformations relate to the underlying geographic coordinate systems only. A transformation is applied only when the geographic coordinate systems are not identical. If layers have a different projected coordinate system than the map they are in, but both the layers and the map reference the same underlying geographic coordinate system, a transformation is not needed.
Transformations in scenes
When you work with global scenes, there are only two available coordinate systems: World Geodetic System 1984 (WGS84) or China Geodetic Coordinate System 2000 (CGCS 2000). In the case of CGCS 2000, there are no publicly available transformations. In the absence of a custom transformation, data added to a scene that uses CGCS 2000 is transformed to WGS84. In or near China, WGS84 closely matches CGCS 2000, but datum shifts may be substantial outside China.
Project data to a new coordinate system
Relying on transformations to project layers in real time is helpful when you are exploring data because everything aligns. However, applying a transformation comes with costs in drawing performance and accuracy. It is a best practice to work with data in the same coordinate system when performing edits or analysis on your data. It is recommended that you use one coordinate system for the map and all the data in it. Use the Project tool to project vector spatial data from one coordinate system to another. If you are working with raster data, use the Project Raster tool.