A geographic datum transformation is a calculation used to convert between two geographic coordinate systems to ensure that data is properly aligned within a map. When the layers in a map use a geographic coordinate system that differs from the geographic coordinate system used by the map itself, a transformation must be applied to ensure the data draws in the correct place. When a transformation is needed, a list of suitable transformations is provided, sorted based on data extents and transformation accuracy. By default, the first transformation in the list is applied. In most cases, this choice represents the most suitable transformation for the data and map combination.

## Understand transformations

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 not a perfect sphere but is actually lumpy and slightly squashed.

In ArcGIS Pro, both maps and their layers have coordinate systems, and they are not always the same. Coordinate systems are either geographic or projected, but all projected coordinate systems include an underlying geographic coordinate system. A transformation is used whenever the geographic coordinate system (or the underlying geographic coordinate system) of layer is not the same as the geographic coordinate system (or underlying geographic coordinate system) of the map. Transformations relate to the underlying geographic coordinate systems only. It converts the geographic coordinate system of a layer to match the geographic coordinate system of the map. This calculation is applied 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.

There are many transformations available. The best transformation for your map is suggested based on the data and its extents, but you can specify a different one. The Transformation tab on the Map Properties dialog box lists transformations used in the map.

##### Note:

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. See Install supplementary coordinate system files to learn more.

## Transformations in scenes

When you work with global scenes, there are only two available coordinate systems: World Geodetic System 1984 (WGS 1984) or China Geodetic Coordinate System 2000 (CGCS 2000). In the case of CGCS 2000, there are no publicly available transformations. In or near China, WGS 1984 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.