# To add station equations

Use Station equations to define the station values for an alignment.

To define an equation for the selected tunnel definition:

1. Tap Station equations.
2. Tap Add.
3. In the Back station field, enter a station value.
4. In the Ahead station field, enter a station value. The True station value is calculated.
5. Continue adding records as required.

6. Tap Store.

   The values entered in the Back station and Ahead station fields are shown.

   The zone is indicated by a number after the colon in each field. The zone up to the first station equation is zone 1.

   The calculated Progression indicates whether the station value increases or decreases after the station equation. The default setting is Increasing. To change the Progression for the last station equation to Decreasing, define and store the last equation and then tap Edit.

7. When you are done, tap Close.

8. Tap Accept.
9. Enter the other tunnel components, or tap Store to store the tunnel definition.