A geographic datum transformation is a calculation that is used to convert between 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.
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 store 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.
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 named for the two geographic coordinate systems they convert between, but they are bidirectional. The order of the coordinate systems in the name doesn't matter, they work in either direction. You can specify a different transformation if necessary. Transformations are sorted by their accuracy and suitability for the location of your data.
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, if a transformation is required, the table shows the geographic coordinate system of a layer in the left column, the transformation applied in the center column, and the geographic coordinate system in the right column. More than one transformation is listed if there is more than one layer in the map with differing geographic coordinate systems. If the geographic coordinate systems of the layers and the map are identical, no transformation is necessary.
- To choose a different transformation, click the name of a transformation in the Transformation path column of the table to open the menu. Repeat this step as necessary for other coordinate systems used by layers in the map.
- Click Apply to apply the transformations, or click OK to apply the changes and close the Map Properties dialog box.
Choose not to transform data
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 in step 2 above. 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.
Ignore the extents of basemaps
The transformations that are automatically applied or listed in the Transformation path drop-down list are dependent on the spatial extent of the layers in the map, and they are sorted in this list by their suitability. When a map includes one or more basemaps, the extent is global. This might skew the appropriate transformation choices. You can choose to ignore the extents of any basemaps when transformations are presented in this list and limit the extents considered to the operational layers of the map only.
To ignore the extents of basemaps when applying and listing transformations, follow these steps:
- On the ribbon, click the Project tab. In the list of side tabs, click Options to open the project and application options.
- In the list of side tabs, under the Application heading, click Map and Scene.
- Under the Spatial Reference heading, check Ignore the extents of basemap layers when listing and automatically choosing transformations.
View transformation details
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 1983 to WGS 1984, or from WGS 1984 to NAD 1983. 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 or the Create Custom Vertical Transformation tool.
To view the details of a transformation, 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, if a transformation is required, the table shows transformations applied in the center column.
- In the center column below a transformation, 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.
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 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.