To Specify Geometric References
Geometric references for a Measure step include the entity to be measured (for example, a planar surface, a cylindrical surface, or a point) and, in some cases (such as cylinder, circle, cone), additional references necessary to perform the measurements. In each case, geometric references that you have to select depend on the type of Measure step that you are creating. They are discussed within the topics describing the specific Measure step types.
You can select either solid surfaces or surface quilts (for example, created as a result of importing geometry from another application). When you select a surface of a quilt, the system will prompt you to select which side of it you want to measure. A red arrow normal to the surface will appear; you can specify the direction by using the Flip and Okay options.