A geographic datum transformation is a calculation that is used to convert between different geographic coordinate systems to ensure that data is properly aligned. Anytime the geographic coordinate systems used by layers in a map differ from the geographic coordinate system of the map, a transformation is required and is automatically applied. The transformation dynamically alters the coordinates of data as it is drawn to ensure everything lines up as expected.
All projected coordinate systems (PCS) have an underlying geographic coordinate system (GSC). A transformation is required only when the specified or underlying geographic coordinate systems differ between the map and the layers. If data is projected into different projected coordinate systems but all layers and the map share the same underlying geographic coordinate system, a transformation is not required.
Tip:
Transformations are dynamically applied to align layers in the map without altering anything about the data. This is helpful when you are exploring data before doing analysis or map authoring. Once you've finalized the data you want to work with, consider choosing an appropriate coordinate system for the map, and projecting the data to that coordinate system if necessary. Transformation conversions are continuously applied, so it is more efficient to do this conversion once, deliberately, by projecting the data. Use the Project tool to project vector data or the Project Raster tool to project raster data.
The transformation that is applied is the optimal one based on its accuracy and the extent of the data, but you can specify a different transformation if necessary. You can also save additional transformations with the map for later use. This is helpful if you need to temporarily remove a layer but don't want to lose the transformation that was required for that layer.
Additional information about each transformation used in a map is outlined on the Transformation Details dialog box. If the transformation is comprised of multiple steps, each is listed. Each step can be applied in either the forward or reverse direction. For example, the WGS 1984 (ITRF00) To NAD 1983 transformation can be used to transform from NAD 83 to WGS 84, or from WGS 84 to NAD 83. When the transformation step is applied in reverse, it is indicated in the transformation details for that step.
The parameters listed under the transformation method vary depending on the method used by the transformation. Certain details such as Accuracy and Area of Use are only available for transformations installed with ArcGIS Pro or with the ArcGIS Coordinate System Data. You can see the details of custom transformations only if the transformation has been defined using the Create Custom Geographic Transformation tool.
Apply transformations to a map
The Transformation tab on the Map Properties dialog box lists the geographic and vertical coordinate systems of the layers and the current geographic and vertical coordinate systems of the map. Between these coordinate systems are the transformations suggested to convert between them.
Transformations are bidirectional. The order of the coordinate systems does not matter. You can specify a different transformation if necessary. Transformations are sorted by their accuracy and suitability for the location of your data. The coordinate systems cannot be changed here. Use the Coordinate System tab on the same dialog box to change the coordinate system.
To view the transformations applied in a map, or to choose different transformations, follow these steps:
- Right-click the map or scene in the Contents pane and click Properties
. On the Map Properties dialog box, click the Transformation tab. - In the Layer and map section, click the name of a transformation in the center column of the table to open the menu and choose a different transformation. Repeat this step as necessary for other coordinate systems used by layers in the map.
If the geographic coordinate systems of the layers and the map are identical, no transformation is necessary.
- Optionally, click the Details link to open the Transformation Details dialog box. The details of the transformation, including the transformation direction, the source and target coordinate systems, and method, are listed. Depending on the transformation, additional details may be listed as well.
- Click Apply to apply the transformations, or click OK to apply the changes and close the Map Properties dialog box.
Dive-in:
When a transformation is necessary, in most cases a suitable one can be found and it is automatically applied. In some situations, you may want to deliberately avoid transforming the data. You may want to temporarily suspend the dynamic transformation process when you are troubleshooting data issues or to confirm the discrepancy that is otherwise handled by the transformation. In this case, choose <Do not transform> from the Transformation path menu. This ensures that no transformation is applied when drawing the data. Keep in mind that your data will not necessarily align properly in this case.
No transformation is available
If you know that your data requires a transformation, but no transformation is available, it is likely because there is no spatial overlap between the geographic coordinate system used by the map and the geographic coordinate system used by the data. Usually this means that the map is not in a coordinate system that makes sense for the data you are trying to show. Consider choosing a more appropriate coordinate system for the map to better match the data, `and then a suitable transformation is likely to become available. The extents of all transformations in ArcGIS Pro can be found in the list of supported geographic and vertical transformations.
Another reason why there are no available transformations might be that the transformation path between the two geographic systems is too complex. Currently, a path is limited to only two transformation steps. When working with vertical coordinate systems, a path allows up to four total transformation steps. Consider choosing a different coordinate system for the map if you think you have encountered this limitation.
If a transformation still cannot be found, it may be that the one you need requires a file that is not included in the ArcGIS Pro installation and must be installed separately. See Install supplementary coordinate system files to learn more.
Store additional transformations in a map
In addition to the transformations currently used in a map, you can store transformations with the map that are not currently in use. These can include transformations that were previously used, for example, by a layer that was removed from the map. Additionally, you can add other transformations if you know they will be necessary when additional data is added to the map or if output circumstances require it—for example, you plan to publish the map to a platform that uses a geographic coordinate system that differs from that of your map. In this storage section, you can specify the geographic and vertical coordinate systems and the transformation between them.
To store additional transformations in a map, follow these steps:
- Right-click the map or scene in the Contents pane and click Properties . On the Map Properties dialog box, click the Transformation tab.
- In the Additional transformations section, click Add.
- Click the Set Coordinate System buttons
to choose geographic and vertical coordinate system pairs. The transformation path is bidirectional, so the order of the geographic and vertical coordinate system pairs doesn't matter.
- Use the drop-down menu between the geographic coordinate systems to specify the transformation to convert between them.
- Click Add to add another transformation, or click Apply to save the transformation in the map.
Show a warning if a transformation has been applied
It is useful to know when a transformation has been applied, especially if you will be creating or editing data in the layer. To ensure that you are always aware of this state, you can enable transformation warnings. With this option enabled, a warning appears anytime you add data to a map or scene that requires a geographic coordinate system transformation. This is an application setting, so it applies to all the projects you work with.
To ensure that warning messages appear in the application when a transformation is applied, follow these steps:
- Click the Project tab and click the Options tab to open the application and project settings.
- On the Options dialog box, in the Application section, click the Map and Scene tab.
- Expand the Spatial Reference heading and check the Warn if transformation between geographic coordinate system is required to align data sources correctly check box.
- Click OK to close the Options dialog box.