A datum is an imaginary construct on a reference object that facilitates comparison to a test object and which can be referenced by other tools. The datums that can be specified are:
Plane Datum - a plane that is superimposed on the object.
Axis Datum - an axis that is superimposed on the object.
Point Datum - a point on the surface of the object.
Point Target Datum - defines the contact point of a
test fixture pin.
Line Target Datum - defines the contact point of a
test fixture “lean bar”.
Tools > Datums > Create Datums presents a dialog for the construction and naming of datums.
The controls are:
Name dropdown and field - specifies the name of the datum that will be created or which is being edited. A datum Name consists of an optional alphabetic prefix (A, B, C, ...) and a “label” part. The two fields that construct a Name are:
Name dropdown (A thru Z, except I, O, and Q) specifies a unique alphabetic prefix for this datum. When an alphabetic prefix has been selected from this list and used in the name of a datum or feature, it does not appear in the list the next time you look. Thus, alphabetic prefixes are guaranteed to be unique among all datums and features on one object.
Name field - specifies the “label” part of this datum’s Name. Based on the Datum Type, the application makes up a label such as “Plane 1”, “Axis 2”. Subsequent datums are labelled consecutively, like “Plane 2”, and “Plane 3”, but the user can override any label.
Datum Type (Plane, Axis, Point, Point Target, or Line Target) - specifies the type of datum to be created.
XYZ Method section (where XYZ is the name of the Datum Type). XYZ can be any one of:
Plane Method - specifies the geometric method by which you will define a plane in the Method Input section below.
CAD Face - by selecting an existing face on a CAD object.
Best Fit - by selecting an almost-planar region on
which the software calculates the best fit for a plane.
Offset - by specifying a distance from any other known
plane.
Axis and Point - by specifying an axis and a point.
Three Points - by specifying three points.
Axis Method - specifies the geometric method by which you will define an axis in the Method Input section below.
CAD Cylinder/Cone - by identifying a cylinder or cone
on a CAD object.
Best Fit - by selecting an almost-linear region on
which the software calculates the best fit for an axis.
Intersect - by defining the intersection of two planes.
Plane and Point - by defining a line that intersects
a point and is perpendicular to a plane.
Two Points - by defining two points.
Point Method - specifies the geometric method by which you will define a point in the Method Input section below.
CAD - by defining a point on a face of a CAD object.
Best Fit - by defining a region in which the point
lies.
Intersect - by defining the intersection of two lines.
Centroid - by defining a group of surrounding points.
One Point - by selecting it precisely.
Point Target Method - specifies the geometric method by which you will define a point target in the Method Input section below.
• 
Best Fit - by selecting
the region in which the point lies.
• 
One Point - by selecting
a point on a surface or at a vertex.
Line Target Method - specifies the geometric method by which you will define a line target in the Method Input section below.
Plane/Direction - by specifying a plane on which the
line lies and a direction in which it “travels”.
Point - by selecting a point on a surface or at a vertex.
Method Input group - Used to specify the exact position of the datum by the method selected. The set of controls in this section differs for every combination of Datum Type and Method. For specific instructions, see:
• Details on the Point Target Datum
• Details on the Line Target Datum
Apply button - generates an image of the datum in the Viewing Area.
Next button - stores the datum in the Current Datums list and the Model Manager, and prepares the dialog for another datum.
Current Datums group -
Datum List - is a list of all datums created so far. To rank a datum as more or less significant in future alignment efforts, highlight its name and click the Up and Down Arrow buttons. To flip the orientation (the normal) of the currently selected Plane or Axis (not a Point or Target), click the Flip button. To delete a datum, click the red X.
Shade Points checkbox (applicable when the object is a point object) - specifies whether to shade the points for added visibility.
Hide Other Objects checkbox - specifies whether to hide objects except the one that is selected in the Model Manager (useful if multiple objects appear in the Viewing Area).
OK button - saves datums to the Model Manager and closes the dialog.
Cancel button - closes the dialog without saving datums.