116 Geometric Dimensioning and Tolerancing Using a Contoured Surface as a Datum Feature Some applications require a compound or contoured surface to be identified as a datum feature. The feature on the part containing the compound surface is defined by a three-dimensional mathematical coordinate system. In this application, the datum feature datum simulator results from the mathematical data and is used to establish the datum reference frame. Aligning the high points of the datum feature with its datum feature simulator restricts movement of the part to the datum reference frame. Example 3-44 shows the use of a contoured surface as a datum feature and the three-dimensional mathematical coordinate system. The phantom oval line is used to define the limit of the datum target area and the hatched area defines datum target area A1. The word TRUE is added before the dimension when the feature displayed on a 2D view is shown foreshortened. In this example, the holes are shown as ellipses and the note TRUE is added before the ∅6.0/6.1 dimension value, as in TRUE ∅6.0/6.1. The 10 mm basic dimension also has the note TRUE before it to clarify that it is the width of the datum target area, because it does not show true width in the displayed view. The note [BSC] placed in the feature control frames means that datum target area A1 is defined by the 3D CADD model as basic. See note 1 in the example referring to the model for complete product definition. In addition, there are basic dimensions from the origin of the mathematically defined data to locate the hole centers with the x, y, and z axis directions and coordinates. This is an example of using a mathematically defined feature or a contoured surface as a datum feature. A practical application is when a part is designed to copy the contours of an actual object that has few or no flat surfaces to use as datums. In this approach, a CADD model that defines the basic shape of the plate is used and the engineer applies a profile tolerance on the shape of the plate to control its size. The bottom surface is datum target area A, and the CADD model defines the basic location of the holes in the plate. A position tolerance relative to datum target area A on the holes is used to control the hole locations. This application has many uses for parts that are not square. Chapter 7 explains location tolerances.