# About transforming a field in AtlasMap

In a data mapping, you can apply a transformation to one or more fields. A transformation is a function that AtlasMap performs on the selected field. For example, suppose you select a source field and specify the `Uppercase` transformation. At runtime, AtlasMap converts the source field value to uppercase and inserts the uppercase value in the target field.

To apply a transformation, select a source or target terminal field, click Add Transformation in the Mapping Details panel on the right, and select the transformation.

You can apply different transformations to different fields in the same mapping.

In a one-to-one mapping, which maps one source field to one target field, it does not matter whether you apply the transformation to the source field or the target field. In a one-to-many or many-to-one mapping, consider what the target field value needs to be when you specify a transformation. For example, consider a many-to-one mapping that combines source fields for number, street, city, and state into one target address field. If you want the strings in the target address field to all be uppercase, select the target address field and apply the uppercase transformation. If only the state needs to be uppercase, select the source state field, and apply the uppercase transformation.

You can think of a source field transformation as performing pre-processing, while a target field transformation performs post-processing.