# Alternatives for supplying source values that are missing

When you are mapping fields, you might find that a source data shape does not provide a value that a target data shape requires. In these situations, AtlasMap provides two alternatives for specifying source values that you can map to target fields:

- Define a property — In AtlasMap, at the top of the Source panel, to the right of Properties, click the + sign to display a dialog. Enter the property name, the property’s default value, and indicate the data type. Note that at runtime, if there is an environment variable or a Java system property with the same name as the property, it is possible for this default value to be overridden. Click Save to create a new terminal source field.

- Define a constant — In AtlasMap, near the top of the Source panel, to the right of Constants, click the + sign to display a dialog. Enter the value of the constant, and indicate the data type. Click Save to create a new terminal source field.

For example, suppose that a target data shape defines a Layout field whose value must be `HORIZONTAL` or `VERTICAL`. The source data shape does not provide this field. You can create a constant. Specify `HORIZONTAL` or `VERTICAL` as the value and accept `String` as the data type. After you save the new constant, you can map it to the Layout target field.